AstrophysicsforPeopleinaHurry

NEILDEGRASSETYSON

ForallthosewhoaretoobusytoreadfatbooksYetnonethelessseekaconduittothecosmos

CONTENTS

Preface

1. TheGreatestStoryEverTold

2. OnEarthasintheHeavens

3. LetThereBeLight

4. BetweentheGalaxies

5. DarkMatter

6. DarkEnergy

7. TheCosmosontheTable

8. OnBeingRound

9. InvisibleLight

10. BetweenthePlanets

11. ExoplanetEarth

12. ReflectionsontheCosmicPerspective

Acknowledgments

Index

PREFACE

Inrecentyears,nomorethanaweekgoesbywithoutnewsofacosmicdiscoveryworthy of banner headlines. While media gatekeepers may have developed aninterestintheuniverse,thisriseincoveragelikelycomesfromagenuineincreaseinthepublic’sappetiteforscience.Evidenceforthisabounds,fromhittelevisionshows inspired or informed by science, to the success of science fiction filmsstarringmarquee actors, and brought to the screen by celebrated producers anddirectors.Andlately,theatricalreleasebiopicsfeaturingimportantscientistshavebecomeagenreuntoitself.There’salsowidespreadinterestaroundtheworldinscience festivals, science fiction conventions, and science documentaries fortelevision.

Thehighestgrossingfilmofalltimeisbyafamousdirectorwhosethisstoryonaplanetorbitingadistantstar.Anditfeaturesafamousactresswhoplaysanastrobiologist.Whilemostbranchesofsciencehaveascendedinthisera,thefieldof astrophysicspersistently rises to the top. I think I knowwhy.Atone timeoranothereveryoneofushaslookedupatthenightskyandwondered:Whatdoesitallmean?Howdoesitallwork?And,whatismyplaceintheuniverse?

If you’re too busy to absorb the cosmos via classes, textbooks, ordocumentaries,andyounonethelessseekabriefbutmeaningfulintroductiontothefield, I offer youAstrophysics forPeople in aHurry. In this slimvolume, youwillearnafoundationalfluencyinallthemajorideasanddiscoveriesthatdriveourmodernunderstandingoftheuniverse.IfI’vesucceeded,you’llbeculturallyconversantinmyfieldofexpertise,andyoujustmaybehungryformore.

Theuniverseisundernoobligationtomakesensetoyou.

—NDT

AstrophysicsforPeopleinaHurry

1.

TheGreatestStoryEverToldTheworldhaspersistedmanyalongyear,havingoncebeensetgoingintheappropriatemotions.Fromtheseeverythingelsefollows.

LUCRETIUS,C.50BC

Inthebeginning,nearlyfourteenbillionyearsago,allthespaceandallthematterandalltheenergyoftheknownuniversewascontainedinavolumelessthanone-trillionththesizeoftheperiodthatendsthissentence.

Conditionswere sohot, thebasic forcesofnature thatcollectivelydescribetheuniversewereunified.Thoughstillunknownhowitcameintoexistence,thissub-pinpoint-sizecosmoscouldonlyexpand.Rapidly.Inwhat todaywecall thebigbang.

Einstein’sgeneraltheoryofrelativity,putforthin1916,givesusourmodernunderstandingof gravity, inwhich thepresenceofmatter and energy curves thefabricofspaceandtimesurrounding it. In the1920s,quantummechanicswouldbe discovered, providing our modern account of all that is small: molecules,atoms, and subatomic particles. But these two understandings of nature areformally incompatible with one another, which set physicists off on a race toblend the theoryof the smallwith the theoryof the large into a single coherenttheory of quantum gravity. Althoughwe haven’t yet reached the finish line, weknowexactlywherethehighhurdlesare.Oneofthemisduringthe“Planckera”oftheearlyuniverse.That’stheintervaloftimefromt=0uptot=10‒43seconds(one ten-million-trillion-trillion-trillionths of a second) after the beginning, andbefore theuniversegrewto10‒35meters (onehundredbillion trillion-trillionthsof a meter) across. The German physicist Max Planck, after whom these

unimaginablysmallquantitiesarenamed,introducedtheideaofquantizedenergyin1900andisgenerallycreditedasthefatherofquantummechanics.

Theclashbetweengravityandquantummechanicsposesnopracticalproblemfor the contemporary universe. Astrophysicists apply the tenets and tools ofgeneral relativity and quantummechanics to very different classes of problems.Butinthebeginning,duringthePlanckera, thelargewassmall,andwesuspecttheremusthavebeenakindofshotgunweddingbetweenthetwo.Alas,thevowsexchangedduringthatceremonycontinuetoeludeus,andsono(known)lawsofphysicsdescribewithanyconfidencethebehavioroftheuniverseoverthattime.

We nonetheless expect that by the end of the Planck era, gravity wriggledloose from the other, still unified forces of nature, achieving an independentidentity nicely described by our current theories. As the universe aged through10‒35 seconds it continued to expand, diluting all concentrations of energy, andwhat remainedof theunified forcessplit into the“electroweak”and the“strongnuclear” forces. Later still, the electroweak force split into the electromagneticandthe“weaknuclear”forces,layingbarethefourdistinctforceswehavecometoknowand love:with theweakforcecontrolling radioactivedecay, thestrongforce binding the atomic nucleus, the electromagnetic force binding molecules,andgravitybindingbulkmatter.

Atrillionthofasecondhaspassedsincethebeginning.

All thewhile, the interplayofmatter in theformofsubatomicparticles,andenergy in theformofphotons(masslessvesselsof lightenergy thatareasmuchwavesastheyareparticles)wasincessant.Theuniversewashotenoughforthesephotonstospontaneouslyconverttheirenergyintomatter-antimatterparticlepairs,which immediately thereafter annihilate, returning their energy back to photons.Yes,antimatter is real.Andwediscovered it,notsciencefictionwriters.ThesetransmogrificationsareentirelyprescribedbyEinstein’smostfamousequation:E=mc2,whichisatwo-wayrecipeforhowmuchmatteryourenergyisworth,andhowmuchenergyyourmatter isworth.Thec2 is the speedof light squared—ahugenumberwhich,whenmultipliedby themass, remindsushowmuchenergyyouactuallygetinthisexercise.

Shortly before, during, and after the strong and electroweak forces partedcompany,theuniversewasaseethingsoupofquarks,leptons,andtheirantimattersiblings, alongwithbosons, theparticles that enable their interactions.Noneof

these particle families is thought to be divisible into anything smaller ormorebasic,thougheachcomesinseveralvarieties.Theordinaryphotonisamemberofthebosonfamily.The leptonsmost familiar to thenon-physicistare theelectronandperhapstheneutrino;andthemostfamiliarquarksare...well,therearenofamiliarquarks.Eachoftheirsixsubspecieshasbeenassignedanabstractnamethatservesnorealphilological,philosophical,orpedagogicalpurpose,excepttodistinguish it from theothers:up anddown,strange andcharmed, and top andbottom.

Bosons,bytheway,arenamedfor theIndianscientistSatyendraNathBose.Theword “lepton” derives from theGreek leptos, meaning “light” or “small.”“Quark,”however,hasa literaryand farmore imaginativeorigin.ThephysicistMurrayGell-Mann,whoin1964proposedtheexistenceofquarksastheinternalconstituentsofneutronsandprotons,andwhoatthetimethoughtthequarkfamilyhadonly threemembers,drewthenamefromacharacteristicallyelusive line inJames Joyce’s Finnegans Wake: “Three quarks for Muster Mark!” One thingquarksdohavegoingfor them:all theirnamesaresimple—somethingchemists,biologists, and especially geologists seem incapable of achievingwhen namingtheirownstuff.

Quarksarequirkybeasts.Unlikeprotons,eachwithanelectricchargeof+1,and electrons,with a charge of –1, quarks have fractional charges that come inthirds.And you’ll never catch a quark all by itself; itwill always be clutchingotherquarksnearby. In fact, the force thatkeeps two(ormore)of themtogetheractuallygrowsstrongerthemoreyouseparatethem—asiftheywereattachedbysomesortofsubnuclearrubberband.Separatethequarksenough,therubberbandsnapsandthestoredenergysummonsE=mc2tocreateanewquarkateachend,leavingyoubackwhereyoustarted.

During the quark–lepton era the universewas dense enough for the averageseparation between unattached quarks to rival the separation between attachedquarks.Underthoseconditions,allegiancebetweenadjacentquarkscouldnotbeunambiguouslyestablished,andtheymovedfreelyamongthemselves, inspiteofbeingcollectivelybound tooneanother.Thediscoveryof this stateofmatter, akind of quark cauldron, was reported for the first time in 2002 by a team ofphysicistsattheBrookhavenNationalLaboratories,LongIsland,NewYork.

Strongtheoreticalevidencesuggeststhatanepisodeintheveryearlyuniverse,perhapsduringoneof the force splits, endowed theuniversewitha remarkableasymmetry, in which particles of matter barely outnumbered particles ofantimatter:byabillion-and-one toabillion.Thatsmalldifference inpopulationwouldhardlygetnoticedbyanybodyamid thecontinuouscreation,annihilation,and re-creation of quarks and antiquarks, electrons and antielectrons (better

knownaspositrons),andneutrinosandantineutrinos.Theoddmanouthadoodlesofopportunitiestofindsomebodytoannihilatewith,andsodideverybodyelse.

Butnotformuchlonger.Asthecosmoscontinuedtoexpandandcool,growinglargerthanthesizeofoursolarsystem,thetemperaturedroppedrapidlybelowatrilliondegreesKelvin.

Amillionthofasecondhaspassedsincethebeginning.

Thistepiduniversewasnolongerhotenoughordenseenoughtocookquarks,andsotheyallgrabbeddancepartners,creatingapermanentnewfamilyofheavyparticlescalledhadrons(fromtheGreekhadros,meaning“thick”).Thatquark-to-hadrontransitionsoonresultedintheemergenceofprotonsandneutronsaswellasother, less familiarheavyparticles,allcomposedofvariouscombinationsofquark species. In Switzerland (back on Earth) the European particle physicscollaboration†usesalargeacceleratortocollidebeamsofhadronsinanattempttore-create theseveryconditions.This largestmachine in theworld issensiblycalledtheLargeHadronCollider.

The slightmatter–antimatter asymmetryafflicting thequark–lepton soupnowpassedtothehadrons,butwithextraordinaryconsequences.

As the universe continued to cool, the amount of energy available for thespontaneouscreationofbasicparticlesdropped.During thehadronera,ambientphotons could no longer invokeE=mc2 tomanufacture quark–antiquark pairs.Notonly that, thephotons thatemerged fromall the remainingannihilations lostenergytotheever-expandinguniverse,droppingbelowthethresholdrequiredtocreatehadron–antihadronpairs.Foreverybillionannihilations—leavingabillionphotonsintheirwake—asinglehadronsurvived.Thoselonerswouldultimatelygettohaveallthefun:servingastheultimatesourceofmattertocreategalaxies,stars,planets,andpetunias.

Without the billion-and-one to a billion imbalance between matter andantimatter,allmassintheuniversewouldhaveself-annihilated,leavingacosmosmadeofphotonsandnothingelse—theultimatelet-there-be-lightscenario.

Bynow,onesecondoftimehaspassed.

Theuniversehasgrowntoafewlight-yearsacross,††aboutthedistancefromtheSuntoitsclosestneighboringstars.Atabilliondegrees,it’sstillplentyhot—and still able to cook electrons, which, along with their positron counterparts,continue topop inandoutofexistence.But in theever-expanding,ever-coolinguniverse,theirdays(seconds,really)arenumbered.Whatwastrueforquarks,andtrueforhadrons,hadbecometrueforelectrons:eventuallyonlyoneelectroninabillionsurvives.Therestannihilatewithpositrons,theirantimattersidekicks,inaseaofphotons.

Right about now, one electron for every proton has been “frozen” intoexistence.As the cosmoscontinues to cool—droppingbelowahundredmilliondegrees—protons fuse with protons as well as with neutrons, forming atomicnuclei and hatching a universe in which ninety percent of these nuclei arehydrogen and ten percent are helium, along with trace amounts of deuterium(“heavy”hydrogen),tritium(evenheavierhydrogen),andlithium.

Twominuteshavenowpassedsincethebeginning.

For another 380,000 years not much will happen to our particle soup.Throughout thesemillennia the temperature remains hot enough for electrons toroam free among the photons, batting them to and fro as they interactwith oneanother.

Butthisfreedomcomestoanabruptendwhenthetemperatureoftheuniversefalls below 3,000 degrees Kelvin (about half the temperature of the Sun’ssurface), and all the free electrons combine with nuclei. The marriage leavesbehindaubiquitousbathofvisiblelight,foreverimprintingtheskywitharecordof where all the matter was in that moment, and completing the formation ofparticlesandatomsintheprimordialuniverse.

Forthefirstbillionyears,theuniversecontinuedtoexpandandcoolasmattergravitated into the massive concentrations we call galaxies. Nearly a hundredbillionofthemformed,eachcontaininghundredsofbillionsofstarsthatundergothermonuclearfusionintheircores.Thosestarswithmorethanabouttentimesthemass of the Sun achieve sufficient pressure and temperature in their cores tomanufacture dozens of elements heavier than hydrogen, including those that

composeplanetsandwhateverlifemaythriveuponthem.Theseelementswouldbestunninglyuselesswere they to remainwhere they

formed. But high-mass stars fortuitously explode, scattering their chemicallyenrichedgutsthroughoutthegalaxy.Afterninebillionyearsofsuchenrichment,inanundistinguishedpartoftheuniverse(theoutskirtsoftheVirgoSupercluster)inanundistinguishedgalaxy(theMilkyWay)inanundistinguishedregion(theOrionArm),anundistinguishedstar(theSun)wasborn.

The gas cloud fromwhich the Sun formed contained a sufficient supply ofheavy elements to coalesce and spawn a complex inventory of orbiting objectsthat includes several rocky and gaseous planets, hundreds of thousands ofasteroids,andbillionsofcomets.Forthefirstseveralhundredmillionyears,largequantitiesofleftoverdebrisinwaywardorbitswouldaccreteontolargerbodies.This occurred in the form of high-speed, high-energy impacts, which renderedmolten the surfaces of the rocky planets, preventing the formation of complexmolecules.

As less and less accretable matter remained in the solar system, planetsurfacesbegantocool.TheonewecallEarthformedinakindofGoldilockszonearoundtheSun,whereoceansremainlargelyinliquidform.HadEarthbeenmuchclosertotheSun,theoceanswouldhaveevaporated.HadEarthbeenmuchfartheraway,theoceanswouldhavefrozen.Ineithercase,lifeasweknowitwouldnothaveevolved.

Within the chemically rich liquid oceans, by a mechanism yet to bediscovered, organicmolecules transitioned to self-replicating life. Dominant inthisprimordialsoupweresimpleanaerobicbacteria—lifethatthrivesinoxygen-empty environments but excretes chemically potent oxygen as one of its by-products. These early, single-celled organisms unwittingly transformed Earth’scarbondioxide-richatmosphereintoonewithsufficientoxygentoallowaerobicorganismstoemergeanddominatetheoceansandland.Thesesameoxygenatoms,normallyfound inpairs (O2),alsocombined in threes to formozone(O3) in theupper atmosphere, which serves as a shield that protects Earth’s surface frommostoftheSun’smolecule-hostileultravioletphotons.

WeowetheremarkablediversityoflifeonEarth,andwepresumeelsewherein theuniverse, to thecosmicabundanceof carbonand thecountlessnumberofsimple and complexmolecules that contain it. There’s no doubt about it: morevarieties of carbon-based molecules exist than all other kinds of moleculescombined.

Butlifeisfragile.Earth’soccasionalencounterswithlarge,waywardcometsand asteroids, a formerly common event, wreaks intermittent havoc upon our

ecosystem.Ameresixty-fivemillionyearsago(lessthantwopercentofEarth’spast), a ten-trillion-ton asteroid hit what is now the Yucatan Peninsula andobliteratedmorethanseventypercentofEarth’sfloraandfauna—includingallthefamous outsized dinosaurs. Extinction. This ecological catastrophe enabled ourmammal ancestors to fill freshly vacant niches, rather than continue to serve ashorsd’oeuvresforT.rex.Onebig-brainedbranchof thesemammals, thatwhichwe call primates, evolved a genus and species (Homo sapiens)with sufficientintelligencetoinventmethodsandtoolsofscience—andtodeducetheoriginandevolutionoftheuniverse.

Whathappenedbeforeallthis?Whathappenedbeforethebeginning?Astrophysicistshavenoidea.Or,rather,ourmostcreativeideashavelittleor

nogroundinginexperimentalscience.Inresponse,somereligiouspeopleassert,with a tinge of righteousness, that something must have started it all: a forcegreaterthanallothers,asourcefromwhicheverythingissues.Aprimemover.Inthemindofsuchaperson,thatsomethingis,ofcourse,God.

Butwhatiftheuniversewasalwaysthere,inastateorconditionwehaveyettoidentify—amultiverse,forinstance,thatcontinuallybirthsuniverses?Orwhatiftheuniversejustpoppedintoexistencefromnothing?Orwhatifeverythingweknowand lovewere justacomputer simulation rendered forentertainmentbyasuperintelligentalienspecies?

These philosophically fun ideas usually satisfy nobody. Nonetheless, theyremind us that ignorance is the natural state of mind for a research scientist.People who believe they are ignorant of nothing have neither looked for, norstumbledupon,theboundarybetweenwhatisknownandunknownintheuniverse.

Whatwedoknow,andwhatwecanassertwithout furtherhesitation, is thattheuniversehad abeginning.Theuniverse continues to evolve.Andyes, everyone of our body’s atoms is traceable to the big bang and to the thermonuclearfurnaceswithinhigh-massstarsthatexplodedmorethanfivebillionyearsago.

Wearestardustbroughttolife,thenempoweredbytheuniversetofigureitselfout—andwehaveonlyjustbegun.

†TheEuropeanCenterforNuclearResearch,betterknownbyitsacronym,CERN.††Alight-yearisthedistancelighttravelsinoneEarthyear—nearlysixtrillionmilesortentrillionkilometers.

2.

OnEarthasintheHeavens

UntilSirIsaacNewtonwrotedowntheuniversallawofgravitation,nobodyhadany reason to presume that the laws of physics at home were the same aseverywhereelseintheuniverse.Earthhadearthlythingsgoingonandtheheavenshadheavenly things going on.According toChristian teachings of the day,Godcontrolled the heavens, rendering them unknowable to our feeblemortalminds.When Newton breached this philosophical barrier by rendering all motioncomprehensible and predictable, some theologians criticized him for leavingnothing for the Creator to do. Newton had figured out that the force of gravitypulling ripe apples from their orchards also guides tossed objects along theircurvedtrajectoriesanddirectstheMooninitsorbitaroundEarth.Newton’slawofgravityalsoguidesplanets,asteroids,andcometsintheirorbitsaroundtheSunandkeepshundredsofbillionsofstarsinorbitwithinourMilkyWaygalaxy.

This universality of physical laws drives scientific discovery like nothingelse.Andgravitywasjustthebeginning.Imaginetheexcitementamongnineteenth-century astronomers when laboratory prisms, which break light beams into aspectrumofcolors,werefirstturnedtotheSun.Spectraarenotonlybeautiful,butcontain oodles of information about the light-emitting object, including itstemperature and composition. Chemical elements reveal themselves by theiruniquepatternsof light or darkbands that cut across the spectrum.Topeople’sdelightandamazement,thechemicalsignaturesontheSunwereidenticaltothoseinthelaboratory.Nolongertheexclusivetoolofchemists,theprismshowedthatas different as the Sun is from Earth in size, mass, temperature, location, andappearance,webothcontain the samestuff:hydrogen,carbon,oxygen,nitrogen,calcium, iron, and so forth. Butmore important than our laundry list of sharedingredientswastherecognitionthatthelawsofphysicsprescribingtheformation

of these spectral signatures on the Sunwere the same laws operating onEarth,ninety-threemillionmilesaway.

Sofertilewasthisconceptofuniversalitythatitwassuccessfullyappliedinreverse. Further analysis of the Sun’s spectrum revealed the signature of anelement that had no known counterpart on Earth. Being of the Sun, the newsubstancewasgivenanamederivedfromtheGreekwordhelios(“theSun”),andwas only later discovered in the lab. Thus, helium became the first and onlyelement in the chemist’s PeriodicTable to be discovered someplace other thanEarth.

Okay,thelawsofphysicsworkinthesolarsystem,butdotheyworkacrossthegalaxy?Acrosstheuniverse?Acrosstimeitself?Stepbystep,thelawsweretested.Nearbystarsalsorevealedfamiliarchemicals.Distantbinarystars,boundinmutualorbit, seemtoknowallaboutNewton’s lawsofgravity.For thesamereason,sodobinarygalaxies.

And, like the geologist’s stratified sediments, which serve as a timeline ofearthlyevents,thefartherawaywelookinspace,thefurtherbackintimewesee.Spectra from the most distant objects in the universe show the same chemicalsignaturesthatweseenearbyinspaceandintime.True,heavyelementswerelessabundantbackthen—theyaremanufacturedprimarilyinsubsequentgenerationsofexplodingstars—butthelawsdescribingtheatomicandmolecularprocessesthatcreatedthesespectralsignaturesremainintact.Inparticular,aquantityknownasthe fine-structure constant, which controls the basic fingerprinting for everyelement,musthaveremainedunchangedforbillionsofyears.

Ofcourse,notall thingsandphenomena in thecosmoshavecounterpartsonEarth.You’veprobablyneverwalkedthroughacloudofglowingmillion-degreeplasma,andI’dbetyou’venevergreetedablackholeonthestreet.Whatmattersistheuniversalityofthephysicallawsthatdescribethem.Whenspectralanalysiswas first applied to the light emitted by interstellar nebulae, a signature wasdiscoveredthat,onceagain,hadnocounterpartonEarth.Atthetime,thePeriodicTable of Elements had no obvious place for a new element to fit. In response,astrophysicists invented thename“nebulium”asaplace-holder,until theycouldfigureoutwhatwasgoingon.Turnedout that in space, gaseousnebulae are sorarefied that atoms go long stretcheswithout colliding. Under these conditions,electronscandothingswithinatomsthathadneverbeforebeenseeninEarthlabs.Nebuliumwassimplythesignatureofordinaryoxygendoingextraordinarythings.

This universality of physical laws tells us that ifwe landon another planetwitha thrivingaliencivilization, theywillberunningon thesamelaws thatwehave discovered and tested here on Earth—even if the aliens harbor differentsocialandpoliticalbeliefs.Furthermore,ifyouwantedtotalktothealiens,you

can bet they don’t speak English or French or evenMandarin. Nor would youknowwhether shaking their hands—if indeed their outstretched appendage is ahand—wouldbeconsideredanactofwarorofpeace.Yourbesthopeistofindawaytocommunicateusingthelanguageofscience.

Suchanattemptwasmadeinthe1970swithPioneer10and11andVoyager1and2.Allfourspacecraftwereendowedwithenoughenergy,aftergravityassistsfromthegiantplanets,toescapethesolarsystementirely.

Pioneerwore a golden etchedplaque that showed, in scientific pictograms,the layout of our solar system, our location in the MilkyWay galaxy, and thestructure of the hydrogen atom.Voyager went further and also included a goldrecordalbumcontainingdiverse sounds frommotherEarth, including thehumanheartbeat,whale“songs,”andmusicalselectionsfromaroundtheworld,includingtheworksofBeethovenandChuckBerry.Whilethishumanizedthemessage,it’snot clear whether alien ears would have a clue what they were listening to—assumingtheyhaveearsinthefirstplace.MyfavoriteparodyofthisgesturewasaskitonNBC’sSaturdayNightLive,shortlyaftertheVoyager launch,inwhichthey showed awritten reply from the alienswho recovered the spacecraft.Thenotesimplyrequested,“SendmoreChuckBerry.”

Science thrivesnotonlyon theuniversalityofphysical lawsbutalsoon theexistence and persistence of physical constants. The constant of gravitation,knownbymostscientistsas“bigG,”suppliesNewton’sequationofgravitywiththemeasure of how strong the forcewill be. This quantity has been implicitlytestedforvariationovereons.Ifyoudothemath,youcandeterminethatastar’sluminosityissteeplydependentonbigG.Inotherwords,ifbigGhadbeenevenslightlydifferentinthepast,thentheenergyoutputoftheSunwouldhavebeenfarmorevariablethananythingthebiological,climatological,orgeologicalrecordsindicate.

Suchistheuniformityofouruniverse.

Amongallconstants,thespeedoflightisthemostfamous.Nomatterhowfastyougo,youwill neverovertakeabeamof light.Whynot?Noexperiment everconducted has ever revealed an object of any form reaching the speed of light.Well-tested laws of physics predict and account for that fact. I know thesestatements sound closed-minded. Some of themost bone-headed, science-basedproclamations in the past have underestimated the ingenuity of inventors andengineers: “We will never fly.” “Flying will never be commercially feasible.”“Wewillneversplittheatom.”“Wewillneverbreakthesoundbarrier.”“Wewill

nevergototheMoon.”Whattheyhaveincommonis thatnoestablishedlawofphysicsstoodinthetheirway.

Theclaim“Wewillneveroutrunabeamoflight”isaqualitativelydifferentprediction.Itflowsfrombasic,time-testedphysicalprinciples.Highwaysignsforinterstellartravelersofthefuturewilljustifiablyread:

TheSpeedofLight:It’sNotJustaGoodIdea

It’stheLaw.

Unlikegetting caught speedingonEarth roads, thegood thingabout the lawsofphysics is that they require no law enforcement agencies to maintain them,althoughIdidonceownageekyT-shirtthatproclaimed,“OBEYGRAVITY.”

All measurements suggest that the known fundamental constants, and thephysical laws that reference them, are neither time-dependent nor location-dependent.They’retrulyconstantanduniversal.

Many natural phenomenamanifestmultiple physical laws operating at once.This fact often complicates the analysis and, in most cases, requires high-performancecomputingtocalculatewhat’sgoingonandtokeeptrackofimportantparameters. When comet Shoemaker-Levy 9 plunged into Jupiter’s gas-richatmosphere in July1994, and then exploded, themost accurate computermodelcombined the laws of fluid mechanics, thermodynamics, kinematics, andgravitation.Climateandweatherrepresentotherleadingexamplesofcomplicated(anddifficult-to-predict)phenomena.Butthebasiclawsgoverningthemarestillatwork.Jupiter’sGreatRedSpot,araginganticyclonethathasbeengoingstrongfor at least 350 years, is driven by identical physical processes that generatestormsonEarthandelsewhereinthesolarsystem.

Anotherclassofuniversaltruthsistheconservationlaws,wheretheamountofsome measured quantity remains unchanged no matter what. The three mostimportantaretheconservationofmassandenergy,theconservationoflinearandangular momentum, and the conservation of electric charge. These laws are inevidenceonEarth,andeverywherewehavethoughttolook—fromthedomainofparticlephysicstothelarge-scalestructureoftheuniverse.

In spite of this boasting, all is not perfect in paradise. It happens that wecannot see, touch, or taste the source of eighty-five percent of the gravity wemeasureintheuniverse.Thismysteriousdarkmatter,whichremainsundetected

except for its gravitational pull onmatter we see, may be composed of exoticparticles that we have yet to discover or identify. A small minority ofastrophysicists,however,areunconvincedandhavesuggestedthatthereisnodarkmatter—you just need to modify Newton’s law of gravity. Simply add a fewcomponentstotheequationsandallwillbewell.

Perhaps one day we will learn that Newton’s gravity indeed requiresadjustment.That’llbeokay.Ithashappenedoncebefore.Einstein’s1916generaltheoryofrelativityexpandedontheprinciplesofNewton’sgravityinawaythatalsoapplied toobjectsofextremelyhighmass.Newton’s lawofgravitybreaksdowninthisexpandedrealm,whichwasunknowntohim.Thelessonhereisthatourconfidenceflowsthroughtherangeofconditionsoverwhichalawhasbeentestedandverified.Thebroaderthatrange,themorepotentandpowerfulthelawbecomesindescribingthecosmos.Forordinaryhouseholdgravity,Newton’slawworksjustfine.ItgotustotheMoonandreturnedussafelytoEarthin1969.Forblack holes and the large-scale structure of the universe, we need generalrelativity.And if you insert lowmass and low speeds intoEinstein’s equationsthey literally (or, rather,mathematically)becomeNewton’s equations—allgoodreasonstodevelopconfidenceinourunderstandingofallweclaimtounderstand.

To the scientist, the universality of physical laws makes the cosmos amarvelously simple place. By comparison, human nature—the psychologist’sdomain—is infinitely more daunting. In America, local school boards vote onsubjectstobetaughtintheclassroom.Insomecases,votesarecastaccordingtothe whims of cultural, political, or religious tides. Around the world, varyingbelief systems lead to political differences that are not always resolvedpeacefully.Thepowerandbeautyofphysicallawsisthattheyapplyeverywhere,whetherornotyouchoosetobelieveinthem.

Inotherwords,afterthelawsofphysics,everythingelseisopinion.Not that scientistsdon’targue.Wedo.A lot.Butwhenwedo,we typically

express opinions about the interpretation of insufficient or ratty data on thebleedingfrontierofourknowledge.Whereverandwheneveraphysical lawcanbeinvokedinthediscussion,thedebateisguaranteedtobebrief:No,yourideaforaperpetualmotionmachinewillneverwork; itviolateswell-tested lawsofthermodynamics.No, you can’t build a timemachine thatwill enableyou togobackandkillyourmotherbeforeyouwereborn—itviolatescausalitylaws.Andwithout violatingmomentum laws, you cannot spontaneously levitate and hoverabovetheground,whetherornotyouareseatedinthelotusposition.†

Knowledgeofphysical lawscan, in somecases,giveyou theconfidence toconfront surly people.A few years ago Iwas having a hot-cocoa nightcap at adessertshopinPasadena,California.Ordereditwithwhippedcream,ofcourse.Whenitarrivedatthetable,Isawnotraceofthestuff.AfterItoldthewaiterthatmycocoahadnowhippedcream,heassertedIcouldn’tseeitbecauseitsanktothe bottom. But whipped cream has low density, and floats on all liquids thathumans consume. So I offered the waiter two possible explanations: eithersomebodyforgottoaddthewhippedcreamtomyhotcocoaortheuniversallawsofphysicsweredifferentinhisrestaurant.Unconvinced,hedefiantlybroughtoveradollopofwhippedcreamtodemonstratehisclaim.Afterbobbingonceortwicethewhippedcreamrosetothetop,safelyafloat.

Whatbetterproofdoyouneedoftheuniversalityofphysicallaw?

†Youcould,inprinciple,performthisstuntifyoumanagedtoletforthapowerfulandsustainedexhaustofflatulence.

3.

LetThereBeLight

Afterthebigbang,themainagendaofthecosmoswasexpansion,everdilutingthe concentration of energy that filled space. With each passing moment theuniverse got a little bit bigger, a little bit cooler, and a little bit dimmer.Meanwhile,matterandenergyco-inhabitedakindofopaquesoup,inwhichfree-rangeelectronscontinuallyscatteredphotonseverywhichway.

For380,000years,thingscarriedonthatway.Inthisearlyepoch,photonsdidn’ttravelfarbeforeencounteringanelectron.

Backthen,ifyourmissionhadbeentoseeacrosstheuniverse,youcouldn’t.Anyphotonyoudetectedhadcareenedoffanelectronrightinfrontofyournose,nano-and picoseconds earlier.† Since that’s the largest distance that information cantravelbeforereachingyoureyes,theentireuniversewassimplyaglowingopaquefog ineverydirectionyou looked.TheSunandallother starsbehave thisway,too.

Asthetemperaturedrops,particlesmovemoreandmoreslowly.Andsorightabout then, when the temperature of the universe first dipped below a red-hot3,000 degrees Kelvin, electrons slowed down just enough to be captured bypassing protons, thus bringing full-fledged atoms into the world. This allowedpreviouslyharassedphotonstobesetfreeandtravelonuninterruptedpathsacrosstheuniverse.

This “cosmic background” is the incarnation of the leftover light from adazzling, sizzling early universe, and can be assigned a temperature, based onwhat part of the spectrum the dominant photons represent. As the cosmoscontinued to cool, the photons that had been born in the visible part of thespectrum lost energy to the expanding universe and eventually slid down thespectrum,morphingintoinfraredphotons.Althoughthevisiblelightphotonshad

becomeweakerandweaker,theyneverstoppedbeingphotons.What’snextonthespectrum?Today,theuniversehasexpandedbyafactorof

1,000fromthetimephotonsweresetfree,andsothecosmicbackgroundhas,inturn, cooled by a factor of 1,000.All the visible light photons from that epochhavebecome1/1,000th as energetic.They’renowmicrowaves,which iswherewe derive the modern moniker “cosmic microwave background,” or CMB forshort.Keepthisupandfiftybillionyearsfromnowastrophysicistswillbewritingaboutthecosmicradiowavebackground.

When something glows from being heated, it emits light in all parts of thespectrum, butwill always peak somewhere. For household lamps that still useglowingmetal filaments, the bulbs all peak in the infrared,which is the singlegreatest contributor to their inefficiencyas a sourceofvisible light.Our sensesdetect infrared only in the form ofwarmth on our skin. The LED revolution inadvancedlightingtechnologycreatespurevisiblelightwithoutwastingwattageoninvisiblepartsofthespectrum.That’showyoucangetcrazy-soundingsentenceslike:“7WattsLEDreplaces60WattsIncandescent”onthepackaging.

Beingtheremnantofsomethingthatwasoncebrilliantlyaglow,theCMBhastheprofileweexpectofaradiantbutcoolingobject: itpeaks inonepartof thespectrumbutradiatesinotherpartsofthespectrumaswell.Inthiscase,besidespeaking in microwaves, the CMB also gives off some radio waves and avanishinglysmallnumberofphotonsofhigherenergy.

In themid-twentieth century, the subfield of cosmology—not to be confusedwithcosmetology—didn’thavemuchdata.Andwheredataaresparse,competingideas abound that are clever and wishful. The existence of the CMB waspredictedbytheRussian-bornAmericanphysicistGeorgeGamowandcolleaguesduringthe1940s.Thefoundationoftheseideascamefromthe1927workoftheBelgianphysicistandpriestGeorgesLemaître,whoisgenerallyrecognizedasthe“father”ofbigbangcosmology.ButitwasAmericanphysicistsRalphAlpherandRobertHermanwho,in1948,firstestimatedwhatthetemperatureofthecosmicbackground ought to be. They based their calculations on three pillars: 1)Einstein’s1916general theoryof relativity;2)EdwinHubble’s1929discoverythat theuniverse is expanding; and3) atomicphysicsdeveloped in laboratoriesbeforeandduringtheManhattanProjectthatbuilttheatomicbombsofWorldWarII.

Herman and Alpher calculated and proposed a temperature of 5 degreesKelvin for the universe.Well, that’s just plain wrong. The precisely measuredtemperatureof thesemicrowaves is2.725degrees,sometimeswrittenassimply2.7degrees,andifyou’renumericallylazy,nobodywillfaultyouforroundingthetemperatureoftheuniverseto3degrees.

Let’s pause for amoment. Herman and Alpher used atomic physics freshlygleanedinalab,andappliedit tohypothesizedconditionsintheearlyuniverse.From this, they extrapolated billions of years forward, calculating whattemperature the universe should be today. That their prediction even remotelyapproximatedtherightanswerisastunningtriumphofhumaninsight.Theycouldhave been off by a factor or ten, or a hundred, or they could have predictedsomething that isn’t even there. Commenting on this feat, the AmericanastrophysicistJ.RichardGottnoted,“Predictingthatthebackgroundexistedandthengettingitstemperaturecorrecttowithinafactorof2,waslikepredictingthataflyingsaucer50feetwidewouldlandontheWhiteHouselawn,butinstead,aflyingsaucer27feetwideactuallyshowedup.”

Thefirstdirectobservationof thecosmicmicrowavebackgroundwasmadeinadvertentlyin1964byAmericanphysicistsArnoPenziasandRobertWilsonofBellTelephoneLaboratories,theresearchbranchofAT&T.Inthe1960severyoneknewaboutmicrowaves,butalmostnoonehadthetechnologytodetectthem.BellLabs,apioneer in thecommunications industry,developedabeefy,horn-shapedantennaforjustthatpurpose.

Butfirst,ifyou’regoingtosendorreceiveasignal,youdon’twanttoomanysources contaminating it. Penzias and Wilson sought to measure backgroundmicrowave interference to their receiver, to enable clean, noise-freecommunication within this band of the spectrum. They were not cosmologists.They were techno-wizards honing a microwave receiver, and unaware of theGamow,Herman,andAlpherpredictions.

What Penzias and Wilson were decidedly not looking for was the cosmicmicrowave background; they were just trying to open a new channel ofcommunicationforAT&T.

PenziasandWilsonrantheirexperiment,andsubtractedfromtheirdataalltheknownterrestrialandcosmicsourcesofinterferencetheycouldidentify,butonepartofthesignaldidn’tgoaway,andtheyjustcouldn’tfigureouthowtoeliminateit.Finallytheylookedinsidethedishandsawpigeonsnestingthere.Andsotheywere worried that a white dielectric substance (pigeon poop) might beresponsible for the signal,because theydetected it nomatterwhatdirection thedetector pointed. After cleaning out the dielectric substance, the interferencedroppeda littlebit,buta leftover signal remained.Thepaper theypublished in1965wasallaboutthisunaccountable“excessantennatemperature.”††

Meanwhile, a team of physicists at Princeton, led by Robert Dicke, was

building a detector specifically to find the CMB. But they didn’t have theresourcesofBellLabs,sotheirworkwentalittleslower.AndthemomentDickeand his colleagues heard about Penzias andWilson’swork, the Princeton teamknewexactlywhat theobservedexcessantenna temperaturewas.Everythingfit:especiallythetemperatureitself,andthatthesignalcamefromeverydirectioninthesky.

In1978,PenziasandWilsonwontheNobelPrizefortheirdiscovery.Andin2006,AmericanastrophysicistsJohnC.MatherandGeorgeF.Smootwouldsharethe Nobel Prize for observing the CMB over a broad range of the spectrum,bringingcosmologyfromanurseryofcleverbutuntestedideasintotherealmofaprecision,experimentalscience.

Becauselighttakestimetoreachusfromdistantplacesintheuniverse,ifwelook out in deep spacewe actually see eons back in time. So if the intelligentinhabitants of a galaxy far, far away were to measure the temperature of thecosmicbackgroundradiationatthemomentcapturedbyourgaze,theyshouldgetareading higher than 2.7 degrees, because they are living in a younger, smaller,hotteruniversethanweare.

Turns out you can actually test this hypothesis. Themolecule cyanogen CN(once used on convicted murderers as the active component of the gasadministeredbytheirexecutioners)getsexcitedbyexposuretomicrowaves.Ifthemicrowaves arewarmer than the ones in our CMB, they excite themolecule alittlemore.Inthebigbangmodel, thecyanogenindistant,youngergalaxiesgetsbathedinawarmercosmicbackgroundthanthecyanogeninourownMilkyWaygalaxy.Andthat’sexactlywhatweobserve.

Youcan’tmakethisstuffup.Whyshouldanyofthisbeinteresting?Theuniversewasopaqueuntil380,000

yearsafterthebigbang,soyoucouldnothavewitnessedmattertakingshapeevenifyou’dbeen sitting front-rowcenter.Youcouldn’t have seenwhere thegalaxyclustersandvoidswerestartingtoform.Beforeanybodycouldhaveseenanythingworthseeing,photonshadtotravel,unimpeded,acrosstheuniverse,ascarriersofthisinformation.

Thespotwhereeachphotonbegan itscross-cosmos journey iswhere ithadsmackedintothelastelectronthatwouldeverstandinitsway—the“pointoflastscatter.”Asmoreandmorephotonsescapeunsmacked,theycreateanexpanding“surface”oflastscatter,some120,000yearsdeep.Thatsurfaceiswherealltheatomsintheuniversewereborn:anelectronjoinsanatomicnucleus,andalittle

pulseofenergyintheformofaphotonsoarsawayintothewildredyonder.By then, some regionsof theuniversehad alreadybegun to coalesceby the

gravitational attractionof their parts.Photons that last scatteredoff electrons inthese regions developed a different, slightly cooler profile than those scatteringoff the less sociable electrons sitting in the middle of nowhere.Where matteraccumulated, the strength of gravity grew, enabling more and more matter togather. These regions seeded the formation of galaxy superclusters while otherregionswereleftrelativelyempty.

Whenyoumapthecosmicmicrowavebackgroundindetail,youfindthatit’snot completely smooth. It’s got spots that are slightly hotter and slightly coolerthanaverage.BystudyingthesetemperaturevariationsintheCMB—thatistosay,bystudyingpatternsinthesurfaceoflastscatter—wecaninferwhatthestructureandcontentofthematterwasintheearlyuniverse.Tofigureouthowgalaxiesandclustersandsuperclustersarose,weuseourbestprobe,theCMB—apotenttimecapsule that empowers astrophysicists to reconstruct cosmic history in reverse.Studying its patterns is like performing some sort of cosmic phrenology, asweanalyzetheskullbumpsoftheinfantuniverse.

When constrained by other observations of the contemporary and distantuniverse, the CMB enables you to decode all sorts of fundamental cosmicproperties.Compare thedistributionof sizes and temperaturesof thewarmandcoolareasandyoucaninferhowstrongtheforceofgravitywasatthetimeandhowquicklymatteraccumulated,allowingyoutothendeducehowmuchordinarymatter,darkmatter,anddarkenergythereisintheuniverse.Fromhere,it’sthenstraightforwardtotellwhetherornottheuniversewillexpandforever.

Ordinarymatteriswhatweareallmadeof.Ithasgravityandinteractswithlight.Darkmatterisamysterioussubstancethathasgravitybutdoesnotinteractwithlightinanyknownway.Darkenergyisamysteriouspressureinthevacuumof space that acts in the opposite direction of gravity, forcing the universe toexpandfasterthanitotherwisewould.

What our phrenological exam says is that we understand how the universebehaved, but that most of the universe is made of stuff about which we areclueless.Ourprofoundareasofignorancenotwithstanding,today,asneverbefore,cosmologyhasananchor,becausetheCMBrevealstheportalthroughwhichweallwalked.It’sapointwhereinterestingphysicshappened,andwherewelearnedabouttheuniversebeforeandafteritslightwassetfree.

Thesimplediscoveryofthecosmicmicrowavebackgroundturnedcosmology

intosomethingmorethanmythology.Butitwastheaccurateanddetailedmapofthecosmicmicrowavebackgroundthatturnedcosmologyintoamodernscience.Cosmologistshaveplentyofego.Howcouldyounotwhenyourjobistodeducewhat brought the universe into existence?Without data, their explanationswerejusthypotheses.Now,eachnewobservation,eachmorselofdata,wieldsatwo-edged sword: it enables cosmology to thrive on the kind of foundation that somuch of the rest of science enjoys, but it also constrains theories that peoplethought up when there wasn’t enough data to say whether they were right orwrong.

Noscienceachievesmaturitywithoutit.

†Onenanosecondisabillionthofasecond.Onepicosecondisatrillionthofasecond.††A.A.PenziasandR.W.Wilson,“AMeasurementofExcessAntennaTemperatureat4080Mc/s,”AstrophysicalJournal142(1965):419–21.

4.

BetweentheGalaxies

Inthegrandtallyofcosmicconstituents,galaxiesarewhattypicallygetcounted.Latestestimatesshowthattheobservableuniversemaycontainahundredbillionof them.Bright and beautiful and packedwith stars, galaxies decorate the darkvoidsofspacelikecitiesacrossacountryatnight.Butjusthowvoidyisthevoidofspace?(Howempty is thecountrysidebetweencities?)Justbecausegalaxiesare in your face, and just because theywould have us believe that nothing elsematters, the universemay nonetheless contain hard-to-detect things between thegalaxies. Maybe those things are more interesting, or more important to theevolutionoftheuniverse,thanthegalaxiesthemselves.

Ourown spiral-shapedgalaxy, theMilkyWay, isnamed for its spilled-milkappearancetotheunaidedeyeacrossEarth’snighttimesky.Indeed,theveryword“galaxy”derivesfromtheGreekgalaxias,“milky.”Ourpairofnearest-neighborgalaxies, 600,000 light-years distant, are both small and irregularly shaped.FerdinandMagellan’sship’slogidentifiedthesecosmicobjectsduringhisfamousround-the-worldvoyageof1519.Inhishonor,wecallthemtheLargeandSmallMagellanicClouds,andtheyarevisibleprimarilyfromtheSouthernHemisphereasapairofcloudlikesplotchesonthesky,parkedbeyondthestars.Thenearestgalaxylargerthanourownistwomillionlight-yearsaway,beyondthestarsthattrace the constellation Andromeda. This spiral galaxy, historically dubbed theGreatNebulainAndromeda,isasomewhatmoremassiveandluminoustwinofthe Milky Way. Notice that the name for each system lacks reference to theexistenceofstars:MilkyWay,MagellanicClouds,AndromedaNebula.Allthreewerenamedbefore telescopeswere invented,so theycouldnotyetberesolvedintotheirstellarconstituencies.

As detailed in chapter 9, without the benefit of telescopes operating inmultiplebandsoflightwemightstilldeclarethespacebetweenthegalaxiestobeempty. Aided by modern detectors, and modern theories, we have probed ourcosmic countryside and revealed all manner of hard-to-detect things: dwarfgalaxies, runaway stars, runaway stars that explode, million-degree X-ray-emittinggas,darkmatter, faintbluegalaxies,ubiquitousgasclouds,super-duperhigh-energychargedparticles,andthemysteriousquantumvacuumenergy.Withalistlikethat,onecouldarguethatallthefunintheuniversehappensbetweenthegalaxiesratherthanwithinthem.

In any reliably surveyed volume of space, dwarf galaxies outnumber largegalaxiesbymorethantentoone.ThefirstessayIeverwroteontheuniverse,intheearly1980s,wastitled“TheGalaxyandtheSevenDwarfs,”referringtotheMilky Way’s diminutive nearby family. Since then, the tally of local dwarfgalaxies has been counted in the dozens. While full-blooded galaxies containhundredsofbillionsofstars,dwarfgalaxiescanhaveasfewasamillion,whichrendersthemahundredthousandtimeshardertodetect.Nowondertheyarestillbeingdiscoveredinfrontofournoses.

Images of dwarf galaxies that no longermanufacture stars tend to look liketiny,boringsmudges.Thosedwarfs thatdoformstarsareall irregularlyshapedand, quite frankly, are a sorry-looking lot. Dwarf galaxies have three thingsworking against their detection: They are small, and so are easily passed overwhen seductive spiral galaxiesvie for your attention.They aredim, and so aremissed inmanysurveysofgalaxies that cutoffbelowaprespecifiedbrightnesslevel.Andtheyhavealowdensityofstarswithinthem,sotheyofferpoorcontrastabovetheglowofsurroundinglightfromEarth’snighttimeatmosphereandfromothersources.Allthisistrue.Butsincedwarfsfaroutnumber“normal”galaxies,perhapsourdefinitionofwhatisnormalneedsrevision.

Youwillfindmost(known)dwarfgalaxieshangingoutnearbiggergalaxies,in orbit around them like satellites. The twoMagellanicClouds are part of theMilky Way’s dwarf family. But the lives of satellite galaxies can be quitehazardous. Most computer models of their orbits show a slow decay thatultimatelyresultsinthehaplessdwarfsgettingrippedapart,andtheneaten,bythemaingalaxy.TheMilkyWayengagedinatleastoneactofcannibalisminthelastbillionyears,whenitconsumedadwarfgalaxywhoseflayedremainscanbeseenas a stream of stars orbiting the galactic center, beyond the stars of theconstellationSagittarius.The system is called theSagittariusDwarf, but shouldprobablyhavebeennamedLunch.

In the high-density environment of clusters, two or more large galaxiesroutinelycollideandleavebehindatitanicmess:spiralstructureswarpedbeyondall recognition, newly induced bursts of star-forming regions spawned from theviolentcollisionofgasclouds,andhundredsofmillionsofstarsstrewnhitherandyonhavingfreshlyescapedthegravityofbothgalaxies.Somestarsreassembletoformblobsthatcouldbecalleddwarfgalaxies.Otherstarsremainadrift.Abouttenpercentofalllargegalaxiesshowevidenceofamajorgravitationalencounterwithanotherlargegalaxy—andthatratemaybefivetimeshigheramonggalaxiesinclusters.

With all this mayhem, how much galactic flotsam permeates intergalacticspace, especiallywithin clusters?Nobody knows for sure. Themeasurement isdifficultbecauseisolatedstarsaretoodimtodetectindividually.Wemustrelyondetecting a faint glow produced by the light of all stars combined. In fact,observationsofclustersdetectjustsuchaglowbetweenthegalaxies,suggestingthattheremaybeasmanyvagabond,homelessstarsastherearestarswithinthegalaxiesthemselves.

Adding ammo to the discussion, we have found (without looking for them)morethanadozensupernovasthatexplodedfarawayfromwhatwepresumetobetheir“host”galaxies.Inordinarygalaxies,foreverystarthatexplodesinthisway,ahundredthousandtoamilliondonot,soisolatedsupernovasmaybetrayentirepopulationsofundetectedstars.Supernovasarestarsthathaveblownthemselvesto smithereens and, in the process, have temporarily (over several weeks)increasedtheirluminosityabillion-fold,makingthemvisibleacrosstheuniverse.Whileadozenhomelesssupernovasisarelativelysmallnumber,manymoremayawait discovery, since most supernova searches systematically monitor knowngalaxiesandnotemptyspace.

There’s more to clusters than their constituent galaxies and their waywardstars.MeasurementsmadewithX-ray-sensitivetelescopesrevealaspace-filling,intra-cluster gas at tens ofmillions of degrees. The gas is so hot that it glowsstronglyintheX-raypartofthespectrum.Theverymovementofgas-richgalaxiesthroughthismediumeventuallystripsthemoftheirowngas,forcingthemtoforfeittheircapacitytomakenewstars.Thatcouldexplainit.Butwhenyoucalculatethetotalmasspresentinthisheatedgas,formostclustersitexceedsthemassofallgalaxies in the cluster by as much as a factor of ten. Worse yet, clusters areoverrunbydarkmatter,whichhappenstocontainuptoanotherfactoroftentimesthemassof everything else. In otherwords, if telescopesobservedmass rather

than light, then our cherished galaxies in clusterswould appear as insignificantblipsamidagiantsphericalblobofgravitationalforces.

Intherestofspace,outsideofclusters,thereisapopulationofgalaxiesthatthrivedlongago.Asalreadynoted,lookingoutintothecosmosisanalogoustoageologistlookingacrosssedimentarystrata,wherethehistoryofrockformationislaidoutinfullview.Cosmicdistancesaresovastthatthetraveltimeforlighttoreachuscanbemillionsorevenbillionsofyears.Whentheuniversewasonehalfits current age, a very blue and very faint species of intermediate-sized galaxythrived.Weseethem.Theyhailfromalongtimeago,representinggalaxiesfar,faraway.Theirbluecomesfromtheglowoffreshlyformed,short-lived,high-mass,high-temperature, high-luminosity stars. The galaxies are faint not only becausetheyaredistantbutbecausethepopulationofluminousstarswithinthemwasthin.Like the dinosaurs that came and went, leaving birds as their only moderndescendant, the faint blue galaxies no longer exist, but presumably have acounterpart in today’s universe.Did all their stars burnout?Have theybecomeinvisible corpses strewn throughout the universe? Did they evolve into thefamiliardwarfgalaxiesoftoday?Orweretheyalleatenbylargergalaxies?Wedonotknow,buttheirplaceinthetimelineofcosmichistoryiscertain.

With all this stuff between the big galaxies, wemight expect some of it toobscure our view of what lies beyond. This could be a problem for the mostdistant objects in the universe, such as quasars. Quasars are super-luminousgalaxycoreswhoselighthastypicallybeentravelingforbillionsofyearsacrossspacebeforereachingourtelescopes.Asextremelydistantsourcesoflight, theymakeidealguineapigsforthedetectionofinterveningjunk.

Sure enough, when you separate quasar light into its component colors,revealingaspectrum,it’sriddledwiththeabsorbingpresenceofinterveninggasclouds.Everyknownquasar,nomatterwhereontheskyit’sfound,showsfeaturesfrom dozens of isolated hydrogen clouds scattered across time and space. Thisuniqueclassofintergalacticobjectwasfirstidentifiedinthe1980s,andcontinuestobeanactiveareaofastrophysicalresearch.Wheredidtheycomefrom?Howmuchmassdotheyallcontain?

Everyknownquasarrevealsthesehydrogenfeatures,soweconcludethatthehydrogencloudsareeverywhereintheuniverse.And,asexpected,thefartherthequasar,themorecloudsarepresentinthespectrum.Someofthehydrogenclouds(less than one percent) are simply the consequence of our line of sight passingthroughthegascontainedinanordinaryspiralorirregulargalaxy.Youwould,ofcourse,expectat leastsomequasars tofallbehindthelightofordinarygalaxiesthatare toodistant todetect.But therestof theabsorbersareunmistakableasaclassofcosmicobject.

Meanwhile, quasar light commonly passes through regions of space thatcontainmonstroussourcesofgravity,whichwreakhavoconthequasar’simage.Theseareoftenhardtodetectbecausetheymaybecomposedofordinarymatterthat is simply toodimanddistant,or theymaybezonesofdarkmatter, suchaswhat occupies the centers and surrounding regions of galaxy clusters. In eithercase, where there ismass there is gravity. Andwhere there is gravity there iscurved space, according to Einstein’s general theory of relativity. And wherespaceiscurveditcanmimicthecurvatureofanordinaryglasslensandalterthepathways of light that pass through. Indeed, distant quasars andwhole galaxieshavebeen“lensed”byobjectsthathappentofallalongthelineofsighttoEarth’stelescopes.Dependingonthemassofthelensitselfandthegeometryoftheline-of-sight alignments, the lensing action can magnify, distort, or even split thebackground source of light into multiple images, just like fun-house mirrors atarcades.

Oneofthemostdistant(known)objectsintheuniverseisnotaquasarbutanordinarygalaxy,whosefeeblelighthasbeenmagnifiedsignificantlybytheactionof an intervening gravitational lens.Wemayhenceforth need to rely upon these“intergalactic” telescopes topeerwhere (andwhen)ordinary telescopes cannotreach,andthusrevealthefutureholdersofthecosmicdistancerecord.

Nobodydoesn’tlikeintergalacticspace,butitcanbehazardoustoyourhealthifyouchoosetogothere.Let’signorethefact thatyouwouldfreezetodeathasyourwarmbody tried to reachequilibriumwith the3-degree temperatureof theuniverse.And let’s ignore the fact that your blood cellswould burstwhile yousuffocated from the lack of atmospheric pressure. These are ordinary dangers.Fromthedepartmentofexotichappenings,intergalacticspaceisregularlypiercedby super-duper high-energy, fast-moving, charged, subatomic particles.We callthem cosmic rays. The highest-energy particles among them have a hundredmillion times the energy that can be generated in the world’s largest particleaccelerators. Their origin continues to be amystery, butmost of these chargedparticles are protons, the nuclei of hydrogen atoms, and are moving at99.9999999999999999999percentofthespeedoflight.Remarkably,thesesinglesubatomicparticlescarryenoughenergytoknockagolfballfromanywhereonaputtinggreenintothecup.

Perhaps themostexotichappeningsbetween(andamong) thegalaxies in thevacuumofspaceandtimeistheseethingoceanofvirtualparticles—undetectablematter and antimatter pairs, popping in and out of existence. This peculiar

prediction of quantum physics has been dubbed the “vacuum energy,” whichmanifestsasanoutwardpressure,actingcountertogravity,thatthrivesinthetotalabsence of matter. The accelerating universe, dark energy incarnate, may bedrivenbytheactionofthisvacuumenergy.

Yes,intergalacticspaceis,andwillforeverbe,wheretheactionis.

5.

DarkMatter

Gravity, themost familiar of nature’s forces, offers us simultaneously thebestandtheleastunderstoodphenomenainnature.Ittookthemindofthemillennium’smost brilliant and influential person, Isaac Newton, to realize that gravity’smysterious “action-at-a-distance” arises from the natural effects of every bit ofmatter,andthattheattractiveforcebetweenanytwoobjectscanbedescribedbyasimplealgebraicequation.Ittookthemindofthelastcentury’smostbrilliantandinfluentialperson,AlbertEinstein,toshowthatwecanmoreaccuratelydescribegravity’saction-at-a-distanceasawarpinthefabricofspace-time,producedbyanycombinationofmatterandenergy.EinsteindemonstratedthatNewton’stheoryrequires some modification to describe gravity accurately—to predict, forexample, howmuch light rays will bend when they pass by a massive object.AlthoughEinstein’sequationsarefancierthanNewton’s,theynicelyaccommodatethematter thatwehave come toknowand love.Matter thatwe can see, touch,feel,smell,andoccasionallytaste.

We don’t know who’s next in the genius sequence, but we’ve now beenwaiting nearly a century for somebody to tell us why the bulk of all thegravitationalforcethatwe’vemeasuredintheuniverse—abouteighty-fivepercentofit—arisesfromsubstancesthatdonototherwiseinteractwith“our”matterorenergy.Ormaybetheexcessgravitydoesn’tcomefrommatterandenergyatall,but emanates from some other conceptual thing. In any case,we are essentiallyclueless.Wefindourselvesnoclosertoananswertodaythanwewerewhenthis“missingmass”problemwasfirstfullyanalyzedin1937bytheSwiss-AmericanastrophysicistFritzZwicky.HetaughtattheCaliforniaInstituteofTechnologyformore than fortyyears,combininghis far-ranging insights into thecosmoswithacolorful means of expression and an impressive ability to antagonize his

colleagues.Zwickystudiedthemovementofindividualgalaxieswithinatitanicclusterof

them, located far beyond the local stars of the Milky Way that trace out theconstellation Coma Berenices (the “hair of Berenice,” an Egyptian queen inantiquity). TheComa cluster, aswe call it, is an isolated and richly populatedensemble of galaxies about 300 million light-years from Earth. Its thousandgalaxiesorbitthecluster’scenter,movinginalldirectionslikebeesswarmingabeehive.Usingthemotionsofafewdozengalaxiesastracersofthegravityfieldthatbindstheentirecluster,Zwickydiscoveredthattheiraveragevelocityhadashockinglyhighvalue.Sincelargergravitationalforcesinducehighervelocitiesintheobjectstheyattract,ZwickyinferredanenormousmassfortheComacluster.Asa realitycheckon thatestimate,youcansumup themassesofeachmembergalaxy that you see. And even though Coma ranks among the largest and mostmassive clusters in the universe, it does not contain enough visible galaxies toaccountfortheobservedspeedsZwickymeasured.

Howbad is the situation?Have our known laws of gravity failed us?Theycertainlyworkwithin the solar system.Newton showed that youcanderive theuniquespeedthataplanetmusthavetomaintainastableorbitatanydistancefromtheSun,lestitdescendbacktowardtheSunorascendtoafartherorbit.Turnsout,if we could boost Earth’s orbital speed to more than the square root of two(1.4142...)timesitscurrentvalue,ourplanetwouldachieve“escapevelocity,”and leave the solar system entirely.We can apply the same reasoning tomuchlargersystems,suchasourownMilkyWaygalaxy,inwhichstarsmoveinorbitsthatrespondtothegravityfromalltheotherstars;orinclustersofgalaxies,whereeachgalaxy likewise feels the gravity fromall the other galaxies. In this spirit,amidapageofformulasinhisnotebook,Einsteinwrotearhyme(moreringinglyinGermanthaninthisEnglishtranslation)inhonorofIsaacNewton:

LookuntothestarstoteachusHowthemaster’sthoughtscanreachusEachonefollowsNewton’smathSilentlyalongitspath.†

WhenweexaminetheComacluster,asZwickydidduringthe1930s,wefindthatitsmembergalaxiesareallmovingmorerapidlythantheescapevelocityforthe cluster. The cluster should swiftly fly apart, leaving barely a trace of itsbeehive existence after just a few hundred million years had passed. But theclusterismorethantenbillionyearsold,whichisnearlyasoldastheuniverseitself. And sowas bornwhat remains the longest-standing unsolvedmystery in

astrophysics.

Across the decades that followed Zwicky’s work, other galaxy clustersrevealed the same problem, so Coma could not be blamed for being peculiar.Then what or who should we blame? Newton? I wouldn’t. Not just yet. Histheorieshadbeenexaminedfor250yearsandpassedalltests.Einstein?No.Theformidable gravity of galaxy clusters is still not high enough to require the fullhammer of Einstein’s general theory of relativity, just two decades old whenZwicky did his research. Perhaps the “missingmass” needed to bind theComacluster’sgalaxiesdoesexist,butinsomeunknown,invisibleform.Today,we’vesettled on themoniker “darkmatter,”whichmakes no assertion that anything ismissing,yetnonethelessimpliesthatsomenewkindofmattermustexist,waitingtobediscovered.

Justasastrophysicistshadcometoacceptdarkmatteringalaxyclustersasamysterious thing, theproblem reared its invisibleheadonce again. In1976, thelate Vera Rubin, an astrophysicist at the Carnegie Institution of Washington,discovered a similarmass anomalywithin spiral galaxies themselves. Studyingthe speeds atwhich stars orbit their galaxy centers,Rubin first foundwhat sheexpected:withinthevisiblediskofeachgalaxy,thestarsfartherfromthecentermove at greater speeds than stars close in. The farther stars havemorematter(stars and gas) between themselves and the galaxy center, enabling their higherorbital speeds. Beyond the galaxy’s luminous disk, however, one can still findsomeisolatedgascloudsandafewbrightstars.Usingtheseobjectsastracersofthegravityfieldexteriortothemostluminouspartsofthegalaxy,wherenomorevisiblematteraddstothetotal,Rubindiscoveredthattheirorbitalspeeds,whichshouldnowbefallingwithincreasingdistanceoutthereinNowheresville,infactremainedhigh.

Theselargelyemptyvolumesofspace—thefar-ruralregionsofeachgalaxy—containtoolittlevisiblemattertoexplaintheanomalouslyhighorbitalspeedsofthe tracers.Rubin correctly reasoned that some form of darkmattermust lie inthesefar-outregions,wellbeyondthevisibleedgeofeachspiralgalaxy.ThankstoRubin’swork,wenowcallthesemysteriouszones“darkmatterhaloes.”

Thishaloproblemexistsunderournoses,rightintheMilkyWay.Fromgalaxyto galaxy and from cluster to cluster, the discrepancy between themass talliedfrom visible objects and the objects’ mass estimated from total gravity rangesfromafactorofafewupto(insomecases)afactorofmanyhundreds.Acrosstheuniverse,thediscrepancyaveragestoafactorofsix:cosmicdarkmatterhasabout

sixtimesthetotalgravityofallthevisiblematter.Further researchhas revealed that thedarkmattercannotconsistofordinary

matter thathappens tobeunder-luminous,ornonluminous.Thisconclusionrestsontwolinesofreasoning.First,wecaneliminatewithnear-certaintyallplausiblefamiliar candidates, like the suspects in a police lineup. Could the darkmatterresideinblackholes?No,wethinkthatwewouldhavedetectedthismanyblackholesfromtheirgravitationaleffectsonnearbystars.Coulditbedarkclouds?No,theywouldabsorborotherwiseinteractwithlightfromstarsbehindthem,whichbonafidedarkmatterdoesn’tdo.Coulditbeinterstellar(orintergalactic)rogueplanets, asteroids, and comets, all ofwhich produce no light of their own? It’shard to believe that the universewouldmanufacture six times asmuchmass inplanets as in stars.Thatwouldmean six thousand Jupiters for every star in thegalaxy,orworseyet, twomillionEarths. Inourownsolarsystem, forexample,everythingthat isnot theSunaddsuptolessthanonefifthofonepercentof theSun’smass.

More direct evidence for the strange nature of darkmatter comes from therelativeamountofhydrogenandheliumintheuniverse.Together, thesenumbersprovide a cosmic fingerprint left behind by the early universe. To a closeapproximation,nuclearfusionduring thefirst fewminutesafter thebigbangleftbehindoneheliumnucleusforeverytenhydrogennuclei(whichare,themselves,simplyprotons).Calculations show that ifmostof thedarkmatterhad involveditselfinnuclearfusion,therewouldbemuchmoreheliumrelativetohydrogenintheuniverse.Fromthisweconcludethatmostofthedarkmatter—hence,mostofthe mass in the universe—does not participate in nuclear fusion, whichdisqualifies it as “ordinary” matter, whose essence lies in a willingness toparticipate in the atomic and nuclear forces that shape matter as we know it.Detailed observations of the cosmic microwave background, which allow aseparatetestofthisconclusion,verifytheresult:Darkmatterandnuclearfusiondon’tmix.

Thus,asbestwecanfigure, thedarkmatterdoesn’tsimplyconsistofmatterthathappenstobedark.Instead,it’ssomethingelsealtogether.Darkmatterexertsgravityaccordingtothesamerulesthatordinarymatterfollows,butitdoeslittleelsethatmightallowustodetectit.Ofcourse,wearehamstrunginthisanalysisbynotknowingwhatthedarkmatterisinthefirstplace.Ifallmasshasgravity,doesallgravityhavemass?Wedon’tknow.Maybethere’snothingwrongwiththematter,andit’sthegravitywedon’tunderstand.

Thediscrepancybetweendark andordinarymatter varies significantly fromone astrophysical environment to another, but it becomes most pronounced forlargeentitiessuchasgalaxiesandgalaxyclusters.Forthesmallestobjects,suchasmoonsandplanets,nodiscrepancyexists.Earth’ssurfacegravity,forexample,canbeexplainedentirelybythestuffthat’sunderourfeet.IfyouareoverweightonEarth,don’tblamedarkmatter.DarkmatteralsohasnobearingontheMoon’sorbitaroundEarth,noron themovementsof theplanetsaround theSun—butaswe’ve already seen, we do need it to explain the motions of stars around thecenterofthegalaxy.

Doesadifferentkindofgravitationalphysicsoperateon thegalactic scale?Probablynot.More likely,darkmatterconsistsofmatterwhosenaturewehaveyet to divine, and which gathers more diffusely than ordinary matter does.Otherwise, we would detect the gravity of concentrated chunks of dark matterdotting the universe—dark matter comets, dark matter planets, dark mattergalaxies.Asfaraswecantell,that’snotthewaythingsare.

Whatweknowis that thematterwehavecometo love in theuniverse—thestuffofstars,planets,andlife—isonlyalightfrostingonthecosmiccake,modestbuoysafloatinavastcosmicoceanofsomethingthatlookslikenothing.

During the first halfmillionyears after thebigbang, amere eyeblink in thefourteen-billion-yearsweepofcosmichistory,matterintheuniversehadalreadybeguntocoalesceintotheblobsthatwouldbecomeclustersandsuperclustersofgalaxies.Butthecosmoswoulddoubleinsizeduringitsnexthalfmillionyears,and continue growing after that. At odds in the universe were two competingeffects:gravitywantstomakestuffcoagulate,buttheexpansionwantstodiluteit.Ifyoudothemath,yourapidlydeducethatthegravityfromordinarymattercouldnotwinthisbattlebyitself.Itneededthehelpofdarkmatter,withoutwhichwewould be living—actually not living—in a universe with no structures: noclusters,nogalaxies,nostars,noplanets,nopeople.

Howmuch gravity from darkmatter did it need? Six times asmuch as thatprovidedbyordinarymatter itself. Just the amountwemeasure in theuniverse.Thisanalysisdoesn’ttelluswhatdarkmatteris,onlythatdarkmatter’seffectsarerealandthat,tryasyoumight,youcannotcreditordinarymatterforit.

So dark matter is our frenemy. We have no clue what it is. It’s kind ofannoying.Butwedesperatelyneedit inourcalculationstoarriveatanaccurate

descriptionof theuniverse.Scientistsaregenerallyuncomfortablewheneverwemustbaseourcalculationsonconceptswedon’tunderstand,butwe’lldoitifwehave to. And dark matter is not our first rodeo. In the nineteenth century, forexample,scientistsmeasuredtheenergyoutputofourSunandshoweditseffectonour seasons and climate, long before anyone knew that thermonuclear fusion isresponsibleforthatenergy.Atthetime,thebestideasincludedtheretrospectivelylaughable suggestion that the Sun was a burning lump of coal. Also in thenineteenthcentury,weobservedstars,obtainedtheirspectra,andclassifiedthemlongbeforethetwentieth-centuryintroductionofquantumphysics,whichgivesusourunderstandingofhowandwhythesespectralookthewaytheydo.

Unrelenting skeptics might compare the dark matter of today to thehypothetical, now-defunct “aether” proposed in the nineteenth century as theweightless, transparent medium permeating the vacuum of space through whichlight moved. Until a famous 1887 experiment in Cleveland showed otherwise,performed by AlbertMichelson and EdwardMorley at CaseWestern ReserveUniversity,scientistsassertedthattheaethermustexist,eventhoughnotashredofevidence supported thispresumption.Asawave, lightwas thought to require amediumthroughwhichtopropagateitsenergy,muchassoundrequiresairorsomeother substance to transmit its waves. But light turns out to be quite happytravelingthroughthevacuumofspace,devoidofanymediumtocarryit.Unlikesoundwaves,whichconsistofairvibrations,lightwaveswerefoundtobeself-propagatingpacketsofenergyrequiringnoassistanceatall.

Dark-matterignorancediffersfundamentallyfromaetherignorance.Theaetherwas a placeholder for our incomplete understanding, whereas the existence ofdarkmatterderivesnotfrommerepresumptionbutfromtheobservedeffectsofitsgravity on visible matter. We’re not inventing dark matter out of thin space;instead,wededuce itsexistencefromobservationalfacts.Darkmatter is justasrealas themanyexoplanetsdiscovered inorbitaroundstarsother than theSun,discoveredsolelythroughtheirgravitationalinfluenceontheirhoststarsandnotfromdirectmeasurementoftheirlight.

Theworstthatcanhappeniswediscoverthatdarkmatterdoesnotconsistofmatteratall,butofsomethingelse.Couldwebeseeingtheeffectsofforcesfromanother dimension? Are we feeling the ordinary gravity of ordinary mattercrossingthemembraneofaphantomuniverseadjacenttoours?Ifso,thiscouldbejustoneofaninfiniteassortmentofuniversesthatcomprisethemultiverse.Soundsexotic and unbelievable.But is it anymore crazy than the first suggestions thatEarthorbits theSun?That theSunisoneofahundredbillionstars in theMilkyWay? Or that the Milky Way is but one of a hundred billion galaxies in theuniverse?

Evenifanyofthesefantasticalaccountsprovetrue,noneofitwouldchangethesuccessfulinvocationofdarkmatter’sgravityintheequationsthatweusetounderstandtheformationandevolutionoftheuniverse.

Other unrelenting skeptics might declare that “seeing is believing”—anapproach to life that works well in many endeavors, including mechanicalengineering, fishing,andperhapsdating. It’salsogood,apparently, for residentsof Missouri. But it doesn’t make for good science. Science is not just aboutseeing,it’saboutmeasuring,preferablywithsomethingthat’snotyourowneyes,whichareinextricablyconjoinedwiththebaggageofyourbrain.Thatbaggageismoreoftenthannotasatchelofpreconceivedideas,post-conceivednotions,andoutrightbias.

Havingresistedattempts todetect itdirectlyonEarthfor three-quartersofacentury, darkmatter remains in play. Particle physicists are confident that darkmatter consists of a ghostly class of undiscovered particles that interact withmatterviagravity,butotherwiseinteractwithmatterorlightonlyweaklyornotatall.Ifyoulikegamblingonphysics,thisoptionisagoodbet.Theworld’slargestparticle accelerators are trying to manufacture dark matter particles amid thedetritus of particle collisions.And specially designed laboratories buried deepunderground are trying to detect dark matter particles passively, in case theywanderinfromspace.Anundergroundlocationnaturallyshieldsthefacilityfromknowncosmicparticlesthatmighttripthedetectorsasdarkmatterimpostors.

Althoughitallcouldbemuchadoaboutnothing, the ideaofanelusivedarkmatterparticlehasgoodprecedence.Neutrinos,forinstance,werepredictedandeventuallydiscovered,eventhoughtheyinteractextremelyweaklywithordinarymatter. The copious flux of neutrinos from the Sun—two neutrinos for everyheliumnucleusfusedfromhydrogenintheSun’sthermonuclearcore—exittheSununfazedbytheSunitself,travelthroughthevacuumofspaceatnearlythespeedoflight,thenpassthroughEarthasthoughitdoesnotexist.Thetally:nightandday,ahundredbillionneutrinosfromtheSunpassthrougheachsquareinchofyourbody,every second,without a traceof interactionwithyourbody’s atoms. In spiteofthis elusivity, neutrinos are nonetheless stoppable under special circumstances.Andifyoucanstopaparticleatall,you’vedetectedit.

Dark matter particles may reveal themselves through similarly rareinteractions, or, more amazingly, they might manifest via forces other than thestrongnuclearforce,weaknuclearforce,andelectromagnetism.Thesethree,plusgravity, complete the fab four forces of the universe,mediating all interactions

between and among all known particles. So the choices are clear. Either darkmatterparticlesmustwaitforustodiscoverandtocontrolanewforceorclassofforcesthroughwhichtheirparticlesinteract,orelsedarkmatterparticlesinteractvianormalforces,butwithstaggeringweakness.

So,darkmatter’seffectsarereal.Wejustdon’tknowwhatitis.Darkmatterseemsnottointeractthroughthestrongnuclearforce,soitcannotmakenuclei.Ithasn’t been found to interact through the weak nuclear force, something evenelusiveneutrinosdo.Itdoesn’tseemtointeractwiththeelectromagneticforce,soit doesn’tmakemolecules and concentrate into dense balls of darkmatter.Nordoes it absorb or emit or reflect or scatter light. As we’ve known from thebeginning, dark matter does, indeed, exert gravity, to which ordinary matterresponds. But that’s it. After all these years, we haven’t discovered it doinganythingelse.

For now, we must remain content to carry dark matter along as a strange,invisiblefriend,invokingitwhereandwhentheuniverserequiresitofus.

†Manuscriptnote,quotedinKárolySimonyi,ACulturalHistoryofPhysics(BocaRaton,FL:CRCPress,2012).

6.

DarkEnergy

Asifyoudidn’thaveenoughtoworryabout,theuniverseinrecentdecadeswasdiscovered towield amysterious pressure that issues forth from the vacuumofspaceandthatactsoppositecosmicgravity.Notonlythat,this“negativegravity”willultimatelywinthetug-of-war,asitforcesthecosmicexpansiontoaccelerateexponentiallyintothefuture.

For the most mind-warping ideas of twentieth-century physics, just blameEinstein.

AlbertEinsteinhardlyeversetfootinthelaboratory;hedidn’ttestphenomenaor use elaborate equipment. He was a theorist who perfected the “thoughtexperiment,”inwhichyouengagenaturethroughyourimagination,byinventingasituation or model and then working out the consequences of some physicalprinciple. In Germany before World War II, laboratory-based physics faroutranked theoretical physics in the minds of most Aryan scientists. Jewishphysicistswereall relegated to the lowly theorists’sandboxandleft tofendforthemselves.Andwhatasandboxthatwouldbecome.

Aswas thecase forEinstein, if aphysicist’smodel intends to represent theentireuniverse,thenmanipulatingthemodelshouldbetantamounttomanipulatingtheuniverseitself.Observersandexperimentalistscanthengooutandlookforthephenomena predicted by that model. If the model is flawed, or if the theoristsmake a mistake in their calculations, the observers will uncover a mismatchbetweenthemodel’spredictionsandthewaythingshappenin therealuniverse.That’s the first cue for a theorist to return to the proverbial drawing board, byeitheradjustingtheoldmodelorcreatinganewone.

Oneof themostpowerful and far-reaching theoreticalmodels everdevised,already introduced in thesepages, isEinstein’sgeneral theoryof relativity—but

youcancallitGRafteryougettoknowitbetter.Publishedin1916,GRoutlinestherelevantmathematicaldetailsofhoweverythingintheuniversemovesundertheinfluenceofgravity.Everyfewyears,labscientistsdeviseevermorepreciseexperimentstotest thetheory,onlytofurtherextendtheenvelopeofthetheory’saccuracy.AmodernexampleofthisstunningknowledgeofnaturethatEinsteinhasgifted us, comes from 2016, when gravitational waves were discovered by aspecially designed observatory tuned for just this purpose.† These waves,predictedbyEinstein,areripplesmovingatthespeedoflightacrossthefabricofspace-time, and are generated by severe gravitational disturbances, such as thecollisionoftwoblackholes.

And that’s exactlywhatwas observed. The gravitational waves of the firstdetection were generated by a collision of black holes in a galaxy 1.3 billionlight-yearsaway,andatatimewhenEarthwasteemingwithsimple,single-celledorganisms.Whiletheripplemovedthroughspaceinalldirections,Earthwould,after another 800 million years, evolve complex life, including flowers anddinosaursandflyingcreatures,aswellasabranchofvertebratescalledmammals.Among the mammals, a sub-branch would evolve frontal lobes and complexthought to accompany them. We call them primates. A single branch of theseprimateswoulddevelopageneticmutationthatallowedspeech,andthatbranch—Homosapiens—would inventagricultureandcivilizationandphilosophyandartandscience.Allinthelasttenthousandyears.Ultimately,oneofitstwentieth-centuryscientistswouldinventrelativityoutofhishead,andpredicttheexistenceofgravitationalwaves.Acenturylater,technologycapableofseeingthesewaveswould finally catch up with the prediction, just days before that gravity wave,which had been traveling for 1.3 billion years, washed over Earth and wasdetected.

Yes,Einsteinwasabadass.

When first proposed, most scientific models are only half-baked, leavingwiggleroomtoadjustparametersforabetterfittotheknownuniverse.IntheSun-based“heliocentric”universe, conceivedby the sixteenth-centurymathematicianNicolaus Copernicus, planets orbited in perfect circles. The orbit-the-Sun partwascorrect,andamajoradvanceon theEarth-based“geocentric”universe,butthe perfect-circle part turned out to be a bit off—all planets orbit the Sun inflattenedcirclescalledellipses,andeventhatshapeisjustanapproximationofamore complex trajectory. Copernicus’s basic ideawas correct, and that’s whatmatteredmost.Itsimplyrequiredsometweakingtomakeitmoreaccurate.

Yet, in the case ofEinstein’s relativity, the founding principles of the entiretheoryrequire thateverythingmusthappenexactlyaspredicted.Einsteinhad, ineffect,builtwhatlooksontheoutsidelikeahouseofcards,withonlytwoorthreesimplepostulatesholdinguptheentirestructure.Indeed,uponlearningofa1931bookentitledOneHundredAuthorsAgainstEinstein,†† he responded that if hewerewrong,thenonlyonewouldhavebeenenough.

Thereinwere sown the seeds of one of themost fascinating blunders in thehistoryofscience.Einstein’snewequationsofgravityincludedatermhecalledthe “cosmological constant,” which he represented by the capital Greek letterlambda: Λ. A mathematically permitted but optional term, the cosmologicalconstantallowedhimtorepresentastaticuniverse.

Backthen,theideathatouruniversewouldbedoinganythingatall,otherthansimplyexisting,wasbeyondanyone’s imagination.So lambda’ssole jobwas toopposegravitywithinEinstein’smodel,keepingtheuniverseinbalance,resistingthenaturaltendencyforgravitytopullthewholeuniverseintoonegiantmass.Inthis way, Einstein invented a universe that neither expands nor contracts,consistentwitheverybody’sexpectationsatthetime.

The Russian physicist Alexander Friedmann would subsequently showmathematicallythatEinstein’suniverse,thoughbalanced,wasinanunstablestate.Likeaball restingonthe topofahill,awaitingtheslightestprovocationtorolldowninonedirectionoranother,orlikeapencilbalancedonitssharpenedpoint,Einstein’s universewas precariously perched between a state of expansion andtotal collapse.Moreover,Einstein’s theorywas new, and just because yougivesomethinganamedoesnotmakeitreal—Einsteinknewthatlambda,asanegativegravityforceofnature,hadnoknowncounterpartinthephysicaluniverse.

Einstein’s general theory of relativity radically departed from all previousthinkingaboutgravitational attraction. Insteadof settling forSir IsaacNewton’sviewof gravity as spooky action-at-a-distance (a conclusion thatmadeNewtonhimselfuncomfortable),GRregardsgravityastheresponseofamasstothelocalcurvatureofspaceandtimecausedbysomeothermassorfieldofenergy.Inotherwords, concentrationsofmass causedistortions—dimples, really—in the fabricofspaceandtime.Thesedistortionsguidethemovingmassesalongstraight-linegeodesics,††† though they look to us like the curved trajectorieswe call orbits.The twentieth-century American theoretical physicist John Archibald Wheelersaiditbest,summingupEinstein’sconceptas,“Mattertellsspacehowtocurve;spacetellsmatterhowtomove.”††††

Attheendoftheday,generalrelativitydescribedtwokindsofgravity.Oneisthefamiliarkind,liketheattractionbetweenEarthandaballthrownintotheair,or between the Sun and the planets. It also predicted another variety—amysterious,anti-gravitypressureassociatedwiththevacuumofspace-timeitself.LambdapreservedwhatEinsteinandeveryotherphysicistofhisdayhadstronglypresumed to be true: the status quo of a static universe—an unstable staticuniverse.Toinvokeanunstableconditionasthenaturalstateofaphysicalsystemviolates scientific credo.You cannot assert that the entire universe is a specialcasethathappenstobebalancedforeverandever.Nothingeverseen,measured,or imagined has behaved this way in the history of science, which makes forpowerfulprecedent.

Thirteen years later, in 1929, theAmerican astrophysicist Edwin P.Hubblediscoveredthattheuniverseisnotstatic.Hehadfoundandassembledconvincingevidence that the more distant a galaxy, the faster the galaxy recedes from theMilkyWay.Inotherwords, theuniverseisexpanding.Now,embarrassedbythecosmologicalconstant,whichcorrespondedtonoknownforceofnature,andbythe lost opportunity to have predicted the expanding universe himself, Einsteindiscarded lambda entirely, calling it his life’s “greatest blunder.” By yankinglambda from the equation he presumed its value to be zero, such as in thisexample:AssumeA=B+C.IfyoulearnlaterthatA=10andB=10,thenAstillequalsBplusC,exceptinthatcaseCequals0andisrenderedunnecessaryintheequation.

But thatwasn’t the end of the story.Off and on over the decades, theoristswouldextractlambdafromthecrypt,imaginingwhattheirideaswouldlooklikein a universe that had a cosmological constant. Sixty-nine years later, in 1998,scienceexhumedlambdaonelasttime.Earlythatyear,remarkableannouncementsweremadebytwocompetingteamsofastrophysicists:oneledbySaulPerlmutterofLawrenceBerkeleyNationalLaboratoryinBerkeley,California,andtheotherco-led by Brian Schmidt ofMount Stromlo and Siding Spring observatories inCanberra, Australia, and Adam Riess of the Johns Hopkins University inBaltimore, Maryland. Dozens of the most distant supernovas ever observedappearednoticeablydimmerthanexpected,giventhewell-documentedbehaviorofthisspeciesofexplodingstar.Reconciliationrequiredthateitherthosedistantsupernovasbehavedunliketheirnearerbrethren,ortheywereasmuchasfifteenpercentfartherawaythanwhere theprevailingcosmologicalmodelshadplacedthem. The only known thing that “naturally” accounts for this acceleration isEinstein’s lambda, thecosmologicalconstant.Whenastrophysicistsdusted itoffandputitbackintoEinstein’soriginalequationsforgeneralrelativity,theknownstateoftheuniversematchedthestateofEinstein’sequations.

The supernovas used in Perlmutter’s and Schmidt’s studies are worth theirweightinfusionablenuclei.Withincertainlimits,eachofthosestarsexplodesthesameway,ignitingthesameamountoffuel,releasingthesametitanicamountofenergy in the same amount of time, thereby reaching the same peak luminosity.Thus they serve as a kind of yardstick, or “standard candle,” for calculatingcosmicdistancestothegalaxiesinwhichtheyexplode,outtothefarthestreachesoftheuniverse.

Standard candles simplify calculations immensely: since the supernovas allhavethesamewattage,thedimonesarefarawayandthebrightonesarecloseby.Aftermeasuringtheirbrightness(asimpletask),youcantellexactlyhowfartheyarefromyouandfromoneanother.Iftheluminositiesofthesupernovaswerealldifferent, you could not use brightness alone to tell how far away one was incomparisonwithanother.Adimonecouldbeeitherahigh-wattagebulbfarawayoralow-wattagebulbcloseup.

All fine.But there’sasecondway tomeasure thedistance togalaxies: theirspeedof recessionfromourMilkyWay—recession that’spartandparcelof theoverall cosmic expansion. As Hubble was the first to show, the expandinguniversemakesdistantobjectsraceawayfromusfasterthannearbyones.So,bymeasuringagalaxy’sspeedofrecession(anothersimpletask),onecandeduceagalaxy’sdistance.

Ifthosetwowell-testedmethodsgivedifferentdistancesforthesameobject,somethingmustbewrong.Eitherthesupernovasarebadstandardcandles,orourmodelfortherateofcosmicexpansionasmeasuredbygalaxyspeedsiswrong.

Well, somethingwaswrong. It turnedout that the supernovaswere splendidstandard candles, surviving the careful scrutiny ofmany skeptical investigators,andsoastrophysicistswereleftwithauniversethathadexpandedfasterthanwethought, placing galaxies farther away than their recession speed would haveotherwise indicated.And therewasno easyway to explain the extra expansionwithoutinvokinglambda,Einstein’scosmologicalconstant.

Here was the first direct evidence that a repulsive force permeated theuniverse,opposinggravity,whichishowandwhythecosmologicalconstantrosefromthedead.Lambdasuddenlyacquiredaphysicalrealitythatneededaname,and so “dark energy” took center stage in the cosmic drama, suitably capturingboth themysteryandourassociated ignoranceof itscause.Perlmutter,Schmidt,andReissjustifiablysharedthe2011NobelPrizeinphysicsforthisdiscovery.

The most accurate measurements to date reveal dark energy as the mostprominent thing in town, currently responsible for 68 percent of all the mass-

energy in the universe; dark matter comprises 27 percent, with regular mattercomprisingamere5percent.

The shape of our four-dimensional universe comes from the relationshipbetweentheamountofmatterandenergythatlivesinthecosmosandtherateatwhich the cosmos is expanding. A convenient mathematical measure of this isomega:Ω,yetanothercapitalGreekletterwithafirmgriponthecosmos.

If you take the matter-energy density of the universe and divide it by thematter-energy density required to just barely halt the expansion (known as the“critical”density),yougetomega.

Sincebothmassandenergycausespace-timetowarp,orcurve,omegatellsustheshapeofthecosmos.Ifomegaislessthanone,theactualmass-energyfallsbelowthecriticalvalue,andtheuniverseexpandsforeverineverydirectionforall of time, taking on the shape of a saddle, in which initially parallel linesdiverge.Ifomegaequalsone,theuniverseexpandsforever,butonlybarelyso.Inthatcase theshape is flat,preservingall thegeometric ruleswe learned inhighschoolaboutparallellines.Ifomegaexceedsone,parallellinesconverge,andtheuniversecurvesbackonitself,ultimatelyrecollapsingintothefireballwhenceitcame.

Atno timesinceHubblediscovered theexpandinguniversehasany teamofobserverseverreliablymeasuredomegatobeanywhereclosetoone.Addingupallthemassandenergytheirtelescopescouldsee,andevenextrapolatingbeyondthese limits,darkmatter included, thebiggestvalues from thebestobservationstoppedout at aboutΩ=0.3.As far asobserverswere concerned, theuniversewas“open”forbusiness,ridingaone-waysaddleintothefuture.

Meanwhile, beginning in 1979, theAmerican physicistAlanH.Guth of theMassachusettsInstituteofTechnology,andothers,advancedanadjustment tothebigbangtheorythatclearedupsomenaggingproblemswithgettingauniversetobe as smoothly filled with matter and energy as ours is known to be. Afundamental by-product of this update to the big bangwas that it drives omegatowardone.Not towardahalf.Not toward two.Not towardamillion.Towardone.

Hardlyatheoristintheworldhadaproblemwiththatrequirement,becauseithelpedgetthebigbangtoaccountfortheglobalpropertiesoftheknownuniverse.Therewas,however,another littleproblem: theupdatepredicted three timesasmuch mass-energy as observers could find. Undeterred, the theorists said theobserversjustweren’tlookinghardenough.

Attheendofthetallies,visiblematteralonecouldaccountfornomorethan5percentofthecriticaldensity.Howaboutthemysteriousdarkmatter?Theyaddedthat,too.Nobodyknewwhatitwas,andwestilldon’tknowwhatitis,butsurelyitcontributedtothetotals.Fromtherewegetfiveorsixtimesasmuchdarkmatterasvisiblematter.Butthat’sstillwaytoolittle.Observerswereataloss,andthetheoristsanswered,“Keeplooking.”

Both camps were sure the other was wrong—until the discovery of darkenergy.Thatsinglecomponent,whenaddedtotheordinarymatterandtheordinaryenergy and dark matter, raised the mass-energy density of the universe to thecriticallevel.Simultaneouslysatisfyingboththeobserversandthetheorists.

For the first time, the theorists and observers kissed andmade up.Both, intheir own way, were correct. Omega does equal one, just as the theoristsdemanded of the universe, even though you can’t get there by adding up all thematter—dark or otherwise—as they had naively presumed. There’s no morematter running around the cosmos today than had ever been estimated by theobservers.

Nobodyhadforeseenthedominatingpresenceofcosmicdarkenergy,norhadanybodyimagineditasthegreatreconcilerofdifferences.

So what is the stuff? Nobody knows. The closest anybody has come is topresumedarkenergyisaquantumeffect—wherethevacuumofspace,insteadofbeingempty,actuallyseetheswithparticlesandtheirantimattercounterparts.Theypopinandoutofexistence inpairs,anddon’t last longenoughtobemeasured.Their transient existence is captured in their moniker: virtual particles. Theremarkable legacy of quantumphysics—the science of the small—demands thatwegivethisideaseriousattention.Eachpairofvirtualparticlesexertsalittlebitofoutwardpressureasiteversobrieflyelbowsitswayintospace.

Unfortunately,whenyouestimatetheamountofrepulsive“vacuumpressure”thatarisesfromtheabbreviatedlivesofvirtualparticles, theresultismorethan10120 timesbigger than theexperimentallydeterminedvalueof thecosmologicalconstant.Thisisastupidlylargefactor,leadingtothebiggestmismatchbetweentheoryandobservationinthehistoryofscience.

Yes,we’reclueless.Butit’snotabjectcluelessness.Darkenergyisnotadrift,withnaryatheorytoanchorit.Darkenergyinhabitsoneofthesafestharborswecan imagine: Einstein’s equations of general relativity. It’s the cosmologicalconstant.It’slambda.Whateverdarkenergyturnsouttobe,wealreadyknowhowtomeasureitandhowtocalculateitseffectsonthepast,present,andfutureofthe

cosmos.Withoutadoubt,Einstein’sgreatestblunderwashavingdeclaredthatlambda

washisgreatestblunder.

And the hunt is on. Now that we know dark energy is real, teams ofastrophysicists have begun ambitious programs to measure distances and thegrowth of structure in the universe using ground-based and space-bornetelescopes.Theseobservationswilltestthedetailedinfluenceofdarkenergyonthe expansion history of the universe, andwill surely keep theorists busy.Theydesperatelyneed toatoneforhowembarrassing theircalculationofdarkenergyturnedouttobe.

Do we need an alternative to GR? Does the marriage of GR and quantummechanicsrequireanoverhaul?Oristheresometheoryofdarkenergythatawaitsdiscoverybyacleverpersonyettobeborn?

A remarkable feature of lambda and the accelerating universe is that therepulsiveforcearisesfromwithinthevacuum,notfromanythingmaterial.Asthevacuum grows, the density of matter and (familiar) energy within the universediminishes, and the greater becomes lambda’s relative influence on the cosmicstate of affairs.With greater repulsive pressure comesmore vacuum, and withmore vacuum comes greater repulsive pressure, forcing an endless andexponentialaccelerationofthecosmicexpansion.

Asaconsequence,anythingnotgravitationallybound to theneighborhoodofthe Milky Way galaxy will recede at ever-increasing speed, as part of theacceleratingexpansionofthefabricofspace-time.Distantgalaxiesnowvisibleinthenight skywillultimatelydisappearbeyondanunreachablehorizon, recedingfromusfasterthanthespeedoflight.Afeatallowed,notbecausethey’removingthroughspaceatsuchspeeds,butbecausethefabricoftheuniverseitselfcarriesthematsuchspeeds.Nolawofphysicspreventsthis.

Inatrillionorsoyears,anyonealiveinourowngalaxymayknownothingofothergalaxies.Ourobservableuniversewillmerelycompriseasystemofnearby,long-lived starswithin theMilkyWay.And beyond this starry nightwill lie anendlessvoid—darknessinthefaceofthedeep.

Darkenergy,afundamentalpropertyofthecosmos,will,intheend,underminethe ability of future generations to comprehend the universe they’ve been dealt.Unless contemporary astrophysicists across the galaxy keep remarkable recordsandburyanawesome,trillion-yeartimecapsule,postapocaplypticscientistswillknow nothing of galaxies—the principal form of organization for matter in our

cosmos—andwillthusbedeniedaccesstokeypagesfromthecosmicdramathatisouruniverse.

Beholdmyrecurringnightmare:Arewe,too,missingsomebasicpiecesoftheuniversethatoncewere?Whatpartof thecosmichistorybookhasbeenmarked“accessdenied”?Whatremainsabsentfromourtheoriesandequationsthatoughttobethere,leavingusgropingforanswerswemayneverfind?

†TheLaserInterferometerGravitational-WaveObservatory(LIGO),twinnedinHanford,Washington,andLiving-ston,Louisiana.††R.Israel,E.Ruckhaber,R.Weinmann,etal.,HundertAutorenGegenEinstein(Leipzig:R.VoigtlandersVerlag,1931).†††“Geodesic”isaneedlesslyfancywordfortheshortestdistancebetweentwopointsalongacurvedsurface—extended,inthiscase,tobetheshortestdistancebetweentwopointsinthecurvedfour-dimensionalfabricofspace-time.††††IngraduateschoolItookJohnWheeler’sclassongeneralrelativity(whereImetmywife)andhesaidthisoften.

7.

TheCosmosontheTable

Trivial questions sometimes require deep and expansive knowledge of thecosmosjusttoanswerthem.Inmiddleschoolchemistryclass,Iaskedmyteacherwhere theelementson thePeriodicTablecome from.He replied,Earth’scrust.I’llgranthimthat.It’ssurelywherethesupplylabgetsthem.ButhowdidEarth’scrust acquire them?The answermust be astronomical.But in this case, do youactually need to know the origin and evolution of the universe to answer thequestion?

Yes,youdo.Only three of the naturally occurring elementsweremanufactured in the big

bang.Therestwereforgedinthehigh-temperatureheartsandexplosiveremainsofdyingstars,enablingsubsequentgenerationsofstarsystemstoincorporatethisenrichment,formingplanetsand,inourcase,people.

Formany, thePeriodicTableofChemicalElements isaforgottenoddity—achartofboxesfilledwithmysterious,crypticletterslastencounteredonthewallof high school chemistry class. As the organizing principle for the chemicalbehaviorofallknownandyet-to-be-discoveredelementsintheuniverse,thetableinsteadoughttobeaculturalicon,atestimonytotheenterpriseofscienceasaninternational human adventure conducted in laboratories, particle accelerators,andonthefrontierofthecosmositself.

Yet everynowand then, even a scientist can’t help thinkingof thePeriodicTableasazooofone-of-a-kindanimalsconceivedbyDr.Seuss.Howelsecouldwe believe that sodium is a poisonous, reactivemetal that you can cut with abutterknife,whilepurechlorineisasmelly,deadlygas,yetwhenaddedtogethertheymake sodium chloride, a harmless, biologically essential compound betterknownastablesalt?Orhowabouthydrogenandoxygen?Oneisanexplosivegas,

and the other promotes violent combustion, yet the two combined make liquidwater,whichputsoutfires.

Amid these chemical confabulations we find elements significant to thecosmos,allowingmetoofferthePeriodicTableasviewedthroughthelensofanastrophysicist.

With only one proton in its nucleus, hydrogen is the lightest and simplestelement, made entirely during the big bang. Out of the ninety-four naturallyoccurringelements,hydrogenlaysclaimtomorethantwo-thirdsofalltheatomsinthehumanbody,andmorethanninetypercentofallatomsinthecosmos,onallscales, right on down to the solar system.Hydrogen in the core of themassiveplanet Jupiter isunder somuchpressure that it behavesmore like a conductivemetal than a gas, creating the strongest magnetic field among the planets. TheEnglish chemist Henry Cavendish discovered hydrogen in 1766 during hisexperimentswithH2O(hydro-genesisGreekfor“water-forming”),butheisbestknown among astrophysicists as the first to calculate Earth’smass after havingmeasured an accurate value for the gravitational constant in Newton’s famousequationforgravity.

Everysecondofeveryday,4.5billiontonsoffast-movinghydrogennucleiareturnedintoenergyastheyslamtogethertomakeheliumwithinthefifteen-million-degreecoreoftheSun.

Helium is widely recognized as an over-the-counter, low-density gas that,when inhaled, temporarily increases thevibrational frequencyofyourwindpipeandlarynx,makingyousoundlikeMickeyMouse.Heliumisthesecondsimplestandsecondmostabundantelement in theuniverse.Althoughadistant second tohydrogeninabundance,there’sfourtimesmoreofitthanallotherelementsintheuniversecombined.Oneofthepillarsofbigbangcosmologyisthepredictionthatin every region of the cosmos, no less than about ten percent of all atoms arehelium,manufactured in thatpercentageacross thewell-mixedprimeval fireballthat was the birth of our universe. Since the thermonuclear fusion of hydrogenwithin stars gives you helium, some regions of the cosmos could easilyaccumulatemorethantheirtenpercentshareofhelium,but,aspredicted,noonehaseverfoundaregionofthegalaxywithless.

SomethirtyyearsbeforeitwasdiscoveredandisolatedonEarth,astronomersdetected helium in the spectrum of the Sun’s corona during the total eclipse of

1868.Asnotedearlier,thenameheliumwasdulyderivedfromHelios,theGreeksun god. And with 92 percent of hydrogen’s buoyancy in air, but without itsexplosive characteristics, helium is the gas of choice for the outsized ballooncharactersof theMacy’sThanksgivingDayparade,making thedepartmentstoresecondonlytotheU.S.militaryasthenation’stopuseroftheelement.

Lithiumisthethirdsimplestelementintheuniverse,withthreeprotonsinitsnucleus.Likehydrogenandhelium,lithiumwasmadeinthebigbang,butunlikehelium,whichcanbemanufacturedinstellarcores,lithiumisdestroyedbyeveryknownnuclearreaction.Anotherpredictionofbigbangcosmologyisthatwecanexpectnomorethanonepercentoftheatomsinanyregionoftheuniversetobelithium.No one has yet found a galaxywithmore lithium than this upper limitsuppliedby thebigbang.Thecombinationofhelium’supper limitand lithium’slowerlimitgivesapotentdual-constraintontestsforbigbangcosmology.

Theelementcarboncanbefoundinmorekindsofmoleculesthanthesumofall other kinds of molecules combined. Given the abundance of carbon in thecosmos—forged in thecoresofstars,churnedup to theirsurfaces,andreleasedcopiously into thegalaxy—abetter elementdoesnot existonwhich tobase thechemistryanddiversityoflife.Justedgingoutcarboninabundancerank,oxygeniscommon,too,forgedandreleasedintheremainsofexplodedstars.Bothoxygenandcarbonaremajoringredientsoflifeasweknowit.

Butwhataboutlifeaswedon’tknowit?Howaboutlifebasedontheelementsilicon?SiliconsitsdirectlybelowcarbononthePeriodicTable,whichmeans,inprinciple, it cancreate the sameportfolioofmolecules that carbondoes. In theend,weexpectcarbontowinbecauseit’stentimesmoreabundantthansiliconinthecosmos.Butthatdoesn’tstopsciencefictionwriters,whokeepexobiologistsontheirtoes,wonderingwhatthefirsttrulyalien,silicon-basedlifeformswouldbelike.

Inadditiontobeinganactiveingredientintablesalt,atthemoment,sodiumisthemostcommonglowinggas inmunicipal street lampsacross thenation.They“burn”brighterandlongerthanincandescentbulbs,althoughtheymayallsoonbereplacedbyLEDs,whichareevenbrighteratagivenwattage,andcheaper.Twovarietiesofsodiumlampsarecommon:high-pressurelamps,whichlookyellow-white,andtherarerlow-pressurelamps,whichlookorange.Turnsout,whilealllightpollution isbad for astrophysics, the low-pressure sodium lampsare least

badbecausetheircontaminationcanbeeasilysubtractedfromtelescopedata.Inamodel of cooperation, the entire city of Tucson, Arizona, the nearest largemunicipality to theKitt PeakNationalObservatory, has, by agreementwith thelocalastrophysicists,convertedallitsstreetlightstolow-pressuresodiumlamps.

AluminumoccupiesnearlytenpercentofEarth’scrustyetwasunknowntotheancients andunfamiliar toourgreat-grandparents.The elementwasnot isolatedand identifieduntil1827anddidnot enter commonhouseholduseuntil the late1960s, when tin cans and tin foil yielded to aluminum cans and, of course,aluminum foil. (I’d bet most old people you know still call the stuff tin foil.)Polished aluminum makes a near-perfect reflector of visible light and is thecoatingofchoicefornearlyalltelescopemirrorstoday.

Titaniumis1.7timesdenserthanaluminum,butit’smorethantwiceasstrong.So titanium, the ninth most abundant element in Earth’s crust, has become amodern darling formany applications, such asmilitary aircraft components andprostheticsthatrequirealight,strongmetalfortheirtasks.

Inmost cosmic places, the number of oxygen atoms exceeds that of carbon.After every carbon atom has latched onto the available oxygen atoms (formingcarbonmonoxideorcarbondioxide),theleftoveroxygenbondswithotherthings,like titanium. The spectra of red stars are riddled with features traceable totitanium oxide,which itself is no stranger to stars onEarth: star sapphires andrubies owe their radiant asterisms to titanium oxide impurities in their crystallattice. Furthermore, thewhite paint used for telescope domes features titaniumoxide,whichhappenstobehighlyreflectiveintheinfraredpartofthespectrum,greatly reducing the heat accumulated from sunlight in the air surrounding thetelescope.Atnightfall,withthedomeopen,theairtemperaturenearthetelescoperapidlyequals thetemperatureof thenighttimeair,allowinglightfromstarsandothercosmicobjects tobesharpandclear.And,whilenotdirectlynamedforacosmic object, titanium derives from the Titans of Greek mythology; Titan isSaturn’slargestmoon.

Bymanymeasures, ironranksas themost importantelement in theuniverse.Massive stars manufacture elements in their core, in sequence from helium tocarbon tooxygen tonitrogen,andso forth,all thewayup thePeriodicTable toiron.Withtwenty-sixprotonsandat leastasmanyneutronsinitsnucleus, iron’sodddistinction comes fromhaving the least total energyper nuclear particle of

any element. This means something quite simple: if you split iron atoms viafission, theywillabsorbenergy.Andifyoucombineironatomsviafusion, theywillalsoabsorbenergy.Stars,however,areinthebusinessofmakingenergy.Ashigh-massstarsmanufactureandaccumulate iron in theircores, theyarenearingdeath.Withoutafertilesourceofenergy,thestarcollapsesunderitsownweightandinstantlyreboundsinastupendousasupernovaexplosion,outshiningabillionsunsformorethanaweek.

Thesoftmetalgalliumhassuchalowmeltingpointthat,likecocoabutter,itwillliquefyoncontactwithyourhand.Apartfromthisparlordemo,galliumisnotinteresting to astrophysicists, except as one of the ingredients in the galliumchlorideexperimentsusedtodetectelusiveneutrinosfromtheSun.Ahuge(100-ton) underground vat of liquid gallium chloride ismonitored for any collisionsbetween neutrinos and gallium nuclei, turning it into germanium. The encounteremitsasparkofX-raylightthatismeasuredeverytimeanucleusgetsslammed.The long-standing solarneutrinoproblem,where fewerneutrinosweredetectedthanpredictedbysolartheory,wassolvedusing“telescopes”suchasthis.

Every form of the element technetium is radioactive. Not surprisingly, it’sfound nowhere on Earth except in particle accelerators, where we make it ondemand.Technetiumcarries thisdistinction in itsname,whichderives from theGreek technetos, meaning “artificial.” For reasons not yet fully understood,technetium lives in the atmospheres of a select subset of red stars. This alonewouldnotbecauseforalarmexceptthattechnetiumhasahalf-lifeofameretwomillionyears,whichismuch,muchshorterthantheageandlifeexpectancyofthestarsinwhichitisfound.Inotherwords,thestarcannothavebeenbornwiththestuff, for if it were, therewould be none left by now. There is also no knownmechanismtocreatetechnetiuminastar’scoreandhaveitdredgeitselfuptothesurfacewhere it is observed,which has led to exotic theories that have yet toachieveconsensusintheastrophysicscommunity.

Alongwithosmiumandplatinum,iridiumisoneofthethreeheaviest(densest)elements on theTable—two cubic feet of itweighs asmuch as aBuick,whichmakesiridiumoneoftheworld’sbestpaperweights,abletodefyallknownoffice

fans.Iridiumisalsotheworld’smostfamoussmokinggun.Athinlayerofitcanbe found worldwide at the famous Cretaceous-Paleogene (K-Pg) boundary† ingeologicalstrata,datingfromsixty-fivemillionyearsago.Notsocoincidentally,that’s when every land species larger than a carry-on suitcase went extinct,includingthelegendarydinosaurs.IridiumisrareonEarth’ssurfacebutrelativelycommoninsix-milemetallicasteroids,which,uponcollidingwithEarth,vaporizeonimpact,scatteringtheiratomsacrossEarth’ssurface.So,whatevermighthavebeen your favorite theory for offing the dinosaurs, a killer asteroid the size ofMountEverestfromouterspaceshouldbeatthetopofyourlist.

Idon’tknowhowAlbertwouldhavefeltaboutthis,butanunknownelementwasdiscoveredinthedebrisofthefirsthydrogenbombtestintheEniwetokatollin the South Pacific, onNovember 1, 1952, andwas named einsteinium in hishonor.Imighthavenameditarmageddiuminstead.

Meanwhile,tenentriesinthePeriodicTablegettheirnamesfromobjectsthatorbittheSun:

Phosphorus comes from the Greek for “light-bearing,” and was the ancientnamefortheplanetVenuswhenitappearedbeforesunriseinthedawnsky.

Seleniumcomesfromselene,whichisGreekfortheMoon,namedsobecauseinores, itwasalwaysassociatedwith theelement tellurium,whichhadalreadybeennamedforEarth,fromtheLatintellus.

OnJanuary1,1801,theItalianastronomerGiuseppePiazzidiscoveredanewplanet orbiting theSun in the suspiciously large gap betweenMars and Jupiter.Keepingwith the tradition of naming planets afterRoman gods, the objectwasnamed Ceres, after the goddess of harvest. Ceres is, of course, the root of theword “cereal.” At the time, there was sufficient excitement in the scientificcommunity for the first element to be discovered after this date to be namedceriuminitshonor.Twoyearslater,anotherplanetwasdiscovered,orbitingtheSuninthesamegapasCeres.ThisonewasnamedPallas,fortheRomangoddessofwisdom,and,likeceriumbeforeit,thefirstelementdiscoveredthereafterwasnamedpalladiumin itshonor.Thenamingpartywouldendafewdecades later.Afterdozensmoreoftheseplanetswerediscoveredsharingthesameorbitalzone,closer analysis revealed that these objects were much, much smaller than thesmallest knownplanets.Anew swathof real estatehadbeendiscovered in thesolar system, populated by small, craggy chunks of rock and metal. Ceres andPallaswerenotplanets;theyareasteroids,andtheyliveintheasteroidbelt,nowknown to contain hundreds of thousands of objects—somewhat more than the

numberofelementsinthePeriodicTable.The metal mercury, liquid and runny at room temperature, and the planet

Mercury, the fastest of all planets in the solar system, are both named for thespeedyRomanmessengergodofthesamename.

Thorium isnamed forThor, thehunky, lightningbolt–wieldingScandinaviangod,whocorrespondswith lightningbolt–wieldingJupiter inRomanmythology.AndbyJove,HubbleSpaceTelescope imagesof Jupiter’spolar regions revealextensiveelectricaldischargesdeepwithinitsturbulentcloudlayers.

Alas,Saturn,my favorite planet,†† has no element named for it, butUranus,Neptune, and Pluto are famously represented. The element uranium wasdiscovered in 1789 and named in honor of the planet discovered by WilliamHerschel just eight years earlier. All isotopes of uranium are unstable,spontaneouslydecayingtolighterelements,aprocessaccompaniedbythereleaseofenergy.Thefirstatomicbombeverusedinwarfarehaduraniumas itsactiveingredient,andwasdroppedbytheUnitedStates,incineratingtheJapanesecityofHiroshima on August 6, 1945. With ninety-two protons packed in its nucleus,uraniumiswidelydescribedasthe“largest”naturallyoccurringelement,althoughtrace amounts of larger elements can be found naturally where uranium ore ismined.

IfUranusdeservedanelementnamedinitshonor,thensodidNeptune.Unlikeuranium,however,whichwasdiscoveredshortlyaftertheplanet,neptuniumwasdiscovered in1940in theBerkeleycyclotron,afullninety-sevenyearsafter theGermanastronomerJohnGallefoundNeptuneinaspotintheskypredictedbytheFrench mathematician Joseph Le Verrier after studying Uranus’s odd orbitalbehavior.JustasNeptunecomesrightafterUranusinthesolarsystem,sotoodoesneptuniumcomerightafteruraniuminthePeriodicTableofelements.

The Berkeley cyclotron discovered (or manufactured?) many elements notfoundinnature,includingplutonium,whichdirectlyfollowsneptuniuminthetableandwasnamedforPluto,whichClydeTombaughdiscoveredatArizona’sLowellObservatory in 1930. Just as with the discovery of Ceres 129 years earlier,excitementprevailed.PlutowasthefirstplanetdiscoveredbyanAmericanand,intheabsenceofbetterdata,waswidelyregardedasanobjectofcommensuratesize and mass to Earth, if not Uranus or Neptune. As our attempts to measurePluto’s size became more and more refined, Pluto kept getting smaller andsmaller. Our knowledge of Pluto’s dimensions did not stabilize until the late1980s.Wenowknowthatcold,icyPlutoisbyfarthesmallestofthenine,withthediminutivedistinctionofbeinglittlerthanthesolarsystem’ssixlargestmoons.Andliketheasteroids,hundredsmoreobjectswerelaterdiscoveredintheoutersolarsystemwithorbitssimilartothatofPluto,signalingtheendofPluto’stenure

asaplanet,andtherevelationofaheretoforeundocumentedreservoirofsmallicybodiescalled theKuiperbeltof comets, towhichPlutobelongs. In this regard,one could argue that Ceres, Pallas, and Pluto slipped into the Periodic Tableunderfalsepretenses.

Unstable weapons-grade plutonium was the active ingredient in the atomicbomb that the United States exploded over the Japanese city of Nagasaki, justthreedaysafterHiroshima,bringingaswiftendtoWorldWarII.Smallquantitiesofnon-weapons-graderadioactiveplutoniumcanbeusedtopowerradioisotopethermoelectric generators (sensibly abbreviated as RTGs) for spacecraft thattravel to the outer solar system, where the intensity of sunlight has diminishedbelowthelevelusablebysolarpanels.Onepoundofplutoniumwillgeneratetenmillionkilowatt-hoursofheatenergy,whichisenoughtopoweranincandescentlightbulbforeleventhousandyears,orahumanbeingforjustaslongifweranonnuclearfuelinsteadofgrocery-storefood.

SoendsourcosmicjourneythroughthePeriodicTableofChemicalElements,right to the edge of the solar system, and beyond. For reasons I have yet tounderstand,manypeopledon’tlikechemicals,whichmightexplaintheperennialmovementtoridfoodsofthem.Perhapssesquipedalianchemicalnamesjustsounddangerous.Butinthatcaseweshouldblamethechemists,andnotthechemicalsthemselves.Personally, I amquite comfortablewith chemicals, anywhere in theuniverse.Myfavoritestars,aswellasmybestfriends,areallmadeofthem.

†Forold-timers,thislayerwasformerlyknownastheCretaceous-Tertiary(K-T)boundary.††Actually,Earthismyfavoriteplanet.ThenSaturn.

8.

OnBeingRound

Apart from crystals and broken rocks, notmuch else in the cosmos naturallycomeswith sharp angles.Whilemany objects have peculiar shapes, the list ofround things is practically endless and ranges from simple soap bubbles to theentire observable universe.Of all shapes, spheres are favored by the action ofsimple physical laws. So prevalent is this tendency that often we assumesomething is spherical in a mental experiment just to glean basic insight evenwhenweknowthattheobjectisdecidedlynon-spherical.Inshort,ifyoudonotunderstand the spherical case, then you cannot claim to understand the basicphysicsoftheobject.

Spheres in nature aremade by forces, such as surface tension, thatwant tomakeobjectssmallerinalldirections.Thesurfacetensionoftheliquidthatmakesa soap bubble squeezes air in all directions. It will, within moments of beingformed, enclose the volume of air using the least possible surface area. Thismakes the strongestpossiblebubblebecause the soapy filmwillnothave tobespreadanythinnerthanisabsolutelynecessary.Usingfreshman-levelcalculusyoucan show that the one and only shape that has the smallest surface area for anenclosedvolume is aperfect sphere. In fact, billionsofdollars couldbe savedannuallyonpackagingmaterialsifallshippingboxesandallpackagesoffoodinthesupermarketwerespheres.Forexample,thecontentsofasuper-jumboboxofCheerios would fit easily into a spherical carton with a four-and-a-half-inchradius. But practical matters prevail—nobody wants to chase packaged fooddowntheaisleafteritrollsofftheshelves.

OnEarth,onewaytomakeballbearingsistomachinethem,ordropmoltenmetalinpre-measuredamountsintothetopofalongshaft.Theblobwilltypicallyundulateuntil it settles into theshapeofasphere,but itneedssufficient time to

hardenbeforehittingthebottom.Onorbitingspacestations,whereeverythingisweightless,yougentlysquirtoutprecisequantitiesofmoltenmetalandyouhaveallthetimeyouneed—thebeadsjustfloattherewhiletheycool,untiltheyhardenasperfectspheres,withsurfacetensiondoingalltheworkforyou.

For large cosmic objects, energy and gravity conspire to turn objects intospheres.Gravity is the force that serves tocollapsematter inalldirections,butgravity does not alwayswin—chemical bonds of solid objects are strong. TheHimalayasgrewagainst theforceofEarth’sgravitybecauseof theresilienceofcrustal rock. But before you get excited about Earth’s mighty mountains, youshouldknow that the spread inheight from thedeepestundersea trenches to thetallest mountains is about a dozen miles, yet Earth’s diameter is nearly eightthousandmiles.So,contrarytowhatitlooksliketoteenyhumanscrawlingonitssurface,Earth,asacosmicobject,isremarkablysmooth.Ifyouhadasuper-duper,jumbo-giganticfinger,andyoudraggeditacrossEarth’ssurface(oceansandall),Earthwould feel as smooth as a cueball.Expensive globes that portray raisedportions of Earth’s landmasses to indicate mountain ranges are grossexaggerationsofreality.Thisiswhy,inspiteofEarth’smountainsandvalleys,aswellasbeingslightlyflattenedfrompoletopole,whenviewedfromspace,Earthisindistinguishablefromaperfectsphere.

Earth’smountainsarealsopunywhencomparedwithsomeothermountainsinthe solar system. The largest onMars, OlympusMons, is 65,000 feet tall andnearly300mileswideatitsbase.ItmakesAlaska’sMountMcKinleylooklikeamolehill.Thecosmicmountain-buildingrecipeissimple: theweakerthegravityonthesurfaceofanobject, thehigheritsmountainscanreach.MountEverestisabout as tall as a mountain on Earth can grow before the lower rock layerssuccumbtotheirownplasticityunderthemountain’sweight.

Ifasolidobjecthasa lowenoughsurfacegravity, thechemicalbonds in itsrockswill resist the force of their ownweight.When this happens, almost anyshapeispossible.Twofamouscelestialnon-spheresarePhobosandDeimos,theIdahopotato–shapedmoonsofMars.Onthirteen-mile-longPhobos,thebiggerofthetwomoons,a150-poundpersonwouldweighamerefourounces.

In space, surface tension always forces a small blob of liquid to form asphere.Wheneveryouseeasmallsolidobjectthatissuspiciouslyspherical,youcanassumeitformedinamoltenstate.Iftheblobhasveryhighmass,thenitcouldbecomposedofalmostanythingandgravitywillensurethatitformsasphere.

Bigandmassiveblobsofgasinthegalaxycancoalescetoformnear-perfect,

gaseousspherescalledstars.Butifastarfindsitselforbitingtooclosetoanotherobject whose gravity is significant, the spherical shape can be distorted as itsmaterial gets stripped away. By “too close,” I mean too close to the object’sRoche lobe—named for the mid-nineteenth-century mathematician ÉdouardRoche,whomadedetailedstudiesofgravityfieldsinthevicinityofdoublestars.TheRochelobeisatheoretical,dumbbell-shaped,bulbous,doubleenvelopethatsurrounds any two objects inmutual orbit. If gaseousmaterial from one objectpasses out of its own envelope, then the material will fall toward the secondobject.Thisoccurrenceiscommonamongbinarystarswhenoneofthemswellstobecome a red giant and overfills its Roche lobe. The red giant distorts into adistinctly non-spherical shape that resembles an elongated Hershey’s kiss.Moreover, every now and then, one of the two stars is a black hole, whoselocationisrenderedvisiblebytheflayingofitsbinarycompanion.Thespiralinggas, after having passed from the giant across itsRoche lobe, heats to extremetemperaturesandisrenderedaglowbeforedescendingoutofsightintotheblackholeitself.

ThestarsoftheMilkyWaygalaxytraceabig,flatcircle.Withadiameter-to-thickness ratio of one thousand to one, our galaxy is flatter than the flattestflapjacksevermade.Infact,itsproportionsarebetterrepresentedbyacrépeoratortilla.No,theMilkyWay’sdiskisnotasphere,butitprobablybeganasone.Wecan understand the flatness by assuming the galaxy was once a big, spherical,slowlyrotatingballofcollapsinggas.Duringthecollapse,theballspunfasterandfaster, just as spinning figure skaters do when they draw their arms inward toincreasetheirrotationrate.Thegalaxynaturallyflattenedpole-to-polewhiletheincreasingcentrifugalforcesinthemiddlepreventedcollapseatmidplane.Yes,ifthePillsburyDoughboywereafigureskater,thenfastspinswouldbeahigh-riskactivity.

AnystarsthathappenedtobeformedwithintheMilkyWaycloudbeforethecollapsemaintainedlarge,plungingorbits.Theremaininggas,whicheasilystickstoitself,likeamid-aircollisionoftwohotmarshmallows,gotpinnedatthemid-planeandisresponsibleforallsubsequentgenerationsofstars,includingtheSun.The current Milky Way, which is neither collapsing nor expanding, is agravitationallymaturesystemwhereonecanthinkoftheorbitingstarsaboveandbelowthediskastheskeletalremainsoftheoriginalsphericalgascloud.

This general flattening of objects that rotate is why Earth’s pole-to-polediameterissmallerthanitsdiameterattheequator.Notbymuch:three-tenthsof

onepercent—abouttwenty-sixmiles.ButEarthissmall,mostlysolid,anddoesn’trotate all that fast. At twenty-four hours per day, Earth carries anything on itsequatoratamere1,000milesperhour.Considerthejumbo,fast-rotating,gaseousplanetSaturn.Completingadayinjusttenandahalfhours,itsequatorrevolvesat22,000milesperhouranditspole-to-poledimensionisafulltenpercentflatterthan itsmiddle,adifferencenoticeableeven througha smallamateur telescope.Flattenedspheresaremoregenerallycalledoblatespheroids,whilespheresthatare elongated pole-to-pole are called prolate. In everyday life, hamburgers andhotdogsmakeexcellent(althoughsomewhatextreme)examplesofeachshape.Idon’tknowaboutyou,buttheplanetSaturnpopsintomymindwitheverybiteofahamburgerItake.

We use the effect of centrifugal forces on matter to offer insight into therotationrateofextremecosmicobjects.Considerpulsars.Withsomerotatingatupwardofathousandrevolutionspersecond,weknowthattheycannotbemadeofhouseholdingredients,ortheywouldspinthemselvesapart.Infact,ifapulsarrotatedanyfaster,say4,500revolutionspersecond,itsequatorwouldbemovingat the speed of light, which tells you that this material is unlike any other. Topicture a pulsar, imagine the mass of the Sun packed into a ball the size ofManhattan.Ifthat’shardtodo,thenmaybeit’seasierifyouimaginestuffingabouta hundredmillion elephants into aChapstick casing. To reach this density, youmust compress all the empty space that atoms enjoy around their nucleus andamongtheirorbitingelectrons.Doingsowillcrushnearlyall(negativelycharged)electronsinto(positivelycharged)protons,creatingaballof(neutrallycharged)neutronswithacrazy-highsurfacegravity.Undersuchconditions,aneutronstar’smountainrangeneedn’tbeanytallerthanthethicknessofasheetofpaperforyouto exert more energy climbing it than a rock climber on Earth would exertascendingathree-thousand-mile-highcliff.Inshort,wheregravityishigh,thehighplaces tend to fall, filling in the lowplaces—aphenomenon that sounds almostbiblical, in preparing the way for the Lord: “Every valley shall be raised up,every mountain and hill made low; the rough ground shall become level, theruggedplacesaplain”(Isaiah40:4).That’sarecipeforasphereifthereeverwasone. For all these reasons, we expect pulsars to be the most perfectly shapedspheresintheuniverse.

For richclustersofgalaxies, theoverall shapecanofferdeepastrophysical

insight.Someareraggedy.Othersarestretchedthininfilaments.Yetothersformvast sheets. None of these have settled into a stable—spherical—gravitationalshape.Somearesoextendedthatthefourteen-billion-yearageoftheuniverseisinsufficienttimefortheirconstituentgalaxiestomakeonecrossingofthecluster.Weconclude that theclusterwasborn thatwaybecause themutualgravitationalencountersbetweenandamonggalaxieshavehadinsufficienttimetoinfluencethecluster’sshape.

Butother systems, suchas thebeautifulComaclusterofgalaxies,whichwemetinourchapterondarkmatter,tellusimmediatelythatgravityhasshapedtheclusterintoasphere.Asaconsequence,youareaslikelytofindagalaxymovingin one direction as in any other. Whenever this is true, the cluster cannot berotating all that fast; otherwise,wewould see some flattening, aswedo inourownMilkyWay.

The Coma cluster, once again like the Milky Way, is also gravitationallymature.Inastrophysicalvernacular,suchsystemsaresaidtobe“relaxed,”whichmeans many things, including the fortuitous fact that the average velocity ofgalaxiesintheclusterservesasanexcellentindicatorofthetotalmass,whetheror not the total mass of the system is supplied by the objects used to get theaveragevelocity.It’sfor thesereasonsthatgravitationallyrelaxedsystemsmakeexcellent probes of non-luminous “dark” matter. Allow me to make an evenstrongerstatement:wereitnotforrelaxedsystems,theubiquityofdarkmattermayhaveremainedundiscoveredtothisday.

Thespheretoendallspheres—thelargestandmostperfectofthemall—istheentireobservableuniverse.Ineverydirectionwelook,galaxiesrecedefromusatspeedsproportionaltotheirdistance.Aswesawinthefirstfewchapters,thisisthe famous signatureof an expandinguniverse, discoveredbyEdwinHubble in1929.When you combine Einstein’s relativity and the velocity of light and theexpandinguniverseandthespatialdilutionofmassandenergyasaconsequenceof that expansion, there is a distance in every direction from us where therecession velocity for a galaxy equals the speed of light. At this distance andbeyond, light from all luminous objects loses all its energy before reaching us.Theuniversebeyondthisspherical“edge”isthusrenderedinvisibleand,asfarasweknow,unknowable.

There’savariationoftheever-popularmultiverseideainwhichthemultipleuniverses that comprise it arenot separateuniverses entirely, but isolated, non-interacting pockets of space within one continuous fabric of space-time—like

multiple ships at sea, far enough away from one another so that their circularhorizons do not intersect. As far as any one ship is concerned (without furtherdata),it’stheonlyshipontheocean,yettheyallsharethesamebodyofwater.

Spheres are indeed fertile theoretical tools that help us gain insight into allmanner of astrophysical problems.But one should not be a sphere-zealot. I amreminded of the half-serious joke about how to increase milk production on afarm:Anexpert inanimalhusbandrymightsay,“Consider the roleof thecow’sdiet...”Anengineermightsay,“Considerthedesignofthemilkingmachines...”Butit’stheastrophysicistwhosays,“Considerasphericalcow...”

9.

InvisibleLightAndthereforeasastrangergiveitwelcome.Therearemorethingsinheavenandearth,Horatio,Thanaredreamtofinyourphilosophy

HAMLET,ACT1,SCENE5

Before 1800 theword “light,” apart from its use as a verb and an adjective,referred just tovisible light.Butearly thatyear theEnglishastronomerWilliamHerschelobservedsomewarmingthatcouldonlyhavebeencausedbyaformoflightinvisibletothehumaneye.Alreadyanaccomplishedobserver,HerschelhaddiscoveredtheplanetUranusin1781andwasnowexploringtherelationbetweensunlight,color,andheat.Hebeganbyplacingaprismin thepathofasunbeam.Nothingnewthere.SirIsaacNewtonhaddonethatbackinthe1600s,leadinghimto name the familiar seven colors of the visible spectrum: red, orange, yellow,green,blue,indigo,andviolet.(Yes,thecolorsdoindeedspellRoyG.Biv.)ButHerschelwas inquisitive enough towonderwhat the temperature of each colormight be. So he placed thermometers in various regions of the rainbow andshowed,ashesuspected,thatdifferentcolorsregistereddifferenttemperatures.†

Well-conducted experiments require a “control”—ameasurementwhere youexpectnoeffectatall,andwhichservesasakindofidiot-checkonwhatyouaremeasuring.Forexample,ifyouwonderwhateffectbeerhasonatulipplant,thenalsonurtureasecondtulipplant,identicaltothefirst,butgiveitwaterinstead.Ifbothplantsdie—ifyoukilledthemboth—thenyoucan’tblamethealcohol.That’sthevalueofacontrolsample.Herschelknewthis,andlaidathermometeroutsideof the spectrum, adjacent to the red, expecting to read no more than room

temperature throughout the experiment. But that’s not what happened. Thetemperatureofhiscontrolthermometerroseevenhigherthaninthered.

Herschelwrote:

[I]conclude, that thefull redfallsstillshortof themaximumofheat;whichperhaps liesevena littlebeyondvisiblerefraction.Inthiscase,radiantheatwillatleastpartly,ifnotchiefly,consist,ifImaybepermittedtheexpression,ofinvisiblelight;thatistosay,ofrayscomingfromthesun,thathavesuchamomentumastobeunfitforvision.††

Holys#%t!Herschel inadvertentlydiscovered“infra” red light, abrand-newpartof the

spectrum found just “below” red, reported in the first of his four papers on thesubject.

Herschel’s revelation was the astronomical equivalent of Antonie vanLeeuwenhoek’sdiscoveryof“manyverylittlelivinganimalcules,veryprettilya-moving”††† in the smallest dropof lakewater.Leeuwenhoekdiscovered single-celled organisms—a biological universe. Herschel discovered a new band oflight.Bothhidinginplainsight.

OtherinvestigatorsimmediatelytookupwhereHerschelleftoff.In1801theGermanphysicistandpharmacistJohannWilhelmRitterfoundyetanotherbandofinvisible light.But insteadof a thermometer,Ritter placed a little pile of light-sensitivesilverchlorideineachvisiblecoloraswellasinthedarkareanexttotheviolet endof the spectrum.Sureenough, thepile in theunlitpatchdarkenedmorethanthepileinthevioletpatch.What’sbeyondviolet?“Ultra”violet,betterknowntodayasUV.

Filling out the entire electromagnetic spectrum, in order of low-energy andlow-frequency to high-energy and high-frequency, we have: radio waves,microwaves, infrared,ROYGBIV, ultraviolet,X-rays, and gamma rays.Moderncivilizationhasdeftlyexploitedeachofthesebandsforcountlesshouseholdandindustrialapplications,makingthemfamiliartousall.

AfterthediscoveryofUVandIR,sky-watchingdidn’tchangeovernight.Thefirsttelescopedesignedtodetectinvisiblepartsoftheelectromagneticspectrumwouldn’t be built for 130 years. That’s well after radio waves, X-rays, andgamma rayshadbeendiscovered, andwell after theGermanphysicistHeinrichHertzhadshownthattheonlyrealdifferenceamongthevariouskindsoflightisthefrequencyofthewavesineachband.Infact,creditHertzforrecognizingthatthere is such a thing as an electromagnetic spectrum. In his honor, the unit offrequency—inwavespersecond—foranythingthatvibrates,includingsound,has

dulybeennamedthehertz.Mysteriously,astrophysicistswereabitslowtomaketheconnectionbetween

the newfound invisible bands of light and the idea of building a telescope thatmightseethosebandsfromcosmicsources.Delaysindetectortechnologysurelymatteredhere.But hubrismust take someof theblame:howcould theuniversepossibly send us light that ourmarvelous eyes cannot see?Formore than threecenturies—fromGalileo’sdayuntilEdwinHubble’s—buildingatelescopemeantonly one thing: making an instrument to catch visible light, enhancing ourbiologicallyendowedvision.

Atelescopeismerelyatooltoaugmentourmeagersenses,enablingustogetbetteracquaintedwithfarawayplaces.Thebigger the telescope, thedimmer theobjectsitbringsintoview;themoreperfectlyshapeditsmirrors,thesharpertheimage it makes; the more sensitive its detectors, the more efficient itsobservations.Butinallcases,everybitofinformationatelescopedeliverstotheastrophysicistcomestoEarthonabeamoflight.

Celestial happenings, however, don’t limit themselves to what’s convenientfor the human retina. Instead, they typically emit varying amounts of lightsimultaneouslyinmultiplebands.Sowithouttelescopesandtheirdetectorstunedacross the entire spectrum, astrophysicists would remain blissfully ignorant ofsomemind-blowingstuffintheuniverse.

Takeanexplodingstar—asupernova.It’sacosmicallycommonandseriouslyhigh-energy event that generates prodigious quantities of X-rays. Sometimes,bursts of gamma rays and flashes of ultraviolet accompany the explosions, andthere’sneverashortageofvisiblelight.Longaftertheexplosivegasescool,theshockwavesdissipate,andthevisiblelightfades,thesupernova“remnant”keepson shining in the infrared, while pulsing in radio waves. That’s where pulsarscomefrom,themostreliabletimekeepersintheuniverse.

Most stellar explosions take place in distant galaxies, but if a starwere toblow up within the Milky Way, its death throes would be bright enough foreveryonetosee,evenwithoutatelescope.ButnobodyonEarthsawtheinvisibleX-rays or gamma rays from the last two supernova spectaculars hosted by ourgalaxy—onein1572andanother in1604—yet theirwondrousvisible lightwaswidelyreported.

The range ofwavelengths (or frequencies) that comprise each band of lightstrongly influences the design of the hardware used to detect it. That’swhy nosinglecombinationoftelescopeanddetectorcansimultaneouslyseeeveryfeatureof such explosions. But the way around that problem is simple: gather allobservationsofyourobject,perhapsobtainedbycolleagues,inmultiplebandsoflight.Thenassignvisiblecolorstoinvisiblebandsofinterest,creatingonemeta,

multi-band image. That’s preciselywhatGeordi from the television seriesStarTrek:TheNextGenerationsees.Withthatpowerofvision,youmissnothing.

Onlyafteryouidentifythebandofyourastrophysicalaffectionscanyoubeginto think about the size of yourmirror, thematerials you’ll need tomake it, theshape and surface it must have, and the kind of detector you’ll need. X-raywavelengths, for example, are extremely short. So if you’re accumulating them,yourmirrorhadbetterbe super-smooth, lest imperfections in the surfacedistortthem. But if you’re gathering long radio waves, yourmirror could bemade ofchickenwire that you’ve bentwith your hands, because the irregularities in thewirewould bemuch smaller than thewavelengths you’re after.Of course, youalsowantplentyofdetail—highresolution—soyourmirrorshouldbeasbigasyoucanafford tomake it. In theend,your telescopemustbemuch,muchwiderthan thewavelength of light you aim to detect.And nowhere is this needmoreevidentthanintheconstructionofaradiotelescope.

Radio telescopes, the earliest non-visible-light telescopes ever built, are anamazingsubspeciesofobservatory.TheAmericanengineerKarlG.Janskybuiltthe first successfulonebetween1929and1930. It lookedabit like themovingsprinkler system on a farmerless farm.Made from a series of tall, rectangularmetalframessecuredwithwoodencross-supportsandflooring,itturnedinplacelike amerry-go-round onwheels built with spare parts from aModel T Ford.Jansky had tuned the hundred-foot-long contraption to a wavelength of aboutfifteen meters, corresponding to a frequency of 20.5 megahertz.†††† Jansky’sagenda,onbehalfofhisemployer,BellTelephoneLaboratories,wastostudyanyhisses from Earth-based radio sources that might contaminate terrestrial radiocommunications.ThisgreatlyresemblesthetaskthatBellLabsgavePenziasandWilson, thirty-fiveyears later, to findmicrowavenoise in their receiver, aswesaw in chapter 3, which led to the discovery of the cosmic microwavebackground.

Byspendingacoupleofyearspainstakinglytrackingandtimingthestatichissthatregisteredonhisjury-riggedantenna,Janskyhaddiscoveredthatradiowavesemanatenotjustfromlocalthunderstormsandotherknownterrestrialsources,butalsofromthecenteroftheMilkyWaygalaxy.Thatregionoftheskyswungbythetelescope’s fieldofviewevery twenty-threehoursand fifty-sixminutes:exactlytheperiodofEarth’srotationinspaceandthusexactlythetimeneededtoreturnthe galactic center to the same angle and elevation on the sky. Karl Janskypublished his results under the title “Electrical Disturbances Apparently of

ExtraterrestrialOrigin.”†††††Withthatobservation,radioastronomywasborn—butminusJanskyhimself.

Bell Labs retasked him, preventing him from pursuing the fruits of his ownseminal discovery. A few years later, though, a self-starting American namedGrote Reber, fromWheaton, Illinois, built a thirty-foot-wide, metal-dish radiotelescopeinhisownbackyard.In1938,undernobody’semploy,ReberconfirmedJansky’sdiscovery,andspent thenextfiveyearsmakinglow-resolutionmapsoftheradiosky.

Reber’stelescope,thoughwithoutprecedent,wassmallandcrudebytoday’sstandards. Modern radio telescopes are quite another matter. Unbound bybackyards, they’re sometimes downright humongous. MK 1, which began itsworking life in 1957, is the planet’s first genuinely gigantic radio telescope—asingle,steerable,250-foot-wide,solid-steeldishattheJodrellBankObservatorynearManchester,England.AcoupleofmonthsafterMK1opened forbusiness,theSovietUnion launchedSputnik1, and JodrellBank’s dish suddenly becamejustthethingtotrackthelittleorbitinghunkofhardware—makingittheforerunneroftoday’sDeepSpaceNetworkfortrackingplanetaryspaceprobes.

Theworld’s largest radio telescope, completed in2016, is called theFive-hundred-meterApertureSpherical radioTelescope,or“FAST”forshort. ItwasbuiltbyChinaintheirGuizhouProvince,andislargerinareathanthirtyfootballfields.Ifaliensevergiveusacall,theChinesewillbethefirsttoknow.

Anothervarietyofradiotelescopeistheinterferometer,comprisingarraysofidentical dish antennas, spread across swaths of countryside and electronicallylinked towork inconcert.Theresult isasingle,coherent,super-high-resolutionimage of radio-emitting cosmic objects. Although “supersize me” was theunwritten motto for telescopes long before the fast food industry coined theslogan,radiointerferometersformajumboclassuntothemselves.Oneofthem,avery largearrayof radiodishesnearSocorro,NewMexico, isofficiallycalledthe Very Large Array, with twenty-seven eighty-two-foot dishes positioned ontracks crossing twenty-two miles of desert plains. This observatory is socosmogenic,ithasappearedasabackdropinthefilms2010:TheYearWeMakeContact(1984),Contact(1997),andTransformers(2007).There’salsotheVeryLongBaselineArray,withteneighty-two-footdishesspanning5,000milesfromHawaii to the Virgin Islands, enabling the highest resolution of any radiotelescopeintheworld.

Inthemicrowaveband,relativelynewtointerferometers,we’vegotthesixty-

sixantennasofALMA,theAtacamaLargeMillimeterArray,intheremoteAndesMountainsofnorthernChile.Tunedforwavelengthsthatrangefromfractionsofamillimeter to several centimeters, ALMA gives astrophysicists high-resolutionaccesstocategoriesofcosmicactionunseeninotherbands,suchasthestructureof collapsing gas clouds as they become nurseries from which stars are born.ALMA’slocationis,byintention,themostaridlandscapeonEarth—threemilesabove sea level and well above the wettest clouds. Water may be fine formicrowave cooking but it’s bad for astrophysicists, because thewater vapor inEarth’satmospherechewsuppristinemicrowavesignalsfromacross thegalaxyand beyond. These two phenomena are, of course, related: water is the mostcommon ingredient in food, andmicrowave ovens primarily heat water. Takentogether,yougetthebestindicationthatwaterabsorbsmicrowavefrequencies.Soifyouwantcleanobservationsofcosmicobjects,youmustminimizetheamountofwatervaporbetweenyourtelescopeandtheuniverse,justasALMAhasdone.

Attheultrashort-wavelengthendoftheelectromagneticspectrumyoufindthehigh-frequency, high-energy gamma rays, with wavelengths measured inpicometers.††††††Discovered in1900, theywerenotdetectedfromspaceuntilanewkindoftelescopewasplacedaboardNASA’sExplorerXIsatellitein1961.

Anybodywhowatchestoomanysci-fimoviesknowsthatgammaraysarebadforyou.Youmightturngreenandmuscular,orspiderwebsmightsquirtfromyourwrists.Butthey’realsohardtotrap.Theypassrightthroughordinarylensesandmirrors.How,then,toobservethem?ThegutsofExplorerXI’stelescopeheldadevicecalledascintillator,whichrespondstoincominggammaraysbypumpingoutelectricallychargedparticles.Ifyoumeasureenergiesoftheparticles,youcantellwhatkindofhigh-energylightcreatedthem.

TwoyearslatertheSovietUnion,theUnitedKingdom,andtheUnitedStatessignedtheLimitedTestBanTreaty,whichprohibitednucleartestingunderwater,in the atmosphere, and in space—where nuclear fallout could spread andcontaminateplacesoutsideyourowncountry’sperimeter.But thiswas theColdWar,atimewhennobodybelievedanybodyaboutanything.Invokingthemilitaryedict“trustbutverify,”theU.S.deployedanewseriesofsatellites,theVelas, toscan for gamma ray bursts that would result from Soviet nuclear tests. Thesatellites indeedfoundburstsofgammarays,almostdaily.ButRussiawasn’t toblame.Thesecamefromdeepspace—andwerelatershowntobethecallingcardofintermittent,distant,titanicstellarexplosionsacrosstheuniverse,signalingthebirthofgammarayastrophysics,anewbranchofstudyinmyfield.

In 1994, NASA’s ComptonGammaRayObservatory detected something asunexpectedas theVelas’ discoveries: frequent flashes of gamma rays right nearEarth’s surface. They were sensibly dubbed “terrestrial gamma-ray flashes.”Nuclear holocaust? No, as is evident from the fact that you’re reading thissentence. Not all bursts of gamma rays are equally lethal, nor are they all ofcosmicorigin.Inthiscase,atleastfiftyburstsoftheseflashesemanatedailynearthe tops of thunderclouds, a split second before ordinary lightning bolts strike.Theiroriginremainsabitofamystery,butthebestexplanationholdsthatintheelectricalstorm,freeelectronsacceleratetonearthespeedoflightandthenslamintothenucleiofatmosphericatoms,generatinggammarays.

Today,telescopesoperateineveryinvisiblepartofthespectrum,somefromthe ground but most from space, where a telescope’s view is unimpeded byEarth’s absorptive atmosphere. We can now observe phenomena ranging fromlow-frequencyradiowavesadozenmeterslong,cresttocrest,tohigh-frequencygamma rays no longer than a quadrillionth of ameter.That rich palette of lightsuppliesnoendofastrophysicaldiscoveries:Curioushowmuchgaslurksamongthestarsingalaxies?Radiotelescopesdothatbest.Thereisnoknowledgeofthecosmicbackground,andnorealunderstandingofthebigbang,withoutmicrowavetelescopes.Wanttopeekatstellarnurseriesdeepinsidegalacticgasclouds?Payattentiontowhatinfraredtelescopesdo.Howaboutemissionsfromthevicinityofordinary black holes and supermassive black holes in the center of a galaxy?Ultraviolet and X-ray telescopes do that best. Want to watch the high-energyexplosionofagiantstar,whosemass isasgreatasfortysuns?Catchthedramaviagammaraytelescopes.

We’ve come a long way since Herschel’s experiments with rays that were“unfitforvision,”empoweringustoexploretheuniverseforwhatitis,ratherthanfor what it seems to be. Herschel would be proud. We achieved true cosmicvisiononlyafterseeingtheunseeable:adazzlinglyrichcollectionofobjectsandphenomena across space and across time that we may now dream of in ourphilosophy.

†Notuntilthemid-1800s,whenthephysicist’sspectrometerwasappliedtoastronomicalproblems,didtheastronomerbecometheastrophysicist.In1895,theprestigiousAstrophysicalJournalwasfounded,withthesubtitle“AnInternationalReviewofSpectroscopyandAstronomicalPhysics.”††WilliamHerschel,“ExperimentsonSolarandontheTerrestrialRaysthatOccasionHeat,”PhilosophicalTransactionsoftheRoyalAstronomicalSociety,1800,17.

†††AntonievanLeeuwenhoek,lettertotheRoyalSocietyofLondon,October10,1676.††††Allwavesfollowthesimpleequation:speed=frequency×wavelength.Ataconstantspeed,ifyouincreasethewavelength,thewaveitselfwillhavesmallerfrequency,andviceversa,sothatwhenyoumultiplythetwoquantitiesyourecoverthesamespeedofthewaveeverytime.Worksforlight,sound,andevenfansdoingthe“Wave”atsportsarenas—anythingthat’satravelingwave.†††††KarlJansky,“ElectricalDisturbancesApparentlyofExtraterrestrialOrigin,”ProceedingsoftheInstituteforRadioEngineers21,no.10(1933):1387.††††††“pico-”isthemetricprefixforone-trillionth.

10.

BetweenthePlanets

Fromadistance,oursolarsystemlooksempty.Ifyouencloseditwithinasphere—one largeenough to contain theorbit ofNeptune, theoutermostplanet†—thenthevolumeoccupiedbytheSun,allplanets,andtheirmoonswouldtakeupalittlemorethanone-trillionththeenclosedspace.Butit’snotempty,thespacebetweentheplanetscontainsallmannerofchunkyrocks,pebbles,iceballs,dust,streamsof charged particles, and far-flung probes. The space is also permeated bymonstrousgravitationalandmagneticfields.

Interplanetary space is so not-empty thatEarth, during its 30 kilometer-per-secondorbitaljourney,plowsthroughhundredsoftonsofmeteorsperday—mostof themno larger thanagrainof sand.Nearlyallof themburn inEarth’supperatmosphere,slammingintotheairwithsomuchenergythat thedebrisvaporizesoncontact.Ourfrailspeciesevolvedunder thisprotectiveblanket.Larger,golf-ball-sizemeteorsheatfastbutunevenly,andoftenshatterintomanysmallerpiecesbeforetheyvaporize.Stilllargermeteorssingetheirsurfacebutotherwisemakeitall theway to theground intact.You’d think that bynow, after 4.6billion tripsaroundtheSun,Earthwouldhave“vacuumed”upallpossibledebrisinitsorbitalpath. But things were once much worse. For a half-billion years after theformationoftheSunanditsplanets,somuchjunkraineddownonEarththatheatfrom the persistent energy of impacts rendered Earth’s atmosphere hot and ourcrustmolten.

OnesubstantialhunkofjunkledtotheformationoftheMoon.Theunexpectedscarcityofironandotherhigher-masselementsintheMoon,derivedfromlunarsamplesreturnedbyApolloastronauts,indicatesthattheMoonmostlikelyburstforth from Earth’s iron-poor crust and mantle after a glancing collision with awayward Mars-sized protoplanet. The orbiting debris from this encounter

coalesced to formour lovely, low-densitysatellite.Apart fromthisnewsworthyevent,theperiodofheavybombardmentthatEarthenduredduringitsinfancywasnotuniqueamongtheplanetsandotherlargebodiesofthesolarsystem.Theyeachsustainedsimilardamage,withtheairless,erosionlesssurfacesoftheMoonandMercurypreservingmuchofthecrateredrecordfromthisperiod.

Notonlyisthesolarsystemscarredbytheflotsamofitsformation,butnearbyinterplanetary space also contains rocks of all sizes that were jettisoned fromMars, the Moon, and Earth by the ground’s recoil from high-speed impacts.Computer studies ofmeteor strikes demonstrate conclusively that surface rocksnearimpactzonescangetthrustupwardwithenoughspeedtoescapethebody’sgravitational tether. At the rate we are discovering meteorites on Earth whoseoriginisMars,weconcludethataboutathousandtonsofMartianrocksraindownon Earth each year. Perhaps the same amount reaches Earth from theMoon. Inretrospect,wedidn’thavetogototheMoontoretrieveMoonrocks.Plentycometous,althoughtheywerenotofourchoosingandwedidn’tyetknowitduringtheApolloprogram.

Mostofthesolarsystem’sasteroidsliveandworkinthemainasteroidbelt,aroughly flat zone between the orbits of Mars and Jupiter. By tradition, thediscoverersgettonametheirasteroidswhatevertheylike.Oftendrawnbyartistsas a region of cluttered,meandering rocks in the plane of the solar system, theasteroidbelt’stotalmassislessthanfivepercentthatoftheMoon,whichisitselfbarely more than one percent of Earth’s mass. Sounds insignificant. Butaccumulated perturbations of their orbits continually create a deadly subset,perhapsa few thousand,whoseeccentricpaths intersectEarth’sorbit.AsimplecalculationrevealsthatmostofthemwillhitEarthwithinahundredmillionyears.Theoneslargerthanaboutakilometeracrosswillcollidewithenoughenergytodestabilize Earth’s ecosystem and put most of Earth’s land species at risk ofextinction.

Thatwouldbebad.AsteroidsarenottheonlyspaceobjectsthatposearisktolifeonEarth.The

Kuiperbeltisacomet-strewnswathofcircularrealestatethatbeginsjustbeyondthe orbit of Neptune, includes Pluto, and extends perhaps as far again fromNeptuneasNeptuneisfromtheSun.TheDutch-bornAmericanastronomerGerardKuiper advanced the idea that in the cold depths of space, beyond the orbit ofNeptune, there reside frozen leftovers from the formation of the solar system.Withoutamassiveplanetuponwhichtofall,mostofthesecometswillorbitthe

Sunforbillionsmoreyears.Asis truefor theasteroidbelt,someobjectsof theKuiperbelt traveloneccentricpaths thatcross theorbitsofotherplanets.PlutoanditsensembleofsiblingscalledPlutinoscrossNeptune’spatharoundtheSun.Other Kuiper belt objects plunge all the way down to the inner solar system,crossing planetary orbits with abandon. This subset includes Halley, the mostfamouscometofthemall.

Far beyond the Kuiper belt, extending halfway to the nearest stars, lives asphericalreservoirofcometscalledtheOortcloud,namedforJanOort,theDutchastrophysicist who first deduced its existence. This zone is responsible for thelong-periodcomets, thosewithorbitalperiods far longer thanahuman lifetime.UnlikeKuiperbelt comets,Oortcloudcometscan raindownon the inner solarsystem from any angle and from any direction. The two brightest of the 1990s,comets Hale-Bopp and Hyakutake, were both from the Oort cloud and are notcomingbackanytimesoon.

Ifwe had eyes that could seemagnetic fields, Jupiterwould look ten timeslargerthanthefullMooninthesky.SpacecraftthatvisitJupitermustbedesignedto remain unaffected by this powerful force. As the English physicist MichaelFaradaydemonstratedinthe1800s,ifyoupassawireacrossamagneticfieldyougenerateavoltagedifferencealongthewire’slength.Forthisreason,fast-movingmetalspaceprobeswillhaveelectricalcurrentsinducedwithinthem.Meanwhile,thesecurrentsgeneratemagneticfieldsoftheirownthatinteractwiththeambientmagneticfieldinwaysthatretardthespaceprobe’smotion.

LastIhadkeptcount,therewerefifty-sixmoonsamongtheplanetsinthesolarsystem. Then I woke up one morning to learn that another dozen had beendiscoveredaroundSaturn.After that incident, Idecidedtonolongerkeeptrack.All I careaboutnow iswhether anyof themwouldbe funplaces tovisitor tostudy.Bysomemeasures,thesolarsystem’smoonsaremuchmorefascinatingthantheplanetstheyorbit.

Earth’sMoonisabout1/400ththediameteroftheSun,butitisalso1/400thasfarfromus,makingtheSunandtheMoonthesamesizeonthesky—acoincidencenotsharedbyanyotherplanet–mooncombinationinthesolarsystem,allowingforuniquelyphotogenictotalsolareclipses.EarthhasalsotidallylockedtheMoon,leaving it with identical periods of rotation on its axis and revolution aroundEarth.Wherever and whenever this happens, the lockedmoon shows only one

facetoitshostplanet.Jupiter’ssystemofmoonsisrepletewithoddballs.Io,Jupiter’sclosestmoon,

is tidally locked and structurally stressed by interactionswith Jupiter andwithothermoons,pumpingenoughheatintothelittleorbtorendermoltenitsinteriorrocks;Ioisthemostvolcanicallyactiveplaceinthesolarsystem.Jupiter’smoonEuropahasenoughH2Othatitsheatingmechanism—thesameoneatworkonIo—hasmeltedthesubsurfaceice,leavingawarmedoceanbelow.Ifevertherewasanext-bestplacetolookforlife,it’shere.(AnartistcoworkerofmineonceaskedwhetheralienlifeformsfromEuropawouldbecalledEuropeans.Theabsenceofanyotherplausibleanswerforcedmetosayyes.)

Pluto’s largest moon, Charon, is so big and close to Pluto that Pluto andCharonhaveeachtidallylockedtheother:theirrotationperiodsandtheirperiodsofrevolutionareidentical.Wecallthisa“doubletidallock,”whichsoundslikeayet-to-be-inventedwrestlinghold.

By convention, moons are named for Greek personalities in the life of theGreekcounterparttotheRomangodafterwhomtheplanetitselfwasnamed.Theclassicalgodsledcomplicatedsociallives,sothereisnoshortageofcharactersfrom which to draw. The lone exception to this rule applies to the moons ofUranus, which are named for assorted protagonists in British lit. The EnglishastronomerSirWilliamHerschelwasthefirstpersontodiscoveraplanetbeyondthoseeasilyvisibletothenakedeye,andhewasreadytonameitaftertheKing,under whom he faithfully served. Had Sir William succeeded, the planet listwould read: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and George.Fortunately,clearerheadsprevailedandtheclassicalnameUranuswasadoptedsomeyearslater.ButhisoriginalsuggestiontonamethemoonsaftercharactersinWilliamShakespeare’splaysandAlexanderPope’spoemsremains the traditiontothisday.Amongitstwenty-sevenmoonswefindAriel,Cordelia,Desdemona,Juliet,Ophelia,Portia,Puck,Umbriel,andMiranda.

TheSunlosesmaterialfromitssurfaceatarateofmorethanamilliontonsper second.We call this the “solarwind,”which takes the formof high-energycharged particles. Traveling up to a thousandmiles per second, these particlesstreamthroughspaceandaredeflectedbyplanetarymagneticfields.Theparticlesspiral down toward the north and southmagnetic poles, forcing collisionswithgas molecules and leaving the atmosphere aglow with colorful aurora. TheHubble Space Telescope has spotted aurora near the poles of both Saturn andJupiter.AndonEarth,theauroraborealisandaustralis(thenorthernandsouthernlights) serve as intermittent reminders of how nice it is to have a protectiveatmosphere.

Earth’s atmosphere is commonly described as extending dozens of milesaboveEarth’ssurface.Satellitesin“low”Earthorbittypicallytravelbetweenonehundredandfourhundredmilesup,completinganorbit inaboutninetyminutes.Whileyoucan’tbreatheatthosealtitudes,someatmosphericmoleculesremain—enoughtoslowlydrainorbitalenergyfromunsuspectingsatellites.Tocombatthisdrag,satellitesinloworbitrequireintermittentboosts,lesttheyfallbacktoEarthand burn up in the atmosphere. An alternative way to define the edge of ouratmosphereistoaskwhereitsdensityofgasmoleculesequalsthedensityofgasmolecules in interplanetary space. Under that definition, Earth’s atmosphereextendsthousandsofmiles.

Orbiting high above this level, twenty-three thousandmiles up (one-tenth ofthe distance to the Moon) are the communications satellites. At this specialaltitude,Earth’satmosphereisnotonlyirrelevant,butthespeedofthesatelliteislowenoughfor it torequireafullday tocompleteonerevolutionaroundEarth.WithanorbitpreciselymatchingtherotationrateofEarth,thesesatellitesappearto hover, which make them ideal for relaying signals from one part of Earth’ssurfacetoanother.

Newton’s laws specifically state that, while the gravity of a planet getsweakerandweakerthefartherfromityoutravel, thereisnodistancewheretheforce of gravity reaches zero. The planet Jupiter, with its mighty gravitationalfield,batsoutofharm’swaymanycometsthatwouldotherwisewreakhavocontheinnersolarsystem.JupiteractsasagravitationalshieldforEarth,aburlybigbrother, allowing long (hundred-million-year) stretches of relative peace andquietonEarth.WithoutJupiter’sprotection,complexlifewouldhaveahardtimebecoming interestingly complex, always living at risk of extinction from adevastatingimpact.

Wehave exploited thegravitational fieldsof planets for nearly everyprobelaunchedintospace.TheCassiniprobe,forexample,whichvisitedSaturn,wasgravitationally assisted twice by Venus, once by Earth (on a return flyby), andoncebyJupiter.Likeamulti-cushionbilliardshot,trajectoriesfromoneplanettoanotherarecommon.Ourtinyprobeswouldnototherwisehaveenoughspeedandenergyfromourrocketstoreachtheirdestination.

Iamnowaccountableforsomeofthesolarsystem’sinterplanetarydebris.InNovember2000,themain-beltasteroid1994KA,discoveredbyDavidLevyandCarolynShoemaker,wasnamed13123–Tyson inmyhonor.While I enjoyed thedistinction, there’s no particular reason to get big-headed about it; plenty of

asteroidshavefamiliarnamessuchasJody,Harriet,andThomas.ThereareevenasteroidsouttherenamedMerlin,JamesBond,andSanta.Nowinthehundredsofthousands, the asteroid count might soon challenge our capacity to name them.Whetherornotthatdayarrives,Itakecomfortknowingthatmychunkofcosmicdebris isnotaloneas it litters thespacebetween theplanets,being joinedbyalonglistofotherchunksnamedforrealandfictionalpeople.

I’malsogladthat,atthemoment,myasteroidisnotheadedtowardEarth.

†No,it’snotPluto.Getoverit.

11.

ExoplanetEarth

Whetheryouprefertosprint,swim,walk,orcrawlfromoneplacetoanotheronEarth,youcanenjoyclose-upviewsofourplanet’sunlimitedsupplyofthingsto notice. You might see a vein of pink limestone on the wall of a canyon, aladybug eating an aphid on the stemof a rose, a clamshell poking out from thesand.Allyouhavetodoislook.

From the window of an ascending jetliner, those surface details rapidlydisappear.Noaphidappetizers.Nocuriousclams.Reachcruisingaltitude,aroundsevenmilesup,andidentifyingmajorroadwaysbecomesachallenge.

Detail continues to vanish as you rise into space. From the window of theInternational Space Station,which orbits at about 250miles up, youmight findParis,London,NewYork,andLosAngelesinthedaytime,butonlybecauseyoulearnedwhere they are in geography class.At night, their sprawling cityscapespresent an obvious glow. By day, contrary to common wisdom, you probablywon’tseetheGreatPyramidsatGiza,andyoucertainlywon’tseetheGreatWallofChina.Theirobscurityispartlytheresultofhavingbeenmadefromthesoilandstoneof thesurroundinglandscape.AndalthoughtheGreatWall is thousandsofmiles long, it’s only about twenty feet wide—much narrower than the U.S.interstatehighwaysyoucanbarelyseefromatranscontinentaljet.

Fromorbit,withtheunaidedeye,youwouldhaveseensmokeplumesrisingfromtheoil-fieldfiresinKuwaitattheendofthefirstPersianGulfWarin1991and smoke from the burningWorld Trade Center towers in NewYork City onSeptember11,2001.Youwill alsonotice thegreen–brownboundariesbetweenswaths of irrigated and arid land. Beyond that shortlist, there’s not much elsemadebyhumansthat’sidentifiablefromhundredsofmilesupinthesky.Youcanseeplentyofnaturalscenery,though,includinghurricanesintheGulfofMexico,

icefloesintheNorthAtlantic,andvolcaniceruptionswherevertheyoccur.FromtheMoon,aquartermillionmilesaway,NewYork,Paris,andtherestof

Earth’s urban glitter doesn’t even show up as a twinkle. But from your lunarvantage you can stillwatchmajorweather frontsmove across the planet. FromMars at its closest, some thirty-fivemillionmiles away,massive snow-cappedmountain chains and the edges ofEarth’s continentswould be visible through alargebackyard telescope.Travelout toNeptune, threebillionmilesaway—justdowntheblockonacosmicscale—andtheSunitselfbecomesathousandtimesdimmer,nowoccupyingathousandththeareaonthedaytimeskythatitoccupieswhenseenfromEarth.AndwhatofEarthitself?It’saspecknobrighterthanadimstar,allbutlostintheglareoftheSun.

Acelebratedphotograph taken in1990 from just beyondNeptune’sorbit bytheVoyager1 spacecraftshows justhowunderwhelmingEarth looksfromdeepspace:a“palebluedot,”astheAmericanastrophysicistCarlSagancalledit.Andthat’sgenerous.Withoutthehelpofacaption,youmightnotevenknowit’sthere.

Whatwouldhappenifsomebig-brainedaliensfromthegreatbeyondscannedtheskieswiththeirnaturallysuperbvisualorgans,furtheraidedbyalienstate-of-the-art optical accessories? What visible features of planet Earth might theydetect?

Bluenesswouldbefirstandforemost.Watercoversmore than two-thirdsofEarth’ssurface;thePacificOceanalonespansnearlyanentiresideoftheplanet.Any beings with enough equipment and expertise to detect our planet’s colorwouldsurelyinferthepresenceofwater,thethirdmostabundantmoleculeintheuniverse.

If the resolution of their equipmentwere high enough, the alienswould seemorethanjustapalebluedot.Theywouldseeintricatecoastlines,too,stronglysuggestingthatthewaterisliquid.Andsmartalienswouldsurelyknowthatifaplanet has liquidwater, then the planet’s temperature and atmospheric pressurefallwithinawell-determinedrange.

Earth’sdistinctivepolar ice caps,whichgrowand shrink from the seasonaltemperature variations, could also be seen using visible light. So could ourplanet’s twenty-four-hour rotation, because recognizable landmasses rotate intoviewatpredictable intervalsof time.Thealienswouldalso seemajorweathersystemscomeandgo;with careful study, theycould readilydistinguish featuresrelated toclouds in theatmosphere fromfeatures related to thesurfaceofEarthitself.

Time for a reality check.The nearest exoplanet—the nearest planet in orbitaroundastarthatisnottheSun—canbefoundinourneighborstarsystemAlphaCentauri, about four light-years from us and visible mostly from our southern

hemisphere.Mostof thecatalogedexoplanets liefromdozensup tohundredsoflight-yearsaway.Earth’sbrightnessislessthanone-billionththatoftheSun,andourplanet’sproximitytotheSunwouldmakeitextremelyhardforanybodytoseeEarthdirectlywithavisiblelighttelescope.It’sliketryingtodetectthelightofafireflyinthevicinityofaHollywoodsearchlight.Soifalienshavefoundus,theyarelikelylookinginwavelengthsotherthanvisiblelight,likeinfrared,whereourbrightness relative to the Sun is a bit better than in visible light—or else theirengineersareadaptingsomeotherstrategyaltogether.

Maybe they’re doing what some of our own planet-hunters typically do:monitoringstarstoseeiftheyjiggleatregularintervals.Astar’speriodicjigglebetrays the existence of an orbiting planet thatmay be too dim to see directly.Contrary to what most people suppose, a planet does not orbit its host star.Instead,both theplanet and itshost star revolvearound their commoncenterofmass.Themoremassivetheplanet,thelargerthestar’sresponsemustbe,andthemoremeasurable the jigglegetswhenyouanalyze thestar’s light.Unfortunatelyforplanet-hunting aliens,Earth is puny, so theSunbarelybudges,whichwouldfurtherchallengealienengineers.

NASA’sKepler telescope,designedandtunedtodiscoverEarth-likeplanetsaroundSun-likestars,invokedyetanothermethodofdetection,mightilyaddingtothe exoplanet catalog. Kepler searched for stars whose total brightness dropsslightly,andatregularintervals.Inthesecases,Kepler’slineofsightisjustrighttoseeastargetdimmer,byatinyfraction,duetooneofitsownplanetscrossingdirectlyinfrontofthehoststar.Withthismethod,youcan’tseetheplanetitself.You can’t even see any features on the star’s surface. Kepler simply trackedchanges in a star’s total light, but added thousandsof exoplanets to the catalog,includinghundredsofmultiplanetstarsystems.Fromthesedata,youalsolearnthesizeoftheexoplanet,itsorbitalperiod,anditsorbitaldistancefromthehoststar.Youcanalsomakeaneducatedinferenceontheplanet’smass.

Ifyou’rewondering,whenEarthpassesinfrontoftheSun—whichisalwayshappeningforsomelineofsightinthegalaxy—weblock1/10,000thoftheSun’ssurface,therebybrieflydimmingtheSun’stotallightby1/10,000thofitsnormalbrightness. Fine as it goes. They’ll discover that Earth exists, but learn nothingabouthappeningsonEarth’ssurface.

Radiowavesandmicrowavesmightwork.Maybeoureavesdroppingalienshave something like the 500-meter radio telescope in the Guizhou province ofChina.Iftheydo,andiftheytunetotherightfrequencies,they’llcertainlynotice

Earth—or rather, they’ll notice our modern civilization as one of the mostluminoussources in the sky.Considereverythingwe’vegot thatgenerates radiowaves and microwaves: not only traditional radio itself, but also broadcasttelevision, mobile phones, microwave ovens, garage-door openers, car-doorunlockers,commercialradar,militaryradar,andcommunicationssatellites.We’reablazeinlong-frequencywaves—spectacularevidencethatsomethingunusualisgoingonhere,becauseintheirnaturalstate,smallrockyplanetsemithardlyanyradiowavesatall.

Soifthosealieneavesdroppersturntheirownversionofaradiotelescopeinourdirection,theymightinferthatourplanethoststechnology.Onecomplication,though: other interpretations are possible. Maybe they wouldn’t be able todistinguishEarth’ssignalsfromthoseofthelargerplanetsinoursolarsystem,allofwhich are sizable sources of radiowaves, especially Jupiter.Maybe they’dthinkwewereanewkindofodd,radio-intensiveplanet.Maybetheywouldn’tbeabletodistinguishEarth’sradioemissionsfromthoseoftheSun,forcingthemtoconcludethattheSunisanewkindofodd,radio-intensivestar.

AstrophysicistsrighthereonEarth,attheUniversityofCambridgeinEngland,were similarly stumped back in 1967.While surveying the skies with a radiotelescope for any source of strong radio waves, Antony Hewish and his teamdiscovered something extremely odd: an object pulsing at precise, repeatingintervals of slightly more than a second. Jocelyn Bell, a graduate student ofHewish’satthetime,wasthefirsttonoticeit.

SoonBell’scolleaguesestablishedthatthepulsescamefromagreatdistance.Thethoughtthatthesignalwastechnological—anotherculturebeamingevidenceof its activities across space—was irresistible. As Bell recounts, “We had noproofthatitwasanentirelynaturalradioemission....HerewasItryingtogetaPh.D.outofanewtechnique,andsomesillylotoflittlegreenmenhadtochoosemy aerial and my frequency to communicate with us.”† Within a few days,however,shediscoveredotherrepeatingsignalscomingfromotherplacesinourMilkyWaygalaxy.Bellandherassociatesrealizedthey’ddiscoveredanewclassofcosmicobject—astarmadeentirelyofneutronsthatpulseswithradiowavesforeveryrotationitexecutes.HewishandBellsensiblynamedthem“pulsars.”

Turnsout, interceptingradiowavesisn’t theonlywaytobesnoopy.There’salsocosmochemistry.Thechemicalanalysisofplanetaryatmosphereshasbecomea lively field of modern astrophysics. As you might guess, cosmochemistrydependson spectroscopy—the analysis of light bymeans of a spectrometer.Byexploiting the tools and tactics of spectroscopists, cosmochemists can infer thepresence of life on an exoplanet, regardless of whether that life has sentience,intelligence,ortechnology.

Themethodworksbecauseeveryelement,everymolecule—nomatterwhereit exists in the universe—absorbs, emits, reflects, and scatters light in a uniqueway.Andasalreadydiscussed,passthatlightthroughaspectrometer,andyou’llfind features that can rightly be called chemical fingerprints. The most visiblefingerprints are made by the chemicals most excited by the pressure andtemperature of their environment. Planetary atmospheres are rich with suchfeatures.Andifaplanet is teemingwithfloraandfauna, itsatmospherewillberichwithbiomarkers—spectralevidenceoflife.Whetherbiogenic(producedbyanyoralllife-forms),anthropogenic(producedbythewidespreadspeciesHomosapiens), or technogenic (producedonlyby technology), such rampant evidencewillbehardtoconceal.

Unlesstheyhappentobebornwithbuilt-inspectroscopicsensors,ourspace-snoopingalienswouldneedtobuildaspectrometertoreadourfingerprints.Butaboveall,EarthwouldhavetocrossinfrontoftheSun(orsomeothersource),permittinglighttopassthroughouratmosphereandcontinueontothealiens.Thatway, the chemicals in Earth’s atmosphere could interact with the light, leavingtheirmarksforalltosee.

Somemolecules—ammonia, carbon dioxide,water—show up abundantly inthe universe, whether life is present or not. But other molecules thrive in thepresence of life itself. Another readily detected biomarker is Earth’s sustainedlevelofthemoleculemethane,two-thirdsofwhichisproducedbyhuman-relatedactivitiessuchasfueloilproduction,ricecultivation,sewage,andtheburpsandfarts of domestic livestock. Natural sources, comprising the remaining third,includedecomposingvegetation inwetlands and termite effluences.Meanwhile,in placeswhere free oxygen is scarce,methane does not always require life toform.At this verymoment, astrobiologists are arguing over the exact origin oftracemethane onMars and the copious quantities ofmethane onSaturn’smoonTitan,wherecowsandtermiteswepresumedonotdwell.

If thealienstrackournighttimesidewhileweorbitourhoststar, theymightnotice a surge of sodium from thewidespread use of sodium-vapor streetlightsthat switch on at dusk in urban and suburban municipalities. Most telling,however,wouldbeallour free-floatingoxygen,whichconstitutes a full fifthofouratmosphere.

Oxygen—which,afterhydrogenandhelium,isthethirdmostabundantelementin thecosmos—ischemicallyactiveandbonds readilywithatomsofhydrogen,carbon,nitrogen,silicon,sulfur, iron,andsoon.Itevenbondswith itself.Thus,foroxygentoexistinasteadystate,somethingmustbeliberatingitasfastasit’sbeingconsumed.HereonEarth,theliberationistraceabletolife.Photosynthesis,carriedoutbyplantsandmanybacteria,createsfreeoxygenintheoceansandin

theatmosphere.Freeoxygen,inturn,enablestheexistenceofoxygen-metabolizinglife,includingusandpracticallyeveryothercreatureintheanimalkingdom.

We Earthlings already know the significance of our planet’s distinctivechemicalfingerprints.Butdistantalienswhocomeuponuswillhavetointerprettheirfindingsandtesttheirassumptions.Musttheperiodicappearanceofsodiumbe technogenic?Freeoxygen is surelybiogenic.Howaboutmethane? It, too, ischemically unstable, and yes, some of it is anthropogenic, but as we’ve seen,methanehasnonlivingagentsaswell.

If the aliens decide thatEarth’s chemical features are sure evidence of life,maybethey’llwonderifthelifeisintelligent.Presumablythealienscommunicatewith one another, and perhaps they’ll presume that other intelligent life-formscommunicate, too.Maybe that’swhen they’lldecide toeavesdroponEarthwiththeir radio telescopes to see what part of the electromagnetic spectrum itsinhabitantshavemastered.So,whetherthealiensexplorewithchemistryorwithradio waves, they might come to the same conclusion: a planet where there’sadvanced technology must be populated with intelligent life-forms, who mayoccupythemselvesdiscoveringhowtheuniverseworksandhowtoapplyitslawsforpersonalorpublicgain.

LookingmorecloselyatEarth’satmospheric fingerprints,humanbiomarkerswill also include sulfuric, carbonic, and nitric acids, and other components ofsmogfromtheburningoffossilfuels.Ifthecuriousalienshappentobesocially,culturally,andtechnologicallymoreadvancedthanweare, then theywillsurelyinterpret these biomarkers as convincing evidence for the absence of intelligentlifeonEarth.

Thefirstexoplanetwasdiscoveredin1995,and,asofthiswriting,thetallyisrising through three thousand, most found in a small pocket of the MilkyWayaroundthesolarsystem.Sothere’splentymorewheretheycamefrom.Afterall,our galaxy containsmore than a hundred billion stars, and the known universeharborssomehundredbilliongalaxies.

Oursearchfor life in theuniversedrives thesearchforexoplanets,someofwhichresembleEarth—not indetail,ofcourse,but inoverallproperties.Latestestimates, extrapolating from the current catalogs, suggests as many as fortybillion Earth-like planets in the Milky Way alone. Those are the planets ourdescendantsmightwanttovisitsomeday,bychoice,ifnotbynecessity.

†JocelynBell,AnnalsoftheNewYorkAcademyofSciences302(1977):685.

12.

ReflectionsontheCosmicPerspectiveOfallthesciencescultivatedbymankind,Astronomyisacknowledgedtobe,andundoubtedlyis,themostsublime,themostinteresting,andthemostuseful.For,byknowledgederivedfromthisscience,notonlythebulkoftheEarthisdiscovered...;butourveryfacultiesareenlargedwiththegrandeuroftheideasitconveys,ourmindsexaltedabove[their]lowcontractedprejudices.

JAMESFERGUSON,1757†

Longbeforeanyoneknewthattheuniversehadabeginning,beforeweknewthatthenearestlargegalaxyliestwomillionlight-yearsfromEarth,beforeweknewhow stars work or whether atoms exist, James Ferguson’s enthusiasticintroduction to his favorite science rang true. Yet his words, apart from theireighteenth-centuryflourish,couldhavebeenwrittenyesterday.

Butwho gets to think thatway?Who gets to celebrate this cosmic view oflife? Not themigrant farmworker. Not the sweatshopworker. Certainly not thehomelesspersonrummagingthroughthetrashforfood.Youneedtheluxuryoftimenotspentonmeresurvival.Youneedtoliveinanationwhosegovernmentvaluesthesearch tounderstandhumanity’splace in theuniverse.Youneedasociety inwhichintellectualpursuitcantakeyoutothefrontiersofdiscovery,andinwhichnewsofyourdiscoveriescanberoutinelydisseminated.Bythosemeasures,mostcitizensofindustrializednationsdoquitewell.

Yet the cosmic view comeswith a hidden cost.When I travel thousands ofmiles to spenda fewmoments in the fast-moving shadowof theMoonduringatotalsolareclipse,sometimesIlosesightofEarth.

WhenIpauseandreflectonourexpandinguniverse,withitsgalaxieshurtlingaway from one another, embedded within the ever-stretching, four-dimensional

fabricofspaceandtime,sometimesIforgetthatuncountedpeoplewalkthisEarthwithout food or shelter, and that children are disproportionately representedamongthem.

WhenIporeoverthedatathatestablishthemysteriouspresenceofdarkmatteranddarkenergythroughouttheuniverse,sometimesIforgetthateveryday—everytwenty-four-hour rotation of Earth—people kill and get killed in the name ofsomeoneelse’sconceptionofGod,and that somepeoplewhodonotkill in thenameofGod,killinthenameofneedsorwantsofpoliticaldogma.

When I track the orbits of asteroids, comets, and planets, each one apirouetting dancer in a cosmic ballet, choreographed by the forces of gravity,sometimesIforget that toomanypeopleact inwantondisregardforthedelicateinterplay of Earth’s atmosphere, oceans, and land, with consequences that ourchildrenandourchildren’schildrenwillwitnessandpayforwiththeirhealthandwell-being.

And sometimes I forget that powerful people rarely do all they can to helpthosewhocannothelpthemselves.

Ioccasionallyforgetthosethingsbecause,howeverbigtheworldis—inourhearts,ourminds,andouroutsizeddigitalmaps—theuniverseisevenbigger.Adepressingthoughttosome,butaliberatingthoughttome.

Consideranadultwhotendstothetraumasofachild:spilledmilk,abrokentoy,ascrapedknee.Asadultsweknowthatkidshavenoclueofwhatconstitutesa genuine problem, because inexperience greatly limits their childhoodperspective.Childrendonotyetknowthattheworlddoesn’trevolvearoundthem.

As grown-ups, dare we admit to ourselves that we, too, have a collectiveimmaturityofview?Dareweadmitthatourthoughtsandbehaviorsspringfromabeliefthattheworldrevolvesaroundus?Apparentlynot.Yetevidenceabounds.Part the curtains of society’s racial, ethnic, religious, national, and culturalconflicts,andyoufindthehumanegoturningtheknobsandpullingthelevers.

Nowimagineaworld inwhicheveryone,butespeciallypeoplewithpowerand influence, holds an expanded view of our place in the cosmos. With thatperspective, our problems would shrink—or never arise at all—and we couldcelebrateourearthlydifferenceswhileshunningthebehaviorofourpredecessorswhoslaughteredoneanotherbecauseofthem.

Back in January 2000, the newly rebuilt Hayden Planetarium in NewYorkCityfeaturedaspaceshowtitledPassporttotheUniverse,††whichtookvisitorsonavirtualzoomfromtheplanetariumouttotheedgeofthecosmos.Enroute,the

audience viewedEarth, then the solar system, thenwatched the hundred billionstars of the Milky Way galaxy shrink, in turn, to barely visible dots on theplanetarium’sdome.

Within a month of opening day, I received a letter from an Ivy Leagueprofessor of psychology whose expertise was in things that make people feelinsignificant. I never knew one could specialize in such a field. He wanted toadministerabefore-and-afterquestionnairetovisitors,assessingthedepthoftheirdepressionafterviewingtheshow.PassporttotheUniverse,hewrote,elicitedthe most dramatic feelings of smallness and insignificance he had everexperienced.

How could that be? Every time I see the space show (and others we’veproduced),Ifeelaliveandspiritedandconnected.Ialsofeellarge,knowingthatthegoings-onwithinthethree-poundhumanbrainarewhatenabledustofigureoutourplaceintheuniverse.

Allowmetosuggestthatit’stheprofessor,notI,whohasmisreadnature.Hisegowasunjustifiablybigtobeginwith,inflatedbydelusionsofsignificanceandfedbyculturalassumptionsthathumanbeingsaremoreimportantthaneverythingelseintheuniverse.

In all fairness to the fellow, powerful forces in society leave most of ussusceptible.AswasI,untilthedayIlearnedinbiologyclassthatmorebacterialiveandworkinonecentimeterofmycolonthanthenumberofpeoplewhohaveeverexisted in theworld.Thatkindof informationmakesyouthink twiceaboutwho—orwhat—isactuallyincharge.

Fromthatdayon,Ibeganto thinkofpeoplenotas themastersofspaceandtimebutasparticipantsinagreatcosmicchainofbeing,withadirectgeneticlinkacrossspeciesbothlivingandextinct,extendingbacknearlyfourbillionyearstotheearliestsingle-celledorganismsonEarth.

Iknowwhatyou’rethinking:we’resmarterthanbacteria.Nodoubtaboutit,we’resmarterthaneveryotherlivingcreaturethateverran,

crawled, or slithered on Earth. But how smart is that?We cook our food.Wecomposepoetryandmusic.Wedoartandscience.We’regoodatmath.Even ifyou’re bad at math, you’re probably much better at it than the smartestchimpanzee,whosegeneticidentityvariesinonlytriflingwaysfromours.Tryastheymight, primatologists will never get a chimpanzee to do long division, ortrigonometry.

Ifsmallgeneticdifferencesbetweenusandourfellowapesaccountforwhatappears to be a vast difference in intelligence, then maybe that difference inintelligenceisnotsovastafterall.

Imaginealife-formwhosebrainpoweristooursasoursistoachimpanzee’s.

To such a species, our highest mental achievements would be trivial. Theirtoddlers, instead of learning their ABCs on Sesame Street, would learnmultivariablecalculusonBooleanBoulevard.†††Ourmostcomplextheorems,ourdeepestphilosophies,thecherishedworksofourmostcreativeartists,wouldbeprojects their schoolkids bring home for Mom and Dad to display on therefrigerator doorwith amagnet.These creatureswould studyStephenHawking(whooccupiesthesameendowedprofessorshiponceheldbyIsaacNewtonattheUniversity of Cambridge) because he’s slightlymore clever than other humans.Why?Hecandotheoreticalastrophysicsandotherrudimentarycalculationsinhishead,liketheirlittleTimmywhojustcamehomefromalienpreschool.

If a huge genetic gap separated us from our closest relative in the animalkingdom,we could justifiably celebrate our brilliance.Wemight be entitled towalkaroundthinkingwe’redistantanddistinctfromourfellowcreatures.Butnosuchgapexists.Instead,weareonewiththerestofnature,fittingneitherabovenorbelow,butwithin.

Needmoreegosofteners?Simplecomparisonsofquantity,size,andscaledothejobwell.

Takewater.It’scommon,andvital.Therearemoremoleculesofwaterinaneight-ouncecupofthestuffthantherearecupsofwaterinalltheworld’soceans.Everycupthatpassesthroughasinglepersonandeventuallyrejoinstheworld’swatersupplyholdsenoughmoleculestomix1,500ofthemintoeveryothercupofwater in theworld.Nowayaround it: someof thewateryou justdrankpassedthroughthekidneysofSocrates,GenghisKhan,andJoanofArc.

How about air?Also vital.A single breathful draws inmore airmoleculesthantherearebreathfulsofairinEarth’sentireatmosphere.Thatmeanssomeofthe air you just breathed passed through the lungs of Napoleon, Beethoven,Lincoln,andBillytheKid.

Timetogetcosmic.Therearemorestarsintheuniversethangrainsofsandonanybeach,more stars than secondshavepassed sinceEarth formed,more starsthanwordsandsoundseverutteredbyallthehumanswhoeverlived.

Want a sweeping view of the past?Our unfolding cosmic perspective takesyouthere.LighttakestimetoreachEarth’sobservatoriesfromthedepthsofspace,andsoyouseeobjectsandphenomenanotastheyarebutastheyoncewere,backalmost to the beginning of time itself.Within that horizon of reckoning, cosmicevolutionunfoldscontinuously,infullview.

Want to knowwhatwe’remade of?Again, the cosmic perspective offers abiggeranswer thanyoumightexpect.Thechemicalelementsof theuniverseareforged in the fires of high-mass stars that end their lives in titanic explosions,enrichingtheirhostgalaxieswiththechemicalarsenaloflifeasweknowit.The

result? The four most common, chemically active elements in the universe—hydrogen,oxygen, carbon, andnitrogen—are the fourmost commonelementsoflifeonEarth,withcarbonservingasthefoundationofbiochemistry.

Wedonotsimplyliveinthisuniverse.Theuniverseliveswithinus.Thatbeingsaid,wemaynotevenbeof thisEarth.Severalseparate linesof

research,whenconsideredtogether,haveforcedinvestigatorstoreassesswhowethinkweareandwherewethinkwecamefrom.Aswe’vealreadyseen,whenalargeasteroid strikesaplanet, the surroundingareascan recoil from the impactenergy,catapultingrocksintospace.Fromthere,theycantravelto—andlandon—otherplanetarysurfaces.Second,microorganismscanbehardy.Extremophileson Earth can survive wide ranges of temperature, pressure, and radiationencounteredduringspace travel. If the rockyejecta froman impacthails fromaplanetwith life, thenmicroscopic fauna could have stowed away in the rocks’nooksandcrannies.Third,recentevidencesuggeststhatshortlyaftertheformationofoursolarsystem,Marswaswet,andperhapsfertile,evenbeforeEarthwas.

Collectively,thesefindingstellusit’sconceivablethatlifebeganonMarsandlaterseededlifeonEarth,aprocessknownaspanspermia.SoallEarthlingsmight—justmight—bedescendantsofMartians.

Again and again across the centuries, cosmicdiscoveries havedemotedourself-image. Earth was once assumed to be astronomically unique, untilastronomers learned that Earth is just another planet orbiting the Sun. ThenwepresumedtheSunwasunique,untilwelearnedthatthecountlessstarsofthenightskyaresuns themselves.Thenwepresumedourgalaxy, theMilkyWay,was theentireknownuniverse,untilweestablished that thecountless fuzzy things in theskyareothergalaxies,dottingthelandscapeofourknownuniverse.

Today,howeasyitistopresumethatoneuniverseisallthereis.Yetemergingtheoriesofmoderncosmology,aswellasthecontinuallyreaffirmedimprobabilitythat anything is unique, require thatwe remain open to the latest assault on ourpleafordistinctiveness:themultiverse.

Thecosmicperspectiveflowsfromfundamentalknowledge.Butit’smorethanaboutwhatyouknow.It’salsoabouthavingthewisdomandinsighttoapplythatknowledgetoassessingourplaceintheuniverse.Anditsattributesareclear:

Thecosmicperspectivecomesfromthefrontiersofscience,yetitisnotsolelythe

provenanceofthescientist.Itbelongstoeveryone.Thecosmicperspectiveishumble.Thecosmicperspectiveisspiritual—evenredemptive—butnotreligious.Thecosmicperspectiveenablesustograsp,inthesamethought,thelargeandthe

small.Thecosmicperspectiveopensourmindstoextraordinaryideasbutdoesnotleave

them so open that our brains spill out, making us susceptible to believinganythingwe’retold.

Thecosmicperspectiveopensoureyestotheuniverse,notasabenevolentcradledesigned tonurture lifebut as a cold, lonely,hazardousplace, forcingus toreassessthevalueofallhumanstooneanother.

ThecosmicperspectiveshowsEarthtobeamote.Butit’sapreciousmoteand,forthemoment,it’stheonlyhomewehave.

Thecosmicperspective findsbeauty in the imagesofplanets,moons, stars,andnebulae,butalsocelebratesthelawsofphysicsthatshapethem.

Thecosmicperspectiveenablesustoseebeyondourcircumstances,allowingustotranscendtheprimalsearchforfood,shelter,andamate.

Thecosmicperspectiveremindsusthatinspace,wherethereisnoair,aflagwillnotwave—an indication that perhaps flag-waving and space explorationdonotmix.

The cosmic perspective not only embraces our genetic kinshipwith all life onEarthbutalsovaluesourchemicalkinshipwithanyyet-to-bediscoveredlifeintheuniverse,aswellasouratomickinshipwiththeuniverseitself.

At leastonceaweek, ifnotonceaday,wemighteachponderwhatcosmictruthslieundiscoveredbeforeus,perhapsawaitingthearrivalofacleverthinker,aningeniousexperiment,oraninnovativespacemissiontorevealthem.WemightfurtherponderhowthosediscoveriesmayonedaytransformlifeonEarth.

Absent such curiosity, we are no different from the provincial farmer whoexpressesnoneedtoventurebeyondthecountyline,becausehisfortyacresmeetall his needs. Yet if all our predecessors had felt that way, the farmer wouldinsteadbeacavedweller,chasingdownhisdinnerwithastickandarock.

DuringourbriefstayonplanetEarth,weoweourselvesandourdescendantstheopportunitytoexplore—inpartbecauseit’sfuntodo.Butthere’safarnoblerreason. The day our knowledge of the cosmos ceases to expand, we riskregressingtothechildishviewthattheuniversefigurativelyandliterallyrevolvesaroundus.Inthatbleakworld,arms-bearing,resource-hungrypeopleandnationswouldbepronetoactontheir“lowcontractedprejudices.”Andthatwouldbethelast gaspof human enlightenment—until the rise of a visionarynewculture that

couldonceagainembrace,ratherthanfear,thecosmicperspective.

†JamesFerguson,AstronomyExplainedUponSirIsaacNewton’sPrinciples,AndMadeEasyToThoseWhoHaveNotStudiedMathematics(London,1757).††PassporttotheUniversewaswrittenbyAnnDruyanandStevenSoter,whoarealsotheco-authorsofthe2014FoxminiseriesCosmos:ASpaceTimeOdyssey,hostedbythisauthor.TheyalsoteamedupwithCarlSaganontheoriginal1980PBSminiseriesCosmos:APersonalVoyage.†††Booleanalgebraisabranchofmathematicsthataddressesvaluesoftrueorfalseinitsvariables,commonlyrepresentedby0and1,andisfoundationaltotheworldofcomputing.Namedfortheeighteenth-centuryEnglishmathematicianGeorgeBoole.

ACKNOWLEDGMENTS

My tireless literary editors over the years these essays were written includedEllenGoldensohn andAvis Lang atNaturalHistory magazine—both of whomensuredthat,atalltimes,IsaidwhatImeantandmeantwhatIsaid.MyscientificeditorwasfriendandPrincetoncolleagueRobertLupton,whoknewmorethanIdid, in all places where it mattered most. I also thank Betsy Lerner, forsuggestionstothemanuscriptthatgreatlyimproveditsarcofcontent.

INDEX

Pagenumberslistedcorrespondtotheprinteditionofthisbook.Youcanuseyourdevice’ssearchfunctiontolocateparticulartermsinthetext.

aerobicorganisms,30aether,88air,202AlphaCentauri,182Alpher,Ralph,52–53,54aluminum,122ammonia,189anaerobicbacteria,30AndesMountains,159Andromedagalaxy,53anthropogenicmarkers,188anti-electrons,24,27antimatter,23,26antineutrinos,24antiquarks,24,25Apolloprogram,168asteroidbelt,129,168,169asteroids,29,35,82,127,128–29,131,168–69,176–77,195,203astronomy,193AstrophysicalJournal,148nAtacamaLargeMillimeterArray(ALMA),159–60AT&T,53AtlanticOcean,180atomicbombs,52,130,132atomicnuclei,27atomicphysics,52–53atoms,194formationof,49

aurora,174

bacteria,199ballbearings,135–36Beethoven,Ludwigvon,39

Bell,Jocelyn,186–87BellTelephoneLaboratories,53,54,55,156,157Berkeleycyclotron,130,131Berry,Chuck,39bigbang,17–18,33,48,52,57,83,108–9,117,120bigG,40binarystars:Rochelobeof,138–39spectraof,36–37

biochemistry,203biogenicmarkers,188,190biomarkers,188–89blackholes,37,44,82,97,139,164Boole,George,200nBooleanalgebra,200nBose,SatyendraNath,22bosons,21–22

calcium,36CaliforniaInstituteofTechnology,76–77Cambridge,Universityof,186,201carbon,31,36,121,123,124,203carbondioxide,189carbonicacid,191CarnegieInstitution,80CaseWesternReserveUniversity,88Cassiniprobe,176causalitylaws,46Cavendish,Henry,117–18centrifugalforce,141–42Ceres,128–29,132cerium,128Charon(moon),172–73children,195,196Chile,159chimpanzees,200chlorine,116–17Christianity,34climate,42ColdWar,161–62ComaBerenices,77Comacluster,77–79,144comets,29,35,82,131–32,169–70,195long-period,170

ComptonGammaRayObservatory,162conservationlaws,45constants,physical,40–42seealsospecificconstantsContact(film),159controls,148–49Copernicus,Nicolaus,98cosmicbackground,49cosmicmicrowavebackground(CMB),50–61,84,156,163

cyanogenand,56–57firstdirectobservationof,53–56peaksin,50–51temperatureof,52–53temperaturevariationsin,58–59

cosmicrays,73–74cosmochemistry,187–88cosmologicalconstant,99–100,102–3,104,106,111,112–13cosmology,51–61,205Cosmos:ASpaceTimeOdyssey,197nCretaceous–Paleogene(K-Pg)boundary,126–27criticaldensity,107,109cyanogen,56–57

darkclouds,82darkenergy,59,94–114,195measurementof,106–7

darkmatter,43–44,59–60,64,68–69,72,75–93,107,108,109,144,195as“aether,”88detectionof,91–92discrepancybetweenordinarymatterand,84–85effectsof,86,88–89nuclearfusionand,83–84andrelativeamountofhydrogenandhelium,83–84totalgravityof,82–83

darkmatterhaloes,81–82DeepSpaceNetwork,158Deimos,138deuterium,27Dicke,Robert,55dinosaurs,31,127Druyan,Ann,197ndwarfgalaxies,64–67,70

E=mc2,20–21,23,25Earth,34,35–36,89,97,128,157,178–92,203–5asteroidstrikeson,31atmosphereof,175auroraon,174formationof,29–31meteorstrikeson,165–68orbitalspeedof,78polaricecapsof,182radioandmicrowavegenerationon,185shapeof,140–41assphere,136–37surfacegravityof,85tidallockingofMoonby,171–72viewfromspaceof,178–81

Einstein,Albert,18,20–21,52,72,75–76,78–79,80,94–95,96,98–101,103,145adjustmenttolawofgravityby,44,76thoughtexperimentsof,94–95

einsteinium,127“ElectricalDisturbancesApparentlyofExtraterrestrialOrigin”(Jansky),157electriccharge,conservationof,43electromagneticspectrum,147–64electromagnetism,18,19–20,92,93electrons,21,22,24,27–28,38,48–49,57–58,142electroweakforce,19–20,21elements,115–33,203heavy,37asrevealedinspectra,35seealsospecificelements

energy,18,20–21,48,75,109,136quantized,19vacuum,64,74seealsodarkenergy

energy,conservationof,43Eniwetokatoll,127escapevelocity,78Europa(moon),172EuropeanCenterforNuclearResearch(CERN),25nEverest,Mount,137exoplanets,89,182–84,187firstdiscovered,192ExplorerXIsatellite,161extinctions,127,169extremophiles,204

faintbluegalaxies,64,69–70Faraday,Michael,170Ferguson,James,193,194fine-structureconstant,37FinnegansWake(Joyce),22Five-hundred-meterApertureSphericalradioTelescope(FAST),158flattenedspheres,139–41frequency,154,156nFriedmann,Alexander,100

galaxies,86,194–95ancient,69collisionsof,66–67darkmatterand,84–85darkmatterhaloesand,81–82formationof,28gas-rich,68–69lensingof,72namederivationof,63numberof,62numberofstarsin,65–66orbitalspeedin,80–81recessionvelocityof,105,145

“GalaxyandtheSevenDwarfs,The”(Tyson),64galaxyclusters,67,77,79formationof,58–59

gravityof,80intra-clustergasand,68–69“relaxed,”144shapeof,145–46

Galle,John,130gallium,124–25galliumchlorideexperiments,125gammarayastrophysics,162gammarays,151,154,161–63,164Gamow,George,51,52,54Gell-Mann,Murray,22germanium,125Germany,95God,32,34,195Goldilockszone,29–30Gott,J.Richard,53gravitation,constantof,40gravitationalfields,176gravitationallens,72–73gravitationalwaves,96–97gravity,18,19,20,34–35,43,58,59,72,86“action-at-a-distance,”75,101darkenergyand,60,94,96,100,106darkmatterand,75,82–83,84–85,89,92–93“excess,”76ofgalaxyclusters,80Newton’slawof,34–35,37,40,44,76,175–76andspheres,136–38,144surface,142

GreatNebulainAndromedagalaxy,53GreatPyramidsatGiza,179GreatRedSpot,42–43GreatWallofChina,179GulfofMexico,180Guth,AlanH.,108–9

hadrons,24–26,27Hale-Bopp(comet),170Halley’scomet,170Hawking,Stephen,201HaydenPlanetarium,197–98Helios(god),119helium,27,36,83,118,124Herman,Robert,52,54Herschel,William,130,147–48,149,150,164,173–74hertz,152Hertz,Heinrich,151Hewish,Antony,186high-energychargedparticles,64,174Himalayas,136Hiroshima,130,132homelessness,195Homosapiens,31,97

Hubble,EdwinP.,52,102,105,145HubbleSpaceTelescope,129,174humannature,45,197–201hunger,195hurricanes,180Hyakutake(comet),170hydrogen,27,36,74,83,117–18,203hydrogenbombtests,127hydrogenclouds,71

icefloes,180infraredlight,150,151,185infraredtelescopes,163intelligence,199–201interferometer,158–60intergalaticspace,62–74InternationalSpaceStation,178–79interplanetaryspace,165–77Io(moon),172iridium,126–27iron,36,124,166

Jansky,KarlG.,155–56,157JodrellBankObservatory,158JohnsHopkinsUniversity,103Joyce,James,22Jupiter(god),129Jupiter(planet),42–43,117,128,129,168,174,176gravitationalfieldof,176magneticfieldof,170moonsof,172asradiowavesource,185

Keplertelescope,183–84KittPeakNationalObservatory,122Kuiper,Gerard,169Kuiperbelt,131–32,169–70Kuwait,179

lambda,99–100,102–3,104,106,111,112–13LargeHadronCollider,25LaserInterferometerGravitational-WaveObservatory(LIGO),96nLawrenceBerkeleyNationalLaboratory,105LEDbulbs,51,121Leeuwenhoek,Antonievan,150Lemaître,Georges,52leptons,21,22,23–25LeVerrier,Joseph,130Levy,David,177light,speedof,21,40–41,145light,visible,88,147–48,151,152temperatureof,148seealsoelectromagneticspectrum

lightbulbs,50–51lightning,162–63lightpollution,121light-year,26nLimitedTestBanTreaty,161–62lithium,27,119–20LowellObservatory,131Lucretius,17

Macy’sThanksgivingDayParade,119Magellan,Ferdinand,63MagellanicClouds,63,66magneticfields,170–71,174mammals,31ManhattanProject,52Mars,128,137,167–68,180,204methaneon,189

mass,Conservationof,43MassachusettsInstituteofTechnology,108Mather,JohnC.,56matter,18,20–21,23,48,59–60,75–76,85,107,109accumulationof,58–59,86ultimatesourceof,26seealsodarkmatter

matter-antimatterpairs,74matter-antimatterparticlepairs,20–21McKinley,Mount,137mercury(element),129Mercury(planet),129,167meteors,165–68methane,189,190Michelson,Albert,88microwaveovens,160microwaves,50,51,52,53–57,151,156,159,163,184–85military,U.S.,119MilkyWay,29,55,63,78,89,114,205dwarfgalaxyconsumedby,66exoplanetsin,192flatnessof,139–40,144radiowavesemittedby,157supernovasin,153–54

“missingmass”problem,76,80MK1(radiotelescope),158momentum,conservationof,43,46Moon,35,128,168,170,171–72formationof,166–67viewofEarthfrom,180

moons,85,171–74Morley,Edward,88mountains,136–37multiverse,32,89,145–46

Nagasaki,132

nanosecond,49nNASA,161,162,185nebulae,spectralanalysisof,38nebulium,38Neptune,129,130,165,169,180neptunium,130,131neutrinos,21,24,91–93,125neutrons,22,24,27,142Newton,Isaac,34–35,37,40,43–44,75,76,78–79,80,100,148,175,201nitricacid,191nitrogen,36,124,203nucleartesting,161

oblatespheroids,141OlympusMons,137omega,107–813123–Tyson(asteroid),177OneHundredAuthorsAgainstEinstein,99Oort,Jan,170Oortcloud,170organicmolecules,30OrionArm,29osmium,126oxygen,30,36,117,121,123,124,190innebulae,38

ozone,30

PacificOcean,181palladium,128Pallas,128–29,132panspermia,204Pasadena,Calif.,46PassporttotheUniverse,197–98Penzias,Arno,53–56,156PeriodicTableofElements,36,38,115–17,121,124,127,130,132Perlmutter,Saul,103,104perpetualmotionmachines,45–46PersianGulfWar(1991),179Phobos,138phosphorus,127photons,20,21,25–26,27–28,48–50,57–58infrared,50–51visiblelight,50

photosynthesis,190physics,laboratory-basedvs.theoretical,95Piazzi,Giuseppe,128picosecond,49nPioneer,39Planck,Max,18–19Planckera,18–19planets,29,35,85,195rogue,82

plasma,37

platinum,126Plutinos,169Pluto,129,131–32,169,172plutonium,131–32pointoflastscatter,57–58Pope,Alexander,173positrons,24,27primates,31,97primemover,32PrincetonUniversity,55prisms,35,147–48prolatespheroids,141protons,22,24,27,49,74,142pulsars,141–43,153,186–87

quantizedenergy,19quantumgravity,18quantummechanics,18–19,112quantumphysics,87quark-antiquarkpairs,25quarks,21–22,23–25,27attractiveforcesbetween,22–23chargeof,22nameoriginof,22subspeciesof,21–22

quasars,70–71lensingof,72

radioastronomy,157radioisotopethermoelectricgenerators(RTGs),132radiotelescopes,155–60,163,185,186radiowaves,51,151,153,155,156–57,163,184–85,186–87Reber,Grote,157–58recession,105redgiants,139redstars:spectraof,123technetiumin,125–26

relativity,generaltheoryof,18,19,44,52,72,96,98–102,104,111resolution,155Riess,Adam,103Roche,Édouard,138Rochelobe,138–39rogueplanets,82rotationrates,141–43rubies,123Rubin,Vera,80–81runawaystars,64

Sagan,Carl,181,197nSagittariusDwarf,66salt,117,121satellites,175

SaturdayNightLive,39–40Saturn,123,129,141,171,174Schmidt,Brian,103,104scientificmodels,98–99scintillator,161selenium,127–28September11,2001terroristattacks,179Shakespeare,William,173Shoemaker,Carolyn,177Shoemaker-Levy9(comet),42silicon,121silverchloride,150single-celledorganisms,150Smoot,GeorgeF.,56soapbubbles,135Socorro,N.Mex.,159sodium,116,121–22,189sodiumchloride,116–17solarsystem,165–77solarwind,174Soter,Stephen,197nsoundwaves,88SovietUnion,158,161–62space-time,75spectra,35,37–38spectrometer,148n,187–88spheres,134–46spiralgalaxies,orbitalspeedin,80–81Sputnik1,158standardcandles,104–5,106stars,54–55,138,194,205elementformationin,28–29,115–16,117–20,124,203extra-galactic,67–68ironmadein,124luminosityof,40numberof,202periodicjiggleof,183Rochelobeof,138–39runaway,64spectraof,36–37spectraofred,123

starsapphires,123StarTrek:TheNextGeneration,154streetlamps,121–22,189strongnuclearforces,19,20–21,92subatomicparticles,20,73–74sulfuricacid,191Sun,29–30,35–36,40,85,89,171–72,180,204–5chemicalsignaturesof,35–36energyoutputof,87heliumproducedin,118–19masslostpersecondby,174

neutrinosfrom,91supernovas,67–68,103–6,124,153–54surfaceoflastscatter,57–58,60surfacetension,135,138

technetium,125–26technogenicmarkers,188,190telescopes,64,151–64radio,155–60resolutionof,155

tellurium,128terrestrialgamma-rayflashes,162–63thermodynamics,lawsof,46thermonuclearfusion,87Thor(god),129thorium,129thoughtexperiments,94–95timemachines,46Titan(moon),123,189titanium,122–23titaniumoxide,123Titans(gods),123Tombaugh,Clyde,131Transformers(film),159tritium,27Tucson,Ariz.,1222010:TheYearWeMakeContact(film),159

ultraviolet(UV)light,150–51,164UnitedKingdom,161–62UnitedStates,161–62universalityofphysicallaws,34–47universe,205asymmetryof,23–25,194–95earlyexpansionandcoolingof,17–33expansionof,48–49,52,59,86,94,102–3,105,107–8,113,145,195intergalacticvoidof,62–74matteraccumulationin,58–59,86matter-energydensityof,107–10,113opacityof,48–49,57Planckeraof,18–19quark-leptoneraof,23–25quark-hadrontransitionin,24–25assphere,145astimemachine,202–3

uranium,130Uranus,129,130,173–74

vacuumenergy,64,74Velas(satellites),162Venus,127,176VeryLargeArray,159VeryLongBaselineArray,159

VirgoSupercluster,29virtualparticles,74,111volcaniceruptions,180Voyager,39–40,180–81

water,117,160,181–82,189,201–2watervapor,160wavelength,154,156nweaknuclearforces,20,92weather,42Wheeler,JohnArchibald,101Wilson,Robert,43–56,156WorldTradeCenter,179WorldWarII,52,132

X-rays,125,151,153,154–55,164

YucatanPeninsula,Mexico,31

Zwicky,Fritz,76–79

ALSOBYNEILDEGRASSETYSON

StarTalk:TheBook

WelcometotheUniverse:AnAstrophysicalTour(withMichaelA.StraussandJ.RichardGottIII)

Cosmos:ASpaceTimeOdysseyHOST,NARRATOR,ANDEXECUTIVESCIENCEEDITOR:

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TheInexplicableUniverseA6-PARTVIDEOLECTURESERIES

SpaceChronicles:FacingtheUltimateFrontier

ThePlutoFiles:TheRiseandFallofAmerica’sFavoritePlanet

DeathByBlackHole:AndOtherCosmicQuandaries

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OneUniverse:AtHomeintheCosmos(withCharlesLiuandRobertIrion)

JustVisitingThisPlanet

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Merlin’sTouroftheUniverse

ABOUTTHEAUTHOR

NeildeGrasseTysonisanastrophysicistwiththeAmericanMuseumofNaturalHistoryinNewYorkCity,wherehealsoservesasDirectoroftheHaydenPlanetarium.AgraduateoftheprestigiousBronxHighSchoolofScience,hehasaBAinphysicsfromHarvardandaPhDinastrophysicsfromColumbia.HelivesinManhattanwithhisfamily.

Chaptersadaptedfrom“Universe”essaysinNaturalHistorymagazine—Chapter1:March1998andSeptember2003;Chapter2:November2000;Chapter3:October2003;Chapter 4: June1999;Chapter5:June2006;Chapter6:October2002;Chapter7:July/August2002;Chapter8:March1997;Chapter9:December2003/January2004;Chapter10:October2001;Chapter11:February2006;Chapter12:April2007.

Copyright©2017byNeildeGrasseTyson

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