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AccordingtotheBigBangmodel,theuniverseexpandedfromanextremelydenseandhotstateandcontinuestoexpandtoday.

BigBangFromWikipedia,thefreeencyclopedia

TheBigBangtheoryistheprevailingcosmologicalmodelfortheuniversefromtheearliestknownperiodsthroughitssubsequentlargescaleevolution.[1][2][3]Itstatesthattheuniversewasinaveryhighdensitystateandthenexpanded.[4][5]Iftheknownlawsofphysicsareextrapolatedbeyondwheretheyarevalidthereisasingularity.Modernmeasurementsplacethismomentatapproximately13.8billionyearsago,whichisthusconsideredtheageoftheuniverse.[6]Aftertheinitialexpansion,theuniversecooledsufficientlytoallowtheformationofsubatomicparticles,andlatersimpleatoms.Giantcloudsoftheseprimordialelementslatercoalescedthroughgravitytoformstarsandgalaxies.

Inthemid20thcentury,threeBritishastrophysicists,StephenHawking,GeorgeEllis,andRogerPenroseturnedtheirattentiontotheTheoryofRelativityanditsimplicationsregardingournotionsoftime.In1968and1970,theypublishedpapersinwhichtheyextendedEinstein'sTheoryofGeneralRelativitytoincludemeasurementsoftimeandspace.[7][8]Accordingtotheircalculations,timeandspacehadafinitebeginningthatcorrespondedtotheoriginofmatterandenergy.

SinceGeorgesLematrefirstnoted,in1927,thatanexpandinguniversemightbetracedbackintimetoanoriginatingsinglepoint,scientistshavebuiltonhisideaofcosmicexpansion.Whilethescientificcommunitywasoncedividedbetweensupportersoftwodifferentexpandinguniversetheories,theBigBangandtheSteadyStatetheory,accumulatedempiricalevidenceprovidesstrongsupportfortheformer.[9]In1929,fromanalysisofgalacticredshifts,EdwinHubbleconcludedthatgalaxiesaredriftingapart,importantobservationalevidenceconsistentwiththehypothesisofanexpandinguniverse.In1964,thecosmicmicrowavebackgroundradiationwasdiscovered,whichwascrucialevidenceinfavoroftheBigBangmodel,sincethattheorypredictedtheexistenceofbackgroundradiationthroughouttheuniversebeforeitwasdiscovered.Morerecently,measurementsoftheredshiftsofsupernovaeindicatethattheexpansionoftheuniverseisaccelerating,anobservationattributedtodarkenergy.[10]Theknownphysicallawsofnaturecanbeusedtocalculatethecharacteristicsoftheuniverseindetailbackintimetoaninitialstateofextremedensityandtemperature.[11][12][13]

Contents

1Overview2TimelineoftheBigBang

2.1Singularity2.2Inflationandbaryogenesis

2.3Cooling



2.3Cooling2.4Structureformation2.5Cosmicacceleration

3Underlyingassumptions3.1Expansionofspace3.2Horizons

4History4.1Etymology4.2Development

5Observationalevidence5.1Hubble'slawandtheexpansionofspace5.2Cosmicmicrowavebackgroundradiation5.3Abundanceofprimordialelements5.4Galacticevolutionanddistribution5.5Primordialgasclouds5.6Otherlinesofevidence

6Relatedissuesinphysics6.1Baryonasymmetry6.2Darkenergy6.3Darkmatter6.4Globularclusterage

7Problems7.1Horizonproblem7.2Flatnessproblem7.3Magneticmonopoles

8ThefutureaccordingtotheBigBangtheory9Before:SpeculativephysicsbeyondtheBigBangtheory10Religiousandphilosophicalinterpretations11Seealso12Notes13References

13.1Books14Furtherreading15Externallinks

Overview

Hubbleobservedthatthedistancestofarawaygalaxieswerestronglycorrelatedwiththeirredshifts.Thiswasinterpretedtomeanthatalldistantgalaxiesandclustersarerecedingawayfromourvantagepointwithanapparentvelocityproportionaltotheirdistance:thatis,thefarthertheyare,thefastertheymoveawayfromus,regardlessofdirection.[18]AssumingtheCopernicanprinciple(thattheEarthisnotthecenteroftheuniverse),theonlyremaininginterpretationisthatallobservableregionsoftheuniversearerecedingfromallothers.Sinceweknowthatthedistancebetweengalaxiesincreasestoday,itmustmeanthatinthepastgalaxieswereclosertogether.Thecontinuousexpansionoftheuniverseimpliesthattheuniversewasdenserandhotterinthepast.

Largeparticleacceleratorscanreplicatetheconditionsthatprevailedaftertheearlymomentsoftheuniverse,resultinginconfirmationandrefinementofthedetailsoftheBigBangmodel.However,theseacceleratorscanonlyprobesofarintohighenergyregimes.Consequently,thestateoftheuniverseintheearliestinstantsoftheBigBangexpansionisstillpoorlyunderstoodandanareaofopeninvestigationandindeed,speculation.



HistoryoftheUniversegravitationalwavesarehypothesizedtoarisefromcosmicinflation,anexpansionjustaftertheBigBang.[14][15][16][17]

AgraphicaltimelineisavailableatGraphicaltimelineoftheBigBang

Thefirstsubatomicparticlesincludedprotons,neutrons,andelectrons.ThoughsimpleatomicnucleiformedwithinthefirstthreeminutesaftertheBigBang,thousandsofyearspassedbeforethefirstelectricallyneutralatomsformed.ThemajorityofatomsproducedbytheBigBangwerehydrogen,alongwithheliumandtracesoflithium.Giantcloudsoftheseprimordialelementslatercoalescedthroughgravitytoformstarsandgalaxies,andtheheavierelementsweresynthesizedeitherwithinstarsorduringsupernovae.

TheBigBangtheoryoffersacomprehensiveexplanationforabroadrangeofobservedphenomena,includingtheabundanceoflightelements,thecosmicmicrowavebackground,largescalestructure,andHubble'sLaw.[19]TheframeworkfortheBigBangmodelreliesonAlbertEinstein'stheoryofgeneralrelativityandonsimplifyingassumptionssuchashomogeneityandisotropyofspace.ThegoverningequationswereformulatedbyAlexanderFriedmann,andsimilarsolutionswereworkedonbyWillemdeSitter.Sincethen,astrophysicistshaveincorporatedobservationalandtheoreticaladditionsintotheBigBangmodel,anditsparametrizationastheLambdaCDMmodelservesastheframeworkforcurrentinvestigationsoftheoreticalcosmology.TheLambdaCDMmodelisthestandardmodelofBigBangcosmology,thesimplestmodelthatprovidesareasonablygoodaccountofvariousobservationsabouttheuniverse.

TimelineoftheBigBang

Singularity

Extrapolationoftheexpansionoftheuniversebackwardsintimeusinggeneralrelativityyieldsaninfinitedensityandtemperatureatafinitetimeinthepast.[20]Thissingularitysignalsthebreakdownofgeneralrelativityandthus,allthelawsofphysics.HowcloselywecanextrapolatetowardsthesingularityisdebatedcertainlynocloserthantheendofthePlanckepoch.Thissingularityissometimescalled"theBigBang",[21]butthetermcanalsorefertotheearlyhot,densephaseitself,[22][notes1]whichcanbeconsideredthe"birth"ofouruniverse.BasedonmeasurementsoftheexpansionusingTypeIasupernovae,measurementsoftemperaturefluctuationsinthecosmicmicrowavebackground,andmeasurementsofthecorrelationfunctionofgalaxies,theuniversehasanestimatedageof13.7980.037billionyears.[23]TheagreementofthesethreeindependentmeasurementsstronglysupportstheCDMmodelthatdescribesindetailthecontentsoftheuniverse.

Inflationandbaryogenesis

TheearliestphasesoftheBigBangaresubjecttomuchspeculation.Inthemostcommonmodelstheuniversewasfilledhomogeneouslyandisotropicallywithanincrediblyhighenergydensityandhugetemperaturesandpressuresandwasveryrapidlyexpandingandcooling.Approximately1037secondsintotheexpansion,aphasetransitioncausedacosmicinflation,duringwhichtheuniversegrewexponentially.[24]Afterinflationstopped,theuniverseconsistedofaquarkgluonplasma,aswellasall



PanoramicviewoftheentirenearinfraredskyrevealsthedistributionofgalaxiesbeyondtheMilkyWay.Galaxiesarecolorcodedbyredshift.

otherelementaryparticles.[25]Temperaturesweresohighthattherandommotionsofparticleswereatrelativisticspeeds,andparticleantiparticlepairsofallkindswerebeingcontinuouslycreatedanddestroyedincollisions.[4]Atsomepointanunknownreactioncalledbaryogenesisviolatedtheconservationofbaryonnumber,leadingtoaverysmallexcessofquarksandleptonsoverantiquarksandantileptonsoftheorderofonepartin30million.Thisresultedinthepredominanceofmatteroverantimatterinthepresentuniverse.[26]

Cooling

Theuniversecontinuedtodecreaseindensityandfallintemperature,hencethetypicalenergyofeachparticlewasdecreasing.Symmetrybreakingphasetransitionsputthefundamentalforcesofphysicsandtheparametersofelementaryparticlesintotheirpresentform.[27]Afterabout1011seconds,thepicturebecomeslessspeculative,sinceparticleenergiesdroptovaluesthatcanbeattainedinparticlephysicsexperiments.Atabout106seconds,quarksandgluonscombinedtoformbaryonssuchasprotonsandneutrons.Thesmallexcessofquarksoverantiquarksledtoa

smallexcessofbaryonsoverantibaryons.Thetemperaturewasnownolongerhighenoughtocreatenewprotonantiprotonpairs(similarlyforneutronsantineutrons),soamassannihilationimmediatelyfollowed,leavingjustonein1010oftheoriginalprotonsandneutrons,andnoneoftheirantiparticles.Asimilarprocesshappenedatabout1secondforelectronsandpositrons.Aftertheseannihilations,theremainingprotons,neutronsandelectronswerenolongermovingrelativisticallyandtheenergydensityoftheuniversewasdominatedbyphotons(withaminorcontributionfromneutrinos).

Afewminutesintotheexpansion,whenthetemperaturewasaboutabillion(onethousandmillion109SIprefixgiga)kelvinandthedensitywasaboutthatofair,neutronscombinedwithprotonstoformtheuniverse'sdeuteriumandheliumnucleiinaprocesscalledBigBangnucleosynthesis.[28]Mostprotonsremaineduncombinedashydrogennuclei.Astheuniversecooled,therestmassenergydensityofmattercametogravitationallydominatethatofthephotonradiation.Afterabout379,000yearstheelectronsandnucleicombinedintoatoms(mostlyhydrogen)hencetheradiationdecoupledfrommatterandcontinuedthroughspacelargelyunimpeded.Thisrelicradiationisknownasthecosmicmicrowavebackgroundradiation.[29]ThechemistryoflifemayhavebegunshortlyaftertheBigBang,13.8billionyearsago,duringahabitableepochwhentheuniversewasonly1017millionyearsold.[30][31][32]

Structureformation

Overalongperiodoftime,theslightlydenserregionsofthenearlyuniformlydistributedmattergravitationallyattractednearbymatterandthusgrewevendenser,forminggasclouds,stars,galaxies,andtheotherastronomicalstructuresobservabletoday.[4]Thedetailsofthisprocessdependontheamountandtypeofmatterintheuniverse.Thefourpossibletypesofmatterareknownascolddarkmatter,warmdarkmatter,hotdarkmatter,andbaryonicmatter.Thebestmeasurementsavailable(fromWMAP)showthatthedataiswellfitbyaLambdaCDMmodelinwhichdarkmatterisassumedtobe



Abell2744galaxyclusterHubbleFrontierFieldsview.[35]

cold(warmdarkmatterisruledoutbyearlyreionization[33]),andisestimatedtomakeupabout23%ofthematter/energyoftheuniverse,whilebaryonicmattermakesupabout4.6%.[34]Inan"extendedmodel"whichincludeshotdarkmatterintheformofneutrinos,thenifthe"physicalbaryondensity"bh2isestimatedatabout0.023(thisisdifferentfromthe'baryondensity'bexpressedasafractionofthetotalmatter/energydensity,whichasnotedaboveisabout0.046),andthecorrespondingcolddarkmatterdensitych2isabout0.11,thecorrespondingneutrinodensityvh2isestimatedtobelessthan

0.0062.[34]

Cosmicacceleration

IndependentlinesofevidencefromTypeIasupernovaeandtheCMBimplythattheuniversetodayisdominatedbyamysteriousformofenergyknownasdarkenergy,whichapparentlypermeatesallofspace.Theobservationssuggest73%ofthetotalenergydensityoftoday'suniverseisinthisform.Whentheuniversewasveryyoung,itwaslikelyinfusedwithdarkenergy,butwithlessspaceandeverythingclosertogether,gravitypredominated,anditwasslowlybrakingtheexpansion.Buteventually,afternumerousbillionyearsofexpansion,thegrowingabundanceofdarkenergycausedtheexpansionoftheuniversetoslowlybegintoaccelerate.DarkenergyinitssimplestformulationtakestheformofthecosmologicalconstantterminEinstein'sfieldequationsofgeneralrelativity,butitscompositionandmechanismareunknownand,moregenerally,thedetailsofitsequationofstateandrelationshipwiththeStandardModelofparticlephysicscontinuetobeinvestigatedbothobservationallyandtheoretically.[10]

AllofthiscosmicevolutionaftertheinflationaryepochcanberigorouslydescribedandmodelledbytheCDMmodelofcosmology,whichusestheindependentframeworksofquantummechanicsandEinstein'sGeneralRelativity.Thereisnowellsupportedmodeldescribingtheactionpriorto1015secondsorso.Apparentlyanewunifiedtheoryofquantumgravitationisneededtobreakthisbarrier.Understandingthisearliestoferasinthehistoryoftheuniverseiscurrentlyoneofthegreatestunsolvedproblemsinphysics.

Underlyingassumptions

TheBigBangtheorydependsontwomajorassumptions:theuniversalityofphysicallawsandthecosmologicalprinciple.Thecosmologicalprinciplestatesthatonlargescalestheuniverseishomogeneousandisotropic.

Theseideaswereinitiallytakenaspostulates,buttodaythereareeffortstotesteachofthem.Forexample,thefirstassumptionhasbeentestedbyobservationsshowingthatlargestpossibledeviationofthefinestructureconstantovermuchoftheageoftheuniverseisoforder105.[36]Also,generalrelativityhaspassedstringenttestsonthescaleoftheSolarSystemandbinarystars.[notes2]

IfthelargescaleuniverseappearsisotropicasviewedfromEarth,thecosmologicalprinciplecanbederivedfromthesimplerCopernicanprinciple,whichstatesthatthereisnopreferred(orspecial)observerorvantagepoint.Tothisend,thecosmologicalprinciplehasbeenconfirmedtoalevelof105viaobservationsoftheCMB.Theuniversehasbeenmeasuredtobehomogeneousonthelargestscalesatthe10%level.[37]



Expansionofspace

Generalrelativitydescribesspacetimebyametric,whichdeterminesthedistancesthatseparatenearbypoints.Thepoints,whichcanbegalaxies,stars,orotherobjects,themselvesarespecifiedusingacoordinatechartor"grid"thatislaiddownoverallspacetime.Thecosmologicalprincipleimpliesthatthemetricshouldbehomogeneousandisotropiconlargescales,whichuniquelysinglesouttheFriedmannLematreRobertsonWalkermetric(FLRWmetric).Thismetriccontainsascalefactor,whichdescribeshowthesizeoftheuniversechangeswithtime.Thisenablesaconvenientchoiceofacoordinatesystemtobemade,calledcomovingcoordinates.Inthiscoordinatesystemthegridexpandsalongwiththeuniverse,andobjectsthataremovingonlyduetotheexpansionoftheuniverseremainatfixedpointsonthegrid.Whiletheircoordinatedistance(comovingdistance)remainsconstant,thephysicaldistancebetweentwosuchcomovingpointsexpandsproportionallywiththescalefactoroftheuniverse.[38]

TheBigBangisnotanexplosionofmattermovingoutwardtofillanemptyuniverse.Instead,spaceitselfexpandswithtimeeverywhereandincreasesthephysicaldistancebetweentwocomovingpoints.Inotherwords,theBigBangisnotanexplosioninspace,butratheranexpansionofspace.[4]BecausetheFLRWmetricassumesauniformdistributionofmassandenergy,itappliestoouruniverseonlyonlargescaleslocalconcentrationsofmattersuchasourgalaxyaregravitationallyboundandassuchdonotexperiencethelargescaleexpansionofspace.[39]

Horizons

AnimportantfeatureoftheBigBangspacetimeisthepresenceofhorizons.Sincetheuniversehasafiniteage,andlighttravelsatafinitespeed,theremaybeeventsinthepastwhoselighthasnothadtimetoreachus.Thisplacesalimitorapasthorizononthemostdistantobjectsthatcanbeobserved.Conversely,becausespaceisexpanding,andmoredistantobjectsarerecedingevermorequickly,lightemittedbyustodaymaynever"catchup"toverydistantobjects.Thisdefinesafuturehorizon,whichlimitstheeventsinthefuturethatwewillbeabletoinfluence.ThepresenceofeithertypeofhorizondependsonthedetailsoftheFLRWmodelthatdescribesouruniverse.Ourunderstandingoftheuniversebacktoveryearlytimessuggeststhatthereisapasthorizon,thoughinpracticeourviewisalsolimitedbytheopacityoftheuniverseatearlytimes.Soourviewcannotextendfurtherbackwardintime,thoughthehorizonrecedesinspace.Iftheexpansionoftheuniversecontinuestoaccelerate,thereisafuturehorizonaswell.[40]

History

Etymology

EnglishastronomerFredHoyleiscreditedwithcoiningtheterm"BigBang"duringa1949BBCradiobroadcast.ItispopularlyreportedthatHoyle,whofavoredanalternative"steadystate"cosmologicalmodel,intendedthistobepejorative,butHoyleexplicitlydeniedthisandsaiditwasjustastrikingimagemeanttohighlightthedifferencebetweenthetwomodels.[41][42][43]:129

Development

TheBigBangtheorydevelopedfromobservationsofthestructureoftheuniverseandfromtheoreticalconsiderations.In1912VestoSliphermeasuredthefirstDopplershiftofa"spiralnebula"(spiralnebulaistheobsoletetermforspiralgalaxies),andsoondiscoveredthatalmostallsuchnebulaewerereceding



HubbleeXtremeDeepField(XDF)

XDFsizecomparedtothesizeofthemoonseveralthousandgalaxies,eachconsistingofbillionsofstars,areinthissmallview.

XDF(2012)vieweachlightspeckisagalaxysomeoftheseareasoldas13.2billionyears[44]

theuniverseisestimatedtocontain200billiongalaxies.

XDFimageshowsfullymaturegalaxiesintheforegroundplanenearlymaturegalaxiesfrom5to9billionyearsagoprotogalaxies,blazingwithyoungstars,beyond9billionyears.

fromEarth.Hedidnotgraspthecosmologicalimplicationsofthisfact,andindeedatthetimeitwashighlycontroversialwhetherornotthesenebulaewere"islanduniverses"outsideourMilkyWay.[45][46]Tenyearslater,AlexanderFriedmann,aRussiancosmologistandmathematician,derivedtheFriedmannequationsfromAlbertEinstein'sequationsofgeneralrelativity,showingthattheuniversemightbeexpandingincontrasttothestaticuniversemodeladvocatedbyEinsteinatthattime.[47]In1924EdwinHubble'smeasurementofthegreatdistancetothenearestspiralnebulaeshowedthatthesesystemswereindeedothergalaxies.IndependentlyderivingFriedmann'sequationsin1927,GeorgesLematre,aBelgianphysicistandRomanCatholicpriest,proposedthattheinferredrecessionofthenebulaewasduetotheexpansionoftheuniverse.[48]

In1931Lematrewentfurtherandsuggestedthattheevidentexpansionoftheuniverse,ifprojectedbackintime,meantthatthefurtherinthepastthesmallertheuniversewas,untilatsomefinitetimeinthepastallthemassoftheuniversewasconcentratedintoasinglepoint,a"primevalatom"whereandwhenthefabricoftimeandspacecameintoexistence.[49]

Startingin1924,Hubblepainstakinglydevelopedaseriesofdistanceindicators,theforerunnerofthecosmicdistanceladder,usingthe100inch(2,500mm)HookertelescopeatMountWilsonObservatory.Thisallowedhimtoestimatedistancestogalaxieswhoseredshiftshadalreadybeenmeasured,mostlybySlipher.In1929HubblediscoveredacorrelationbetweendistanceandrecessionvelocitynowknownasHubble'slaw.[18][50]Lematrehadalreadyshownthatthiswasexpected,giventheCosmologicalPrinciple.[10]

Inthe1920sand1930salmosteverymajorcosmologistpreferredaneternalsteadystateuniverse,andseveralcomplainedthatthebeginningoftimeimpliedbytheBigBangimportedreligiousconceptsintophysicsthisobjectionwaslaterrepeatedbysupportersofthesteadystatetheory.[51]ThisperceptionwasenhancedbythefactthattheoriginatoroftheBigBangtheory,MonsignorGeorgesLematre,wasaRomanCatholicpriest.[52]ArthurEddingtonagreedwithAristotlethattheuniversedidnothaveabeginningintime,viz.,thatmatteriseternal.Abeginningintimewas"repugnant"tohim.[53][54]Lematre,however,thoughtthat

Iftheworldhasbegunwithasinglequantum,thenotionsofspaceandtimewouldaltogetherfailtohaveanymeaningatthebeginningtheywouldonlybegintohaveasensiblemeaningwhentheoriginalquantumhadbeendividedintoa



Artist'sdepictionoftheWMAPsatellitegatheringdatatohelpscientistsunderstandtheBigBang

LawrenceKrauss[66]

sufficientnumberofquanta.Ifthissuggestioniscorrect,thebeginningoftheworldhappenedalittlebeforethebeginningofspaceandtime.[55]

Duringthe1930sotherideaswereproposedasnonstandardcosmologiestoexplainHubble'sobservations,includingtheMilnemodel,[56]theoscillatoryuniverse(originallysuggestedbyFriedmann,butadvocatedbyAlbertEinsteinandRichardTolman)[57]andFritzZwicky'stiredlighthypothesis.[58]

AfterWorldWarII,twodistinctpossibilitiesemerged.OnewasFredHoyle'ssteadystatemodel,wherebynewmatterwouldbecreatedastheuniverseseemedtoexpand.Inthismodeltheuniverseisroughlythesameatanypointintime.[59]TheotherwasLematre'sBigBangtheory,advocatedanddevelopedbyGeorgeGamow,whointroducedbigbangnucleosynthesis(BBN)[60]andwhoseassociates,RalphAlpherandRobertHerman,predictedthecosmicmicrowavebackgroundradiation(CMB).[61]Ironically,itwasHoylewhocoinedthephrasethatcametobeappliedtoLematre'stheory,referringtoitas"thisbigbangidea"duringaBBCRadiobroadcastinMarch1949.[43]:129[notes3]Forawhile,supportwassplitbetweenthesetwotheories.Eventually,theobservationalevidence,mostnotablyfromradiosourcecounts,begantofavorBigBangoverSteadyState.Thediscoveryandconfirmationofthecosmicmicrowavebackgroundradiationin1964[63]securedtheBigBangasthebesttheoryoftheoriginandevolutionoftheuniverse.MuchofthecurrentworkincosmologyincludesunderstandinghowgalaxiesforminthecontextoftheBigBang,understandingthephysicsoftheuniverseatearlierandearliertimes,andreconcilingobservationswiththebasictheory.

SignificantprogressinBigBangcosmologyhavebeenmadesincethelate1990sasaresultofadvancesintelescopetechnologyaswellastheanalysisofdatafromsatellitessuchasCOBE,[64]theHubbleSpaceTelescopeandWMAP.[65]CosmologistsnowhavefairlypreciseandaccuratemeasurementsofmanyoftheparametersoftheBigBangmodel,andhavemadetheunexpecteddiscoverythattheexpansionoftheuniverseappearstobeaccelerating.

Observationalevidence

TheearliestandmostdirectobservationalevidenceofthevalidityofthetheoryaretheexpansionoftheuniverseaccordingtoHubble'slaw(asindicatedbytheredshiftsofgalaxies),discoveryandmeasurementofthecosmicmicrowavebackgroundandtherelativeabundancesoflightelementsproducedbyBigBangnucleosynthesis.Morerecentevidenceincludesobservationsofgalaxyformationandevolution,andthedistributionoflargescalecosmicstructures,[67]Theseare

sometimescalledthe"fourpillars"oftheBigBangtheory.[68]

PrecisemodernmodelsoftheBigBangappealtovariousexoticphysicalphenomenathathavenotbeenobservedinterrestriallaboratoryexperimentsorincorporatedintotheStandardModelofparticlephysics.Ofthesefeatures,darkmatteriscurrentlysubjectedtothemostactivelaboratoryinvestigations.[69]Remainingissuesincludethecuspyhaloproblemandthedwarfgalaxyproblemof

"[The]bigbangpictureistoofirmlygroundedindatafromeveryareatobeprovedinvalidinitsgeneralfeatures."



colddarkmatter.Darkenergyisalsoanareaofintenseinterestforscientists,butitisnotclearwhetherdirectdetectionofdarkenergywillbepossible.[70]InflationandbaryogenesisremainmorespeculativefeaturesofcurrentBigBangmodels.Viable,quantitativeexplanationsforsuchphenomenaarestillbeingsought.Thesearecurrentlyunsolvedproblemsinphysics.

Hubble'slawandtheexpansionofspace

Observationsofdistantgalaxiesandquasarsshowthattheseobjectsareredshiftedthelightemittedfromthemhasbeenshiftedtolongerwavelengths.Thiscanbeseenbytakingafrequencyspectrumofanobjectandmatchingthespectroscopicpatternofemissionlinesorabsorptionlinescorrespondingtoatomsofthechemicalelementsinteractingwiththelight.Theseredshiftsareuniformlyisotropic,distributedevenlyamongtheobservedobjectsinalldirections.IftheredshiftisinterpretedasaDopplershift,therecessionalvelocityoftheobjectcanbecalculated.Forsomegalaxies,itispossibletoestimatedistancesviathecosmicdistanceladder.Whentherecessionalvelocitiesareplottedagainstthesedistances,alinearrelationshipknownasHubble'slawisobserved:[18]

v=H0D,

where

vistherecessionalvelocityofthegalaxyorotherdistantobject,Disthecomovingdistancetotheobject,andH0isHubble'sconstant,measuredtobe70.4

+1.31.4km/s/MpcbytheWMAPprobe.

[34]

Hubble'slawhastwopossibleexplanations.EitherweareatthecenterofanexplosionofgalaxieswhichisuntenablegiventheCopernicanprincipleortheuniverseisuniformlyexpandingeverywhere.ThisuniversalexpansionwaspredictedfromgeneralrelativitybyAlexanderFriedmannin1922[47]andGeorgesLematrein1927,[48]wellbeforeHubblemadehis1929analysisandobservations,anditremainsthecornerstoneoftheBigBangtheoryasdevelopedbyFriedmann,Lematre,Robertson,andWalker.

Thetheoryrequirestherelationv=HDtoholdatalltimes,whereDisthecomovingdistance,vistherecessionalvelocity,andv,H,andDvaryastheuniverseexpands(hencewewriteH0todenotethepresentdayHubble"constant").Fordistancesmuchsmallerthanthesizeoftheobservableuniverse,theHubbleredshiftcanbethoughtofastheDopplershiftcorrespondingtotherecessionvelocityv.However,theredshiftisnotatrueDopplershift,butrathertheresultoftheexpansionoftheuniversebetweenthetimethelightwasemittedandthetimethatitwasdetected.[71]

ThatspaceisundergoingmetricexpansionisshownbydirectobservationalevidenceoftheCosmologicalprincipleandtheCopernicanprinciple,whichtogetherwithHubble'slawhavenootherexplanation.Astronomicalredshiftsareextremelyisotropicandhomogeneous,[18]supportingtheCosmologicalprinciplethattheuniverselooksthesameinalldirections,alongwithmuchotherevidence.Iftheredshiftsweretheresultofanexplosionfromacenterdistantfromus,theywouldnotbesosimilarindifferentdirections.

Measurementsoftheeffectsofthecosmicmicrowavebackgroundradiationonthedynamicsofdistantastrophysicalsystemsin2000provedtheCopernicanprinciple,that,onacosmologicalscale,theEarthisnotinacentralposition.[72]RadiationfromtheBigBangwasdemonstrablywarmeratearliertimes



9yearWMAPimageofthecosmicmicrowavebackgroundradiation(2012).[73][74]Theradiationisisotropictoroughlyonepartin100,000.[75]

ThecosmicmicrowavebackgroundspectrummeasuredbytheFIRASinstrumentontheCOBEsatelliteisthemostpreciselymeasuredblackbodyspectruminnature.[76]Thedatapointsanderrorbarsonthisgraphareobscuredbythetheoreticalcurve.

throughouttheuniverse.Uniformcoolingofthecosmicmicrowavebackgroundoverbillionsofyearsisexplainableonlyiftheuniverseisexperiencingametricexpansion,andexcludesthepossibilitythatweareneartheuniquecenterofanexplosion.

Cosmicmicrowavebackgroundradiation

In1964ArnoPenziasandRobertWilsonserendipitouslydiscoveredthecosmicbackgroundradiation,anomnidirectionalsignalinthemicrowaveband.[63]TheirdiscoveryprovidedsubstantialconfirmationofthegeneralCMBpredictions:theradiationwasfoundtobeconsistentwithanalmostperfectblackbodyspectruminalldirectionsthisspectrumhasbeenredshiftedbytheexpansionoftheuniverse,andtodaycorrespondstoapproximately2.725K.ThistippedthebalanceofevidenceinfavoroftheBigBangmodel,andPenziasandWilsonwereawardedaNobelPrizein1978.

ThesurfaceoflastscatteringcorrespondingtoemissionoftheCMBoccursshortlyafterrecombination,theepochwhenneutralhydrogenbecomesstable.Priortothis,theuniversecomprisedahotdensephotonbaryonplasmaseawherephotonswerequicklyscatteredfromfreechargedparticles.Peakingataround372 14kyr,[33]themeanfreepathforaphotonbecomeslongenoughtoreachthepresentdayandtheuniversebecomestransparent.

In1989NASAlaunchedtheCosmicBackgroundExplorersatellite(COBE).ItsfindingswereconsistentwithpredictionsregardingtheCMB,findingaresidualtemperatureof2.726K(morerecentmeasurementshaverevisedthisfiguredownslightlyto2.725K)andprovidingthefirstevidenceforfluctuations(anisotropies)intheCMB,atalevelofaboutonepartin105.[64]JohnC.MatherandGeorgeSmootwereawardedtheNobelPrizefortheirleadershipinthiswork.Duringthefollowingdecade,CMBanisotropieswerefurtherinvestigatedby

alargenumberofgroundbasedandballoonexperiments.In20002001severalexperiments,mostnotablyBOOMERanG,foundtheshapeoftheuniversetobespatiallyalmostflatbymeasuringthetypicalangularsize(thesizeonthesky)oftheanisotropies.

Inearly2003thefirstresultsoftheWilkinsonMicrowaveAnisotropyProbe(WMAP)werereleased,yieldingwhatwereatthetimethemostaccuratevaluesforsomeofthecosmologicalparameters.Theresultsdisprovedseveralspecificcosmicinflationmodels,butareconsistentwiththeinflationtheoryingeneral.[65]ThePlanckspaceprobewaslaunchedinMay2009.Othergroundandballoonbasedcosmicmicrowavebackgroundexperimentsareongoing.

Abundanceofprimordialelements



UsingtheBigBangmodelitispossibletocalculatetheconcentrationofhelium4,helium3,deuterium,andlithium7intheuniverseasratiostotheamountofordinaryhydrogen.[28]Therelativeabundancesdependonasingleparameter,theratioofphotonstobaryons.ThisvaluecanbecalculatedindependentlyfromthedetailedstructureofCMBfluctuations.Theratiospredicted(bymass,notbynumber)areabout0.25for

4He/H,about103for

2H/H,about104for

3He/Handabout109for

7Li/H.[28]

Themeasuredabundancesallagreeatleastroughlywiththosepredictedfromasinglevalueofthebaryontophotonratio.Theagreementisexcellentfordeuterium,closebutformallydiscrepantfor

4He,

andoffbyafactoroftwofor7Liinthelattertwocasestherearesubstantialsystematicuncertainties.

Nonetheless,thegeneralconsistencywithabundancespredictedbyBigBangnucleosynthesisisstrongevidencefortheBigBang,asthetheoryistheonlyknownexplanationfortherelativeabundancesoflightelements,anditisvirtuallyimpossibleto"tune"theBigBangtoproducemuchmoreorlessthan2030%helium.[77]IndeedthereisnoobviousreasonoutsideoftheBigBangthat,forexample,theyounguniverse(i.e.,beforestarformation,asdeterminedbystudyingmattersupposedlyfreeofstellarnucleosynthesisproducts)shouldhavemoreheliumthandeuteriumormoredeuteriumthan

3He,andin

constantratios,too.

Galacticevolutionanddistribution

DetailedobservationsofthemorphologyanddistributionofgalaxiesandquasarsareinagreementwiththecurrentstateoftheBigBangtheory.AcombinationofobservationsandtheorysuggestthatthefirstquasarsandgalaxiesformedaboutabillionyearsaftertheBigBang,andsincethenlargerstructureshavebeenforming,suchasgalaxyclustersandsuperclusters.Populationsofstarshavebeenagingandevolving,sothatdistantgalaxies(whichareobservedastheywereintheearlyuniverse)appearverydifferentfromnearbygalaxies(observedinamorerecentstate).Moreover,galaxiesthatformedrelativelyrecentlyappearmarkedlydifferentfromgalaxiesformedatsimilardistancesbutshortlyaftertheBigBang.Theseobservationsarestrongargumentsagainstthesteadystatemodel.Observationsofstarformation,galaxyandquasardistributionsandlargerstructuresagreewellwithBigBangsimulationsoftheformationofstructureintheuniverseandarehelpingtocompletedetailsofthetheory.[78][79]

Primordialgasclouds

In2011astronomersfoundwhattheybelievetobepristinecloudsofprimordialgas,byanalyzingabsorptionlinesinthespectraofdistantquasars.Beforethisdiscovery,allotherastronomicalobjectshavebeenobservedtocontainheavyelementsthatareformedinstars.Thesetwocloudsofgascontainnoelementsheavierthanhydrogenanddeuterium.[80][81]Sincethecloudsofgashavenoheavyelements,theylikelyformedinthefirstfewminutesaftertheBigBang,duringBigBangnucleosynthesis.TheircompositionmatchesthecompositionpredictedfromBigBangnucleosynthesis.Thisprovidesdirectevidencethattherewasaperiodinthehistoryoftheuniversebeforetheformationofthefirststars,whenmostordinarymatterexistedintheformofcloudsofneutralhydrogen.

Otherlinesofevidence

TheageofuniverseasestimatedfromtheHubbleexpansionandtheCMBisnowingoodagreementwithotherestimatesusingtheagesoftheoldeststars,bothasmeasuredbyapplyingthetheoryofstellarevolutiontoglobularclustersandthroughradiometricdatingofindividualPopulationIIstars.[82]



FocalplaneofBICEP2telescopeunderamicroscopemayhavedetectedgravitationalwavesfromtheinfantuniverse.[14][15][16][17]

ThepredictionthattheCMBtemperaturewashigherinthepasthasbeenexperimentallysupportedbyobservationsofverylowtemperatureabsorptionlinesingascloudsathighredshift.[83]ThispredictionalsoimpliesthattheamplitudeoftheSunyaevZel'dovicheffectinclustersofgalaxiesdoesnotdependdirectlyonredshift.Observationshavefoundthistoberoughlytrue,butthiseffectdependsonclusterpropertiesthatdochangewithcosmictime,makingprecisemeasurementsdifficult.[84][85]

On17March2014,astronomersattheHarvardSmithsonianCenterforAstrophysicsannouncedtheapparentdetectionofprimordialgravitationalwaves,which,ifconfirmed,mayprovidestrongevidenceforinflationandtheBigBang.[14][15][16][17]However,on19June2014,loweredconfidenceinconfirmingthefindingswasreported[86][87][88]andon19September2014,evenmoreloweredconfidence.[89][90]

Relatedissuesinphysics

Baryonasymmetry

Itisnotyetunderstoodwhytheuniversehasmorematterthanantimatter.[91]Itisgenerallyassumedthatwhentheuniversewasyoungandveryhot,itwasinstatisticalequilibriumandcontainedequalnumbersofbaryonsandantibaryons.However,observationssuggestthattheuniverse,includingitsmostdistantparts,ismadealmostentirelyofmatter.Aprocesscalledbaryogenesiswashypothesizedtoaccountfortheasymmetry.Forbaryogenesistooccur,theSakharovconditionsmustbesatisfied.Theserequirethatbaryonnumberisnotconserved,thatCsymmetryandCPsymmetryareviolatedandthattheuniversedepartfromthermodynamicequilibrium.[92]AlltheseconditionsoccurintheStandardModel,buttheeffectisnotstrongenoughtoexplainthepresentbaryonasymmetry.

Darkenergy

MeasurementsoftheredshiftmagnituderelationfortypeIasupernovaeindicatethattheexpansionoftheuniversehasbeenacceleratingsincetheuniversewasabouthalfitspresentage.Toexplainthisacceleration,generalrelativityrequiresthatmuchoftheenergyintheuniverseconsistsofacomponentwithlargenegativepressure,dubbed"darkenergy".[10]Darkenergy,thoughspeculative,solvesnumerousproblems.Measurementsofthecosmicmicrowavebackgroundindicatethattheuniverseisverynearlyspatiallyflat,andthereforeaccordingtogeneralrelativitytheuniversemusthavealmostexactlythecriticaldensityofmass/energy.Butthemassdensityoftheuniversecanbemeasuredfromitsgravitationalclustering,andisfoundtohaveonlyabout30%ofthecriticaldensity.[10]Sincetheorysuggeststhatdarkenergydoesnotclusterintheusualwayitisthebestexplanationforthe"missing"energydensity.Darkenergyalsohelpstoexplaintwogeometricalmeasuresoftheoverallcurvatureoftheuniverse,oneusingthefrequencyofgravitationallenses,andtheotherusingthecharacteristicpatternofthelargescalestructureasacosmicruler.

Negativepressureisbelievedtobeapropertyofvacuumenergy,buttheexactnatureandexistenceofdarkenergyremainsoneofthegreatmysteriesoftheBigBang.Possiblecandidatesincludeacosmologicalconstantandquintessence.ResultsfromtheWMAPteamin2008areinaccordancewithauniversethatconsistsof73%darkenergy,23%darkmatter,4.6%regularmatterandlessthan1%



Chartshowstheproportionofdifferentcomponentsoftheuniverseabout95%isdarkmatteranddarkenergy.

neutrinos.[34]Accordingtotheory,theenergydensityinmatterdecreaseswiththeexpansionoftheuniverse,butthedarkenergydensityremainsconstant(ornearlyso)astheuniverseexpands.Thereforemattermadeupalargerfractionofthetotalenergyoftheuniverseinthepastthanitdoestoday,butitsfractionalcontributionwillfallinthefarfutureasdarkenergybecomesevenmoredominant.

Darkmatter

Duringthe1970sand80s,variousobservationsshowedthatthereisnotsufficientvisiblematterintheuniversetoaccountfortheapparentstrengthofgravitationalforceswithinandbetweengalaxies.Thisledtotheideathatupto90%ofthematterintheuniverseisdarkmatterthatdoesnotemitlightorinteractwithnormalbaryonicmatter.Inaddition,theassumptionthattheuniverseismostlynormalmatterledtopredictionsthatwerestronglyinconsistentwithobservations.Inparticular,theuniversetodayisfarmorelumpyandcontainsfarlessdeuteriumthancanbeaccountedforwithoutdarkmatter.Whiledarkmatterhasalwaysbeencontroversial,itisinferredbyvariousobservations:theanisotropiesintheCMB,galaxyclustervelocitydispersions,largescalestructuredistributions,gravitationallensingstudies,andXraymeasurementsofgalaxyclusters.[93]

Indirectevidencefordarkmattercomesfromitsgravitationalinfluenceonothermatter,asnodarkmatterparticleshavebeenobservedinlaboratories.Manyparticlephysicscandidatesfordarkmatterhavebeenproposed,andseveralprojectstodetectthemdirectlyareunderway.[94]

Globularclusterage

Inthemid1990sobservationsofglobularclustersappearedtobeinconsistentwiththeBigBangtheory.Computersimulationsthatmatchedtheobservationsofthestellarpopulationsofglobularclusterssuggestedthattheywereabout15billionyearsold,whichconflictedwiththe13.8billionyearageoftheuniverse.Thisissuewaspartiallyresolvedinthelate1990swhennewcomputersimulations,whichincludedtheeffectsofmasslossduetostellarwinds,indicatedamuchyoungerageforglobularclusters.[95]Thereremainsomequestionsastohowaccuratelytheagesoftheclustersaremeasured,butitisclearthatobservationsofglobularclustersnolongerappearinconsistentwiththeBigBangtheory.

Problems

TherearegenerallyconsideredtobethreeoutstandingproblemswiththeBigBangtheory:thehorizonproblem,theflatnessproblem,andthemagneticmonopoleproblem.Themostcommonanswertotheseproblemsisinflationarytheoryhowever,sincethiscreatesnewproblems,otheroptionshavebeenproposed,suchastheWeylcurvaturehypothesis.[96][97]

Horizonproblem



TheoverallgeometryoftheuniverseisdeterminedbywhethertheOmegacosmologicalparameterislessthan,equaltoorgreaterthan1.Shownfromtoptobottomareacloseduniversewithpositivecurvature,ahyperbolicuniversewithnegativecurvatureandaflatuniversewithzerocurvature.

Thehorizonproblemresultsfromthepremisethatinformationcannottravelfasterthanlight.Inauniverseoffiniteagethissetsalimittheparticlehorizonontheseparationofanytworegionsofspacethatareincausalcontact.[98]TheobservedisotropyoftheCMBisproblematicinthisregard:iftheuniversehadbeendominatedbyradiationormatteratalltimesuptotheepochoflastscattering,theparticlehorizonatthattimewouldcorrespondtoabout2degreesonthesky.Therewouldthenbenomechanismtocausewiderregionstohavethesametemperature.

Aresolutiontothisapparentinconsistencyisofferedbyinflationarytheoryinwhichahomogeneousandisotropicscalarenergyfielddominatestheuniverseatsomeveryearlyperiod(beforebaryogenesis).Duringinflation,theuniverseundergoesexponentialexpansion,andtheparticlehorizonexpandsmuchmorerapidlythanpreviouslyassumed,sothatregionspresentlyonoppositesidesoftheobservableuniversearewellinsideeachother'sparticlehorizon.TheobservedisotropyoftheCMBthenfollowsfromthefactthatthislargerregionwasincausalcontactbeforethebeginningofinflation.

Heisenberg'suncertaintyprinciplepredictsthatduringtheinflationaryphasetherewouldbequantumthermalfluctuations,whichwouldbemagnifiedtocosmicscale.Thesefluctuationsserveastheseedsofallcurrentstructureintheuniverse.InflationpredictsthattheprimordialfluctuationsarenearlyscaleinvariantandGaussian,whichhasbeenaccuratelyconfirmedbymeasurementsoftheCMB.

Ifinflationoccurred,exponentialexpansionwouldpushlargeregionsofspacewellbeyondourobservablehorizon.

Flatnessproblem

Theflatnessproblem(alsoknownastheoldnessproblem)isanobservationalproblemassociatedwithaFriedmannLematreRobertsonWalkermetric.[98]Theuniversemayhavepositive,negative,orzerospatialcurvaturedependingonitstotalenergydensity.Curvatureisnegativeifitsdensityislessthanthecriticaldensity,positiveifgreater,andzeroatthecriticaldensity,inwhichcasespaceissaidtobeflat.Theproblemisthatanysmalldeparturefromthecriticaldensitygrowswithtime,andyettheuniversetodayremainsveryclosetoflat.[notes4]GiventhatanaturaltimescalefordeparturefromflatnessmightbethePlancktime,1043seconds,[4]thefactthattheuniversehasreachedneitheraheatdeathnoraBigCrunchafterbillionsofyearsrequiresanexplanation.Forinstance,evenattherelativelylateageofafewminutes(thetimeofnucleosynthesis),theuniversedensitymusthavebeenwithinonepartin1014ofitscriticalvalue,oritwouldnotexistasitdoestoday.[99]

Magneticmonopoles

Themagneticmonopoleobjectionwasraisedinthelate1970s.Grandunificationtheoriespredictedtopologicaldefectsinspacethatwouldmanifestasmagneticmonopoles.Theseobjectswouldbeproducedefficientlyinthehotearlyuniverse,resultinginadensitymuchhigherthanisconsistentwith



observations,giventhatnomonopoleshavebeenfound.Thisproblemisalsoresolvedbycosmicinflation,whichremovesallpointdefectsfromtheobservableuniverse,inthesamewaythatitdrivesthegeometrytoflatness.[98]

ThefutureaccordingtotheBigBangtheory

Beforeobservationsofdarkenergy,cosmologistsconsideredtwoscenariosforthefutureoftheuniverse.Ifthemassdensityoftheuniverseweregreaterthanthecriticaldensity,thentheuniversewouldreachamaximumsizeandthenbegintocollapse.Itwouldbecomedenserandhotteragain,endingwithastatesimilartothatinwhichitstartedaBigCrunch.[40]Alternatively,ifthedensityintheuniversewereequaltoorbelowthecriticaldensity,theexpansionwouldslowdownbutneverstop.Starformationwouldceasewiththeconsumptionofinterstellargasineachgalaxystarswouldburnoutleavingwhitedwarfs,neutronstars,andblackholes.Verygradually,collisionsbetweenthesewouldresultinmassaccumulatingintolargerandlargerblackholes.TheaveragetemperatureoftheuniversewouldasymptoticallyapproachabsolutezeroaBigFreeze.Moreover,iftheprotonwereunstable,thenbaryonicmatterwoulddisappear,leavingonlyradiationandblackholes.Eventually,blackholeswouldevaporatebyemittingHawkingradiation.Theentropyoftheuniversewouldincreasetothepointwherenoorganizedformofenergycouldbeextractedfromit,ascenarioknownasheatdeath.

Modernobservationsofacceleratingexpansionimplythatmoreandmoreofthecurrentlyvisibleuniversewillpassbeyondoureventhorizonandoutofcontactwithus.Theeventualresultisnotknown.TheCDMmodeloftheuniversecontainsdarkenergyintheformofacosmologicalconstant.Thistheorysuggeststhatonlygravitationallyboundsystems,suchasgalaxies,willremaintogether,andtheytoowillbesubjecttoheatdeathastheuniverseexpandsandcools.Otherexplanationsofdarkenergy,calledphantomenergytheories,suggestthatultimatelygalaxyclusters,stars,planets,atoms,nuclei,andmatteritselfwillbetornapartbytheeverincreasingexpansioninasocalledBigRip.[100]

Before:SpeculativephysicsbeyondtheBigBangtheory

WhiletheBigBangmodeliswellestablishedincosmology,itislikelytoberefined.TheBigBangTheory,builtupontheequationsofclassicalgeneralrelativity,indicatesasingularityattheoriginofcosmictimethisinfiniteenergydensityisregardedasimpossibleinphysics.Still,itisknownthattheequationsarenotapplicablebeforethetimewhenuniversecooleddowntothePlancktemperature,andthisconclusiondependsonvariousassumptions,ofwhichsomecouldneverbeexperimentallyverified.

Oneproposedrefinementtoavoidthiswouldbesingularityistodevelopacorrecttreatmentofquantumgravity.[101]

Itisnotknownwhatcouldhaveprecededthehotdensestateoftheearlyuniverseorhowandwhyitoriginated,thoughspeculationaboundsinthefieldofcosmogony.

Someproposals,eachofwhichentailsuntestedhypotheses,are:

ModelsincludingtheHartleHawkingnoboundarycondition,inwhichthewholeofspacetimeisfinitetheBigBangdoesrepresentthelimitoftimebutwithoutanysingularity.[102]BigBanglatticemodel,statesthattheuniverseatthemomentoftheBigBangconsistsofaninfinitelatticeoffermions,whichissmearedoverthefundamentaldomainsoithasrotational,translationalandgaugesymmetry.Thesymmetryisthelargestsymmetrypossibleandhencethelowestentropyofanystate.[103]Branecosmologymodels,inwhichinflationisduetothemovementofbranesinstringtheorythe



Thisisanartist'sconceptofthemetricexpansionofspace,wherespace(includinghypotheticalnonobservableportionsoftheuniverse)isrepresentedateachtimebythecircularsections.Noteontheleftthedramaticexpansion(nottoscale)occurringintheinflationaryepoch,andatthecentertheexpansionacceleration.TheschemeisdecoratedwithWMAPimagesontheleftandwiththerepresentationofstarsattheappropriatelevelofdevelopment.

preBigBangmodeltheekpyroticmodel,inwhichtheBigBangistheresultofacollisionbetweenbranesandthecyclicmodel,avariantoftheekpyroticmodelinwhichcollisionsoccurperiodically.InthelattermodeltheBigBangwasprecededbyaBigCrunchandtheuniversecyclesfromoneprocesstotheother.[104][105][106][107]Eternalinflation,inwhichuniversalinflationendslocallyhereandthereinarandomfashion,eachendpointleadingtoabubbleuniverse,expandingfromitsownbigbang.[108][109]

Proposalsinthelasttwocategories,seetheBigBangasaneventineitheramuchlargerandolderuniverseorinamultiverse.

Religiousandphilosophicalinterpretations

Asadescriptionoftheoriginoftheuniverse,theBigBanghassignificantbearingonreligionandphilosophy.[110][111]Asaresult,ithasbecomeoneoftheliveliestareasinthediscoursebetweenscienceandreligion.[112]SomebelievetheBigBangimpliesacreator,[113][114]andsomeseeitsmentionintheirholybooks,[115]whileothersarguethatBigBangcosmologymakesthenotionofacreatorsuperfluous.[111][116]

Seealso

BigCrunchCosmicCalendarShapeoftheuniverse

Notes1. ThereisnoconsensusabouthowlongtheBigBangphaselasted.Forsomewritersthisdenotesonlythe

initialsingularity,forothersthewholehistoryoftheuniverse.Usually,atleastthefirstfewminutes(duringwhichheliumissynthesized)aresaidtooccur"duringtheBigBang".

2. Detailedinformationofandreferencesfortestsofgeneralrelativityaregiveninthearticletestsofgeneralrelativity.

3. ItiscommonlyreportedthatHoyleintendedthistobepejorative.However,Hoylelaterdeniedthat,sayingthatitwasjustastrikingimagemeanttoemphasizethedifferencebetweenthetwotheoriesforradiolisteners.[62]

4. Strictly,darkenergyintheformofacosmologicalconstantdrivestheuniversetowardsaflatstatehowever,ouruniverseremainedclosetoflatforseveralbillionyears,beforethedarkenergydensitybecamesignificant.

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Books

Farrell,John(2005).TheDayWithoutYesterday:Lemaitre,Einstein,andtheBirthofModernCosmology.NewYork,NY:Thunder'sMouthPress.ISBN1560256605.Kolb,E.Turner,M.(1988).TheEarlyUniverse.AddisonWesley.ISBN0201116049.Masters,Ken(Ed.)(2015).Origins:BeforetheBigBang.Lulu.com.ISBN9781312753266.Ostriker,JeremiahP.Mitton,Simon(2013).HeartofDarkness:UnravelingthemysteriesoftheinvisibleUniverse.Princeton,NJ:PrincetonUniversityPress.ISBN9780691134307.Peacock,J.(1999).CosmologicalPhysics.CambridgeUniversityPress.ISBN0521422701.Woolfson,M.(2013).Time,Space,StarsandMan:TheStoryofBigBang(2ndedition).WorldScientificPublishing.ISBN9781848169333.

Furtherreading

Foranannotatedlistoftextbooksandmonographs,seephysicalcosmology.

Alpher,R.A.Herman,R.(1988)."Reflectionsonearlyworkon'bigbang'cosmology".PhysicsToday8(8):2434.Bibcode:1988PhT....41h..24A(http://adsabs.harvard.edu/abs/1988PhT....41h..24A).doi:10.1063/1.881126(https://dx.doi.org/10.1063%2F1.881126)."CosmicJourney:AHistoryofScientificCosmology"(http://www.aip.org/history/cosmology/index.htm).AmericanInstituteofPhysics.Barrow,J.D.(1994).TheOriginoftheUniverse.Weidenfeld&Nicolson.ISBN0297814974.Davies,P.C.W.(1992).TheMindofGod:Thescientificbasisforarationalworld.Simon&Schuster.ISBN0671710699.Feuerbacher,B.Scranton,R.(2006)."EvidencefortheBigBang"(http://www.talkorigins.org/faqs/astronomy/bigbang.html).TalkOrigins.Mather,J.C.Boslough,J.(1996).Theveryfirstlight:thetrueinsidestoryofthescientificjourneybacktothedawnoftheUniverse.BasicBooks.p.300.ISBN0465015751.Riordan,MichaelZajc,William(May2006)."TheFirstFewMicroseconds"(http://rhig.physics.yale.edu/M_article_11_2005.pdf)(PDF).ScientificAmerican.Singh,S.(2004).BigBang:Theoriginsoftheuniverse.FourthEstate.ISBN0007162200."MisconceptionsabouttheBigBang"(http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf)(PDF).ScientificAmerican.March2005.Weinberg,S.(1993).TheFirstThreeMinutes:AModernViewOfTheOriginOfTheUniverse.BasicBooks.ISBN0465024378.

Externallinks

bigbangmodel(http://www.britannica.com/EBchecked/topic/64893)atEncyclopdiaBritannicaTheStoryoftheBigBang(http://onceuponauniverse.com/about/inthebeginning/)STFCfundedprojectexplainingthehistoryoftheuniverseineasytounderstandlanguageBigBangCosmology(http://map.gsfc.nasa.gov/universe/bb_theory.html)WMAPTheBigBang(http://science.nasa.gov/astrophysics/focusareas/whatpoweredthebigbang/)NASAScienceBigbangmodelwithanimatedgraphics(http://www.scientificblogging.com/hammock_physicist/big_bang_big_bewilderment)Cosmology(https://www.dmoz.org/Science/Astronomy/Cosmology/)atDMOZEvidencefortheBigBang(http://www.talkorigins.org/faqs/astronomy/bigbang.html)Universe(http://www.cosmosportal.org/articles/view/138892)ACosmicHistoryoftheUniverse(http://logiclaw.com/index.php?title=A_Cosmic_History_of_Human_Evolution)



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