PhysicsoftheCosmosProgramAnalysisGroup(PhysPAG)ReportonFlagshipMissionConceptstoStudyfor

the2020DecadalSurvey

8October2015

PhysicsoftheCosmosExecutiveCommitteeJamesJ.Bock Caltech/JPLMarshallW.Bautz MITRachelBean CornellJayA.Bookbinder SAOJohnW.Conklin U.FloridaNeilJ.Cornish MontanaStateOlivierP.Doré NASAJPLHenricKrawczynski WashingtonU.St.LouisMarkL.McConnell U.NewHampshireAmberD.Miller ColumbiaJohnA.Nousek PennStateAngelaV.Olinto U.ChicagoEun‐SukSeo U.MarylandEdwardJ.Wollack NASAGSFC

WithcontributionsfromSteveW.Allen(Stanford),EmanueleBerti(U.Mississippi),CharlesM.Bradford(NASAJPL),JohnBaker(NASAGSFC),LauraW.Brenneman(Harvard),DeeptoChakrabarty(MIT),EricD.Feigelson(PennState),WilliamR.Forman(CfA),JessicaA.Gaskin(NASAMSFC),AshleyL.King(Stanford),SeanMcWilliams(WestVirginiaU.),GuidoMueller(U.Florida),JohnA.Nousek(PennState),MichaelA.Nowak(MIT),RobinStebbins(NASAGSFC),HarveyD.Tananbaum(CfA),AlexeyVikhlinin(CfA),MartinC.Weisskopf(NASAMSFC),NicolasYunes(MontataStateU.)andIrinaZhuravleva(Stanford)

Table of Contents 1.JointPAGStatement............................................................................................................................1

2.SummaryofPhysPAGFindings......................................................................................................2

2a.FindingsonFlagshipMissions.................................................................................................2

2b.PreparingtheL3Gravitational‐WaveMissionforthe2020Decadal.....................3

2c.PreparingtheInflationProbeMissionforthe2020Decadal.....................................4

2d.ImportanceofProbe‐ClassMissions.....................................................................................5

3.PCOSScienceThemesinFlagshipMissions..............................................................................6

a.X‐RaySurveyorMission.................................................................................................................6

b.Far‐InfraredSurveyorMission...................................................................................................9

c.UV/Optical/IRSurveyorMission............................................................................................12

d.Habitable‐ExoplanetImagingMission.................................................................................14

e.TableofFlagshipMissionParameters..................................................................................15

4.PhysPAGSciencefortheL3GravitationalWaveandInflationProbeMissions.....16

a.L3GravitationalWaveMissionScience...............................................................................16

b.InflationProbeScience...............................................................................................................17

5.PhysPAGInterestinProbeMissions.........................................................................................19

Acknowledgements................................................................................................................................20

Appendix.....................................................................................................................................................21

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1.  Joint PAG Statement ThisisajointsummaryofthereportsfromthethreeAstrophysicsProgram

Analysis Groups (PAGs) in response to the charge given to the PAG ExecutiveCommittees by theAstrophysicsDivisionDirector, PaulHertz, in thewhite paper"Planningforthe2020DecadalSurvey",issuedJanuary4,2015.ThisjointexecutivesummaryiscommontoallthreePAGreports,andcontainspointsofconsensusacrossallthreePAGs,achievedthroughextensivediscussionandvettingwithinandbetweenour respective communities. Additional information and findings specific to theindividual PAG activities related to this charge are reported separately in theremainderoftheindividualreports.Theseadditionalfindingsarenotnecessarilyincontradictiontomaterialintheotherreports,butrathergenerallyfocusonfindingsspecifictotheindividualPAGs.

ThePAGsconcurthatallfourlargemissionconceptsidentifiedinthewhitepaperascandidatesformissionconceptmaturationpriortothe2020DecadalSurveyshould be studied in detail. These include the Far‐IR Surveyor, the Habitable‐Exoplanet Imaging Mission, the UV/Optical/IR Surveyor, and the X‐ray Surveyor.Other flagship mission concepts were considered, but none achieved sufficientlybroadcommunitysupporttobeelevatedtothelevelofthesefourprimarycandidatemissions.

Thisfindingispredicateduponassumptionsoutlinedinthewhitepaperandsubsequent charge, namely that 1) major development of future large flagshipmissionsunderconsiderationaretofollowtheimplementationphasesoftheJamesWebb Space Telescope (JWST) and the Wide‐Field InfraRed Survey Telescope(WFIRST); 2) NASA will partner with the European Space Agency on its L3Gravitational Wave Surveyor, participate in preparatory studies leading to thisobservatory,andconductthenecessarytechnologydevelopmentandotheractivitiesleadingtotheL3mission, includingpreparationsthatwillbeneededfor the2020decadalreview;and3)thattheInflationProbebeclassifiedasaprobe‐classmissiontobedevelopedaccordingtothetechnologyandmissionplanningrecommendationsin the2010Decadal Survey report. Physics of theCosmosPAG (PhysPAG) soughtinputonthemissionsizecategoryforthismissionandfindsthatitisappropriatelyclassifiedasaProbe‐classmission.Ifthesekeyassumptionsweretochange,thisPAGfindingwouldneedtobere‐evaluatedinlightofthechanges.

ThePAGsfindthatthereisstrongcommunitysupportforthesecondphaseofthis activity ‐maturationof the fourproposedmission concept studies.ThePAGsbelieve that theseconceptstudiesshouldbeconductedbyscientistsandtechnicalexpertsassignedtotherespectiveScienceandTechnologyDefinitionTeams(STDTs).ThePAGsfindthatthecommunityisconcernedaboutthecompositionoftheseSTDTsand that there is strong consensus that all of the STDTs contain broad andinterdisciplinaryrepresentationofthesciencecommunity.ThePAGsalsofindthatthe community expects cross‐STDT cooperation and exchange of informationwheneverpossibletofacilitatethesharingofexpertise,especiallyinthecaseoftheUVOIR Surveyor and theHabitable‐Exoplanet ImagingMission,which share some

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sciencegoalsandtechnologicalneeds.ThePAGsconcurthatafreeandopenprocessshouldbeusedtocompetitivelyselecttheSTDTs.

Finally, the PAGs find that there is community support for a line of probe‐classmissionswithintheAstrophysicsDivisionmissionportfolio.ThePAGswouldbewillingtocollectfurtherinputonprobemissionsfromthecommunityasafollowingstrategicplanningchargeifaskedtodosobytheAstrophysicsDivisionDirector.

2.  Summary of PhysPAG Findings ToaddressthechargeonflagshipmissionsgiventothePAGsbyPaulHertz

(http://science.nasa.gov/media/medialibrary/2015/01/28/White_Paper_‐_Planning_for_the_2020_Decadal_Survey_‐signed.pdf), the PhysPAG collectedcommunity input through conference and town hall sessions, and PhysPAG SIGsplintermeetingsanddescribedintheAppendix.2a. Findings on Flagship Missions 

ThefindingsofthePhysPAGagreewiththoseofthejointPAGstatementgivenin § 1. These flagship missions offer exciting opportunities for astrophysics,including scientific themes of particular interest to the Physics of the Cosmoscommunity.Beyondtheseuniversaljointfindings,thePhysPAGseparatelyfindsthefollowing:1) TheESAL3gravitationalwavemissioniscompellingandwelookforwardtoseeing

thismissionbeingfullypreparedfortheUS2020Decadalreview.Appropriatedevelopmentsaredescribed in§2cand the revolutionaryscienceenabledbythismissionisdescribedin§4a.

2) WeagreewiththeassumptiongiveninthechargethattheInflationProbemissionshouldbeplannedasprobe‐class.RecentstudiesofcomparableimplementationsinEurope(CORE+)andJapan(LITEBIRD)broadlycorrespondtotheNASAprobecostcategory. Furthermore a spectro‐polarimetric implementationwith low spatialresolution(PIXIE)wasproposedasaMIDEXmission.WenotethatamoredefinitivestatementaboutthecostofaNASAmissionrequiresadedicatedstudy,whichwillbenecessary for the2020DecadalReview,andwhichmayhelpbetterdefine theparametersofaprobemissioncategoryorcategories.Issuesforplanningfortheinflationprobearediscussedin§2danditsscienceisdescribedin§4b.

3) WeassumethattheESAL2ATHENAmissionwillprogresswithNASAparticipationtoa stageofdevelopment such that itwillnotbereviewedby the2020DecadalReview.Ifthisisnotthecase,thenweurgeNASA,incoordinationwithESA,tomakeappropriatepreparationsforpresentingATHENAtothe2020DecadalReview.

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4) Theflagshipmissionstudiesshouldfulfillscientificobjectivesbroadly,totheextentpossible.WeencouragetheSTDTstobuilduponthesePCOSthemesinplanningthescientificcapabilitieswhendevelopingtheflagshipmissions.WehavedescribedscientificthemesofparticularinteresttothePCOScommunityin§3.

5) Forpossiblefuturedevelopmentofprobemissions,wenotethehistoricalsuccessoftheNASAExplorermissionlineofcompetedmissions,anditsstrongsupportinthescientificcommunity.WeanticipatethatthescientificcommunitywillbenefitfromNASAguidanceindefiningtheprobe‐classmissioncategoryorcategoriesaspartoftheprocessofdevelopingprobemissionsforthe2020DecadalReview.

Issuesdiscussedinthecommunityforplanningforprobesarediscussedin§2eand examples of potential PCOS science that can be accomplished as probemissionsarediscussedin§5.

2b. Preparing the L3 Gravitational‐Wave Mission for the 2020 Decadal 

Witha longhistoryofstrongsciencerecommendations,LISA is thehighestranked largemission of the Astrophysics Division afterWFIRST. Changes in thebudgetarylandscapehavemeantthatNASAhasbeenunabletoinitiateanewstartforLISA inthe2010decade,andfinancialsupport forLISAhas fallensignificantlybelowthatrecommendedinthe2010Decadalreport.FollowingtheterminationofthejointNASA‐ESALISAproject,ESAdecidedtopursueasimilarmissionwiththeselectionofthe"GravitationalUniverse"themefortheL3launchopportunityintheCosmicVisionsprogram.

TheAstrophysicsDivisionwhitepaper“Planningforthe2020DecadalSurvey”mentionsplansforindependentNASAstudiesandtechnologydevelopmentactivitiesin this decade thatwill lead toward realization of a GravitationalWave Surveyorthrough U.S. participation in ESA’s L3 “Gravitational Universe” mission. The GWcommunity endorses these plans for NASA investment in a LISA‐like mission,acknowledgingthataNASApartnershipinL3representsanimportantopportunity.Thecommunityfeelsstrongly(andunanimously)thataviablepartnershipwithESAneedsimmediateactionfromNASA.

WhileitiscrucialtocloselyengageinESA’sLISAPathfindermissionandL3technology preparation in pursuit of a vigorous U.S. role, realization of NASAparticipationinsuchaGravitationalWaveSurveyormissionwillalsorequireastrongrecommendation in the2020DecadalSurvey.Thenextdecadal committeewillbeabletodrawonmaterialspreparedforthepreviousdecadalsurvey,andtheresultsofESA‐ledactivities. But furtherstudyisrequiredtoassessnewdevelopments inscience and technology that impact the science case, cost, risks and technicalreadiness, particularly in relation to theU.S. role in L3. The goals of such a studyshouldencompassthefollowing:

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1. Evaluate all technologies needed to realize the gravitational wave missionarchitecture guided by ESA’s and NASA’s latest studies and define thecombinationsoftechnologiesthatarecompatiblewiththeneedsandcapabilitiesofthespaceagenciesinvolved.

2. Define a range of options for various potential NASA contributions to the L3

mission.ThestudyshouldevaluateasetofperhapsthreeorfouroptionswhichspanalargerangeofpotentialNASAcontributionstoevaluatetheirsciencereturn,costandrisk.

3. AssessthesciencecasesforU.S.participationoptions,andupdatethesciencecase

forLISAbasedonknowledgegainedsincethepreparationforthe2010DecadalSurvey.

4. AssessthesizeandscopeoftheU.S.LISAScienceTeamandneedsforaU.S.Data

CentertosupporttheL3collaborationinscience,technology,anddataanalysis.2c. Preparing the Inflation Probe Mission for the 2020 Decadal 

The Inflation Probe requires appropriate NASA investment as well ascontinuedscientificandtechnical leadershipbytheU.S.scientificcommunity. The2010DecadalSurveyprovidesguidancetoNASAregardingtechnologiesrequiredtodevelopthenextgenerationorbitalmission,referredtoastheInflationProbe.Thismissionwasidentifiedasastrategicastrophysicsobjectiveinthe2014Roadmapandasasciencegoalinthe2010DecadalSurvey.TheDecadalSurveycalledforarobusttechnologydevelopmentprograminpreparation for themissionaswellasamid‐decade review of the science and technology requirements to support theseobjectives.Inparticular,thedecadalcommitteeidentifiedsystemsfordetectingthepolarization of the CMB as a high‐priority science need for mid‐term technologyinvestments,andrecommendedanaugmentationofsupporttorampuptowardtheendof thedecade. Thisplanbuildsuponthe field’s longandsuccessfulhistoryofdemonstratingnewtechnologiesinground‐basedandsub‐orbitalexperiments.

TheconsensusviewoftheU.S.communityisthattheappropriateclassfortheInflationProbeCMBpolarizationsurveyorisaprobeclassmission,whichshouldbestudied for presentation to the 2020 Decadal Review. It will be important inconjunctionwiththisstudytoremaincognizantofrelateddevelopmentstowardaspace‐based CMB polarization observations including the status of the JAXA andNASAMOLitebird,NASAMidex,andESAM‐classmissionstudies.

InpreparationfortheInflationProbe,investmentsshouldbemadetoupdatedesignsandcost‐estimatesforaprobe‐classmissioninadvanceofthe2020DecadalReview to refine the detailed mission specifications. Realizing the robust andincreasingsupportfortechnologydevelopmentcalledoutinthe2010DecadalReportwill be also be essential. Finally, in addition to the complementary ground‐basedtechnologydevelopmentandobservations,theballoon‐borneprogram,alsoexplicitlyendorsedbythe2010DecadalReview,mustcontinueinordertoaccessfrequency

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bandsnotavailablefromthegroundandtocompletetheadvancementofmission‐criticaltechnologiestothefinalstage.

Finally,wenotethatasegmentoftheCMBcommunityhasbeenadvocatingforanewground‐basedCMBpolarizationprogramcalledS4.WefeelS4isnaturallycomplementarywiththeInflationProbe,asground‐basedexperimentsarerequiredformeasuringfineangularscales,whileasatelliteoffersunsurpassedmeasurementsof large angular scales with complete spectral coverage for comprehensiveforeground removal. NASA planning should be coordinatedwith these efforts tomaximize the natural synergies between ground‐based and space‐basedobservations.Theoverlapinobjectivesprovideanaturalopportunitytoenhancethesciencereturnsfrombothintermsofchallengessuchascontrolsystematicerrors,foreground component separation, and cross‐calibration. Technological synergieswithS4shouldalsobeactivelyinvestigatedandleveragedfortheInflationProbe.2d. Importance of Probe‐Class Missions 

Ouroutreachforflagshipmissionplanninguncoveredstronginterestinprobemissionsinthescientificcommunity.WeunderstandthatNASAmaydevelopprobemissions for the 2020Decadal following the current charge on flagshipmissions.Thereforewesummarizetheinputwereceivedwhileavoidinginterpretation,inthehopethatthesecommunityreactionswillhelpNASA’splanningprocess.

Enthusiasmfordevelopingprobemissionsasavitalcomponentforplanningthe

nextdecadewaswidespreadandstronglyexpressed.Thecommunityfindsboththecostandscheduleofprobemissionsattractive. Comparedtothepriceanddevelopment time for a flagship, several probe missions could be flown in adecade,leadingtorapidsciencereturnacrossabroadscientificspectrum.Thishigherrateofmissionsmayofferscientificsynergies,suchasmulti‐wavelengthobservationalcapability.

Manyinthecommunityareinterestedindevelopingspecificprobemissions,andanumberofnewprobe‐classconceptswerebroughtbeforethePhysPAG.Thesegenerallycovernewscientificterritoryoutsideofthereachofforeseeableflagshipmissions.Wegiveageneralizedlistofthesenewmissionconceptsfromtheinputwereceivedin§5.

Many in the community stressed the importance of the cost and schedule

disciplineof theNASAExplorerprogram,whichhasreturnedexcellentsciencewhilecarefullymanagingcosts.TheseproponentsreasonthatExplorermissionsarelesssusceptibletothelargeandunfortunatecost,scopeandschedulegrowthencounteredinrecentflagshipmissions.Thisgroupadvocatesforanexpansionof the Explorer program to largermission categories, and that developing theparameters of a category (or categories) of larger competed Explorers is asimportantasdefiningparticularscientificconcepts.

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Somepointed out that Explorermissions are not currently integrated into thestrategicscientificplanningprocess,inthattheyfollowanopenproposalprocessoutsideof thescientific investigationsdirectedby theDecadalReview. Othersnoted that the planetary community has incorporated a degree of strategicplanningfortheirlargercompetedDiscoveryandNewFrontiersmissionsintheirmostrecentdecadalreview.

Asafollow‐ontotheflagshipmissioncharge,thePhysPAGwouldlookforward

toassistingNASAinplanninghowprobemissionsaredevelopedforthe2020DecadalReview.Whileitseemsthatdevelopingspecificpointdesignswouldbeappropriate,itmay be useful to first clearly define the parameters of a competed probe‐classmissioncategory(orcategories).Thesepreparationsshouldbeplannedtogivethe2020DecadalReviewsufficientinformationtoevaluate1)whetherprobemissionsfollowtheflagshipmodeloraremanagedascost‐cappedcompetedmissions,and2)whatbalancebetweenflagshipandprobemissionsisoptimalforthenextdecade.

3.  PCOS Science Themes in Flagship Missions a. X‐Ray Surveyor Mission 

TheNASAAstrophysicsRoadmap“EnduringQuests,DaringVisions”findsthatanX‐raySurveyorisrequiredtoaddressavarietyoffundamentalquestionsaboutcosmic astrophysics. The Roadmap characterizes the X‐ray Surveyor as an X‐raytelescope with ~3 square meters of collecting area and sub‐arcsecond angularresolution,equippedwithinstrumentsdeliveringimagesandhighresolutionspectra(resolving power ~3000) over a broad spectral band (0.1 – 10 keV). Notionaltelescope and instrument parameters are listed in the Table of Flagship Missionparametersin§2e.1

TheRoadmapdiscussesalargenumberofastrophysicalproblemsthatX‐raySurveyor couldattack.These include, amongothers, thephysicsof starsof super‐nucleardensity;thenatureofsupernova‐drivenfeedbackmechanismsthataffectthechemicalcompositionanddynamicsofgalaxies; thehistoryof supermassiveblackholes as encoded in the demographics of black hole spin; the behavior ofmatteraccretingatblackholeeventhorizons;andthemechanismsproducingrelativisticjetsextendingovermillionsof light years.Herewehighlight several other compellingscientific drivers for X‐ray Surveyor of relevance to the Physics of the Cosmosprogram and to broader astrophysics: the origin of the first supermassive blackholes; thephysicsof feedback linkingblackholes to thegalaxiesandclusters that

1Inresponsetothereportofthe2010DecadalSurvey,NASA’sAstrophysicsDivisionhas stated its intention to participate in the development of the European SpaceAgency’slargeATHENAX‐raymission,andisnowtakingstepstodoso.TheX‐raySurveyormissionconceptdiscussedherewouldprovideanorderofmagnitudebetterangular resolution than ATHENA and would have high‐resolution, soft X‐rayspectroscopic capabilities that ATHENA lacks. These differences enable X‐raySurveyortoaddressverydifferentscience.

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surround them; and the growth of cosmic structure and its influence on galaxyevolution.

Theoriginandgrowthof the first supermassiveblackholes. A driving scienceobjectiveforX‐raySurveyoristounderstandtheoriginandenigmaticgrowthofthefirstsupermassiveblackholes,andtotracetheirco‐evolutionwiththeirhostgalaxies.Blackholesaslargeas109Msunhaveevolvedtobequasarsbyredshiftsof6to7(asearlyas0.75GyraftertheBigBang),butwedonotknowhowblackholessomassivecametoexistsoearlyincosmichistory.Nordoweunderstandthenatureandmassoftheseedblackholesfromwhichthesesupermassiveobjectsmusthavegrown.X‐RaySurveyorwilldetectthecentralblackholesintheearliestgalaxiesdetectedbyJWSTatredshiftsoftenandhigher. Itwillrevealthephysicalprocessesbywhichthese objects have grown by observing black holes in their youth,with sufficientsensitivity to detect seed black holes as small as 104 Msun at z=10 (under thereasonable assumptions of Eddington‐limited accretion, with 10% of bolometricluminosityinthehardband). Moremassiveseedswillbedetectedevenearlier incosmictime.X‐raysareoptimalprobesofthesehigh‐redshift,moderate‐mass(M<106Msun) seedblackholesbecause the spectralpeakof their emission shiftswithdecreasingmasstowavelengthsshort‐wardoftheUV,becauseX‐rayspenetratehost‐galaxydustthatwouldobscureUVandopticalemission,andbecauseatz≳10theIRsignatures of AGN activity are red‐shifted out of the JWST band. This scientificobjectiveisakeydriverofX‐RaySurveyor’sangularresolution,whichiscrucialtoitscapabilitytostudysuchfaintobjects.X‐raySurveyor’ssourcedetectionfluxlimitisalmost100timesfainterthanATHENA’s,asthelatterislimitedbysourceconfusion(seeFigure1).

Figure1SimulatedDeepFieldswithJWST(left)andX‐raySurveyor(Center).Eachis2arcminonside. JWSTdetects~2x106galaxiesdeg‐2.X‐raySurveyorresolvesthesewithoutconfusion(0.03galaxiesper0.5”beam).A4MsX‐raySurveyorexposure(sameastheChandraDeepField)detectsa104Msunblackholeatz=10(Lx1041ergs‐1).Atelescopewiththesameareabut5”resolution(right)isconfusionlimitedatthisdepth.AGNareshowninmagenta;normalgalaxiesingreen.[Gaskin,etal.2015].

X‐raySurveyorwillalsoprobethehostgalaxiesoftheM~109Msunblackholespoweringz~6‐7Sloanquasars.Thesemustbethemostmassivegalaxies,andhenceresideinthemostmassivedarkmatterhalos(M~1012Msun)tohavecollapsedbythatepoch.TheexpectedX‐rayluminosityofthehothalogassurroundingoneofthesegalaxies is only a small fraction of the quasar’s, and X‐ray Surveyor’s angular

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resolution is necessary to separate quasar from halo emission, enablingmeasurementofthegastemperatureandtherebyhalomass.ThePhysicsofFeedbackandAccretioninGalaxiesandClusters.X‐raySurveyor’scapabilitiesarealsodrivenbytheneedtounderstandthephysicsofgalaxyassemblyatmorerecentepochs.InMilky‐Waysizedgalaxies,modelspredictthatasubstantialfraction(asmuchas1/3)ofthebaryonsresideinhot(T≳106K)gasextendingfarbeyondthestellarcomponent.Thesehotcoronaemustplayasignificantrolein,andcontainquantitativeinformationabout,thefeedbackprocessesthatbalancegravityandcoolingtoregulategasaccretionand,ultimately,starformationasgalaxiesevolve.Atpresentourobservationalknowledgeofthesehotcomponentsofspiralgalaxiesislimited to a few nearby objects. X‐ray Surveyor will characterize the quantity,compositionandenergycontentofthehotgasinMilky‐Way(andlarger)sizedgalaxycoronaeout toz~1. Itsangularresolution iscrucial to itscapability tomapthisemissionwhile clearlydistinguishing it fromany centralAGNand fromunrelatedbackgroundandforegroundsources.

IntheimmediatevicinityoftheAGN,high‐resolution,time‐resolvedsoftX‐raygrating spectrawill measuremass outflow rates inwinds, providing quantitativeinformationaboutthematterandenergyinjectedbysupermassiveblackholesintotheir host galaxies. X‐ray Surveyor observations of spectral variability will alsoilluminate the physics of supermassive black hole accretion and jet formation,collimationandreacceleration.

X‐raySurveyor’sspectralmaps,withanunprecedentedcombinationofspatialandspectralresolution,willyieldnewinsightintocosmicplasmaphysicsinawidevarietyof astrophysical settings. Spectral imagesof galaxy clusterswill reveal thekinematicsandcharacterize thephysicalmechanismsresponsible for thebubbles,ripples,andfrontsdetectedbyChandra.Thesesmall‐scale(fewarcsecond)featuresmustberesolvedtounderstandturbulentheating,radiativecoolingandmatterflows,andplasmatransportpropertiesinclusters.X‐raySurveyorwillresolvescalesdowntotheCoulombmean‐free‐pathinclustercoolcores,providingquantitativephysicalunderstandingoftheenormousclusterfeedbackloopsthatoperateoverlengthscalesspanningmorethantenordersofmagnitude.Inpulsarwindnebulaeandsupernovaeremnants, X‐ray Surveyor will enable high‐statistics spectroscopy on fine spatialscales,contributingtoourunderstandingoftheenergeticparticleacceleration.GalaxyEvolutionand theGrowthofCosmicStructure: Justasgalaxiesco‐evolvewith the black holes they host, so also are they shaped by the growing cosmicstructuresenvelopingthem.Withtheformationofgalaxygroups,galaxiesexperiencemore frequent mergers, and interact via ram pressure with the hot intragroupmedium. These processes can in turn induce nuclear activity and modulate starformation.X‐raySurveyorwilldetectthefirstgroupsatredshiftsaslargeasz~6,and,crucially,willresolveAGNfromtheextendedintragroupmedium.Itwilldetectanddistinguishthethermalemissionof intragroupplasmafrom(non‐thermal) inverseComptonX‐rayemissionfromradiojets.

X‐ray Surveyor will open a new window onto the enrichment of the IGM,including the roles ofdifferent typesof supernovae, andhowwinds andoutflows

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drivenbysupernovaeandAGNhelpedseedthe intergalacticmediumwithmetals.Critically,itwilltrackalloftheseprocessesfromearlytimes,throughthepeaksofcosmic star formation and AGN activity, to the present epoch, providing acomprehensiveviewofthephysicslinkingblackholes,galaxiesandcosmicstructure.

X‐raySurveyorwillalsorevealandstudyasubstantialfractionoftheas‐yetunobserved,un‐virializedbaryonsinthelocalUniverse,andinsodoingwilltracethecosmicwebofdarkandbaryonicmatterlinkinggalaxies,groupsandclusters.ModelspredictthatasmuchashalfofthehotbaryonsinthecosmicwebwillbedetectableinemissionatX‐raySurveyor’ssurfacebrightnesslimit(about1/30thatofChandra’s)in the0.5‐2keVbandeven in theabsenceof significantmetalenrichment. Theseimages will trace density enhancements as low as ρweb/ρmean = 30. If the web isenriched to the extent expected from simulations and suggested by currentlyavailableobservationsinclusteroutskirts,X‐raySurveyor’shigh‐resolutiongratingspectroscopy will detect the baryonic web in absorption spectra along manysightlines to background sources. These spectra will illuminate the history andphysics of the web’s baryons by diagnosing their composition, ionization state,temperatureandkinematics.b. Far‐Infrared Surveyor Mission 

Star formation and black hole accretion are two of the most importantprocessesthatshapetheevolutionoftheUniverse,andtheybothoccurembeddeddeeply within interstellar gas and dust. The Far‐Infrared Surveyor probes theseregions indistantgalaxieswith infrared linesandcontinuumatwavelengths>30microns. The FIR Surveyor can answer fundamental questions about growth andevolutionofstructureintheuniverse,astracedbydusty,star‐forminggalaxies,andthereby obtain clues to the nature of dark energy, measure certain cosmologicalparameters,testgravityatcosmologicallengthscales,andstudythenon‐Gaussianityofthedensitydistribution.ThekeycosmologicaldatasetsfromtheFar‐IRSurveyorwillbe1)confusion‐limitedsurveysinbroadcontinuumbandsoverafewthousandsquaredegreestothewholesky,and2)blindspectroscopicsurveyscoveringtenstothousandssquaredegreeswithcomplete3‐Dinformationprovidedbythebrightfar‐IR emission lines of [CII] 158μm, [OI] 63μm and [OIII] 52μm and 88μm. Thespectroscopicsurveyisparticularlyunique,andweanticipate2milliongalaxiestobedetectedblindly(automaticallyincludingredshifts)inthismode.Theirclusteringcanbeusedtoconstraincosmology,asistraditionallydoneingalaxysurveys.

The FIR Surveyor survey complements Euclid and WFIRST by extendingclustering measurements to early times. The FIR Surveyor dataset of 2 milliongalaxiesisadequatetodetectthebaryonacousticoscillation(BAOs)in4‐5redshiftbinsbetweenz=1to3.5toafewpercentaccuracy.EuclidandWFIRSTwillperformBAO measurements using redshifts secured from grism spectra in the near‐IR,primarilycoveringthez=1‐2withHαand,inthecaseofWFIRST,tozof3with[OIII].In comparison to the FIR surveyor, Euclid will conduct a survey over 15,000 sq.degreeswithanumberdensityofabout0.4galaxiespersq.arcmin,whileWFIRST‐AFTAwillextendthedepthdowntothelevelof3galaxiespersq.arcminover2000sq. degrees. The FIR Surveyor will reach a shallower depth at higher redshifts,

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compensatedby thehigherbias factorof thedusty, star‐forminggalaxiesatz>2.Moreover,weexpecttheredshiftdistributionoftheFIRSurveyortopeakatz>2,maximizingitssensitivitytoclusteringmeasurementsovertherangez=2‐3whereWFIRSTwillbesub‐optimalduetosingle‐linelineconfusionassociatedwith[OIII]emittersin1.35‐1.95umwavelengthrangeofitsgrism.

ClusteringmeasurementswiththeFIRSurveyoratzof2‐3willbridgethegapbetweenthedistancetoCMBandgalaxysurveys.The1‐2%distancemeasurementsinfourbroadzbinswillprovideanimprovedtestoftheevolutionofthedarkenergyequation of state, w(z), and a constraint on the early dark energy models thatpostulateaverylowlevelofdarkenergydensitythatremainsaconstantatz>2.TheBAO‐baseddistancescorrespondtoaconstraintonanearlydark‐energycomponentatz>2below0.01.Forcomparison,thecurrentconstraint,withPlanckCMBandCMBlensingis0.06(68%confidencelevel).

The3DclusteringfromtheFIRSurveyor,andintensitymappingin3Dspectralcubesthatmakeuseofalldatanotjusthigh‐significancelinedetections,alsooffersthepotentialtoprobeprimordialnon‐Gaussianitywithpeaksensitivityatz=2‐3.Theconstraintcomesfromthefactthatthegalaxybiasonlargescaleshasacorrectionarisingfromthenon‐Gaussianityparameter.Thisisbesttestedatverylowredshiftsbutameasurementorevenaconstraintatz>2ishelpfulsincesurveyscapableofstudyingthisathighredshiftsarecurrentlylimited.TheexpecteddepthoftheFIRSurveyor dataset is such that we are able to constrain fnl, the non‐Gaussianity

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parameter,downtoalevelof9(68%confidence).Thecombinationoffnlparametersmeasuredwith CMB at z of 1100, galaxy surveys at z < 1,with the FIR surveyorconstraintatzof2to3,willprovidetheredshiftevolutionofthenon‐Gaussianityandrule out extrememodels that have significant redshift evolution over the cosmicepoch. Far‐IR sources,witha redshiftdistribution thatpeaksat z=2 ‐3, arealsousefulforacross‐correlationstudies.Inparticulardusty,star‐forminggalaxiestracethe foreground dark matter potential that is responsible for lensing of the CMB.Studiessofarfocusonfieldsoforder100sq.degrees,withSPTandPOLARBEARandHerschelmaps(Holderetal.2013;Adeetal.2014).TheFIRSurveyorwillextendsuchstudiesto1,000‐10,000sq.degreesandwithCMBpolarizationB‐modelensingdatacoming from next‐generation ground‐based CMB‐S4 experiments. The cross‐correlationcanbeperformedwitheitherthefar‐IRgalaxylocations,selectedbasedoncolorandfluxcuts,orwithCIBintensitymapsasawhole,againwithsomecolorbasedonthecombinedcontinuummaps.Theredshiftdistributionforfar‐IRsourcescan be calibrated through the FIR Surveyor spectral surveys. The CMB‐S lensingstudieshavethegoalofmeasuringthesumoftheneutrinomassesdowntothesub‐0.05eVleveltodistinguishbetweennormalandinvertedmasshierarchies fortheneutrinos(Abazajian etal.2015).Thecross‐correlationwithanexternaltracerasproposedherealsohastheadvantagethattheimpactofsystematicscanbereducedfortheproposedmeasurement.

Moving beyond clustering, the FIR Surveyor is capable of providing anindependentmeasureofcosmologicalparametersthroughtwoadditionaltracersofcosmology. First will be the large sample of bright gravitational lenses that it isexpectedatfar‐IR/sub‐mmwavelengths.UnlikethecasewithHerschelthatrequiredsignificantground‐based follow‐up, theFIRSurveyorwillhave theadvantage thattheselensingeventswillalsohaveaccompanyingredshiftmeasurements.AndiftheFIRwideareasurvey isconductedona fieldsuchasWFIRSTHLSwithsignificantancillarydatawewillalsohaveredshiftsfortheforegroundgalaxies.Thelensingrate,and the distribution of foreground lens and background source redshifts, arecosmologydependent.Withacompletesampleoflensingevents,downtoafixedfluxdensity,weareabletomeasuredarkenergyandothercosmologicalparameters(e.g.,MERLIN radio lens surveywith 11 radio lenses over 5000 snapshots; Chae et al.2002). Finally, the statistics of proto‐clusters of galaxies thatwill appear as star‐forming galaxy clumps (e.g., Casey et al. 2015) will also trace the underlyingcosmology.Ascurrentdetectionsarelimitedtoafew,theyhavenotbeendiscussedwidelyascosmologicalprobesinthesamewaygalaxyclustersareused.WeexpecttheFIRSurveyorwillencouragethesimulationandtheorycommunitytostudytheconnectionbetweencosmologyandthestatisticsofproto‐clusterssotheexpecteddatacanleadtocosmologicalconstraints.

Moving to the epoch of reionization and pre‐reionization sources, the Far‐Infrared Surveyor is capable of searching for the molecular hydrogen cooling inprimordialhalos thataresitesof first stars in theuniverse.Suchhaloshavevirialtemperaturesbelow104Kandmolecularhydrogenistheonlycoolingmechanism.Duringreionization,H2duetoturbulentenergydissipationinproto‐galaxiescouldalso be detected. These generally require deep observations of strongest lensing

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clusters, similar to a Hubble Frontiers Fields program. From reionization, thestrongestH2lineisat10um(Gongetal.2012).Withmultiplelinesdetected,H2willallowadeterminationofmass,excitation,andtotalcoolingpower,keyingredientsnecessarytounderstandtheconditionsofprimordialgasconvertingtostarsinanextremeenvironmentoftheprimordialuniverse.c. UV/Optical/IR Surveyor Mission  TheLUVOIRsurveyorisaconceptforan8‐meterto16‐meterUVOIRspaceobservatory.LUVOIRwillallowastronomerstoanswerfundamentalquestionsattheforefrontofmodernastrophysics.WediscussheretherelevanceoftheLUVOIRforthe PhysPAG science by highlighting few particularly promising scientificopportunities.UnravelingGalacticDynamicswithProperMotions.ThemysteriousdarkmatterthatdominatesthemassoftheUniversebindsandorganizesalllarge‐scalecosmicstructure,includinggalaxiesthattracethem.Thenatureofthedarkmatterremainsunknown,andyetitsgravitationalinfluenceonthedynamicsofstarsandgalaxiesisclear. Themotions of stars on the sky (their propermotion) are sculpted by theunderlyingdark‐matter‐dominatedgravitationalpotential,andthustheyrevealthetruestructureandmovementsofthegalaxiesthosestarsinhabit.Hubblehaspushedthis astrometric proper‐motion technique to its limits, searching nearby stellarclustersforsignsofblackholesandtrackingtheorbitsofgalaxiesintheLocalGroup.JWST’scomparableprecisionwillallowthisworktocontinue,yieldinglongertimebaselinesforfieldspreviouslystudiedwithHubble.WhiletheforthcomingGaiaandWFIRST space missions will measure proper motions over huge fields, revealinginternalMilkyWaydynamicswithbillionsofstars,eventhesemajoradvancesarenotenoughtorevealthetruenatureofdarkmatterortoresolvethedetailedmotionsofdistantgalaxies.Incontrast,LUVOIRhasthespatialresolutionatvisiblewavelengthsto achieve an entirely new level of precision inmeasuring the effects of dark onnormalmatter.Thekeytothesegainsisitsaperture:at12meters,with10masspatialresolution,millipixelastrometry,andrapidhigh‐S/Nphotometry,HDSTforexample,willbeabletomeasurethemotionsofstarsontheskytounprecedentedlowlimits.AtthelimitsachievableintheMilkyWay,virtuallyeverystarcanbeseentomove.Measurementsofthisprecisioncandirectlyconstrainthenatureofdarkmatter.Inthe ultra‐faint dwarf satellites of the MilkyWay (and the hundreds of additionaldwarfs thatLSST is likely toreveal),baryonscontributeessentiallynothing to themass of the galaxy, making their stars ideal tracer particles of the dark matterpotential.PrecisionMeasurementsoftheGrowthofStructure.Thebulkofthestellarmassassembledingalaxiesbetweenz~3andz~1,bywhichtimetheHubblesequencewasessentiallyinplace.Topreciselytestthegravitationalinstabilityparadigmwemustfollowthegrowthofmassandstellarpopulationsoverthiskeycosmicperiod.Thetotal(dynamical)massesandmetallicitiesofgalaxiescanbemeasuredbymeansofgaskinematicsfromabsorptionspectroscopyofbackgroundgalaxies,ifsufficient

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spectroscopicsensitivity isavailable,essentiallyusinggalaxies in thesamewayasquasarshavebeenusedtostudytheIGM.Galaxiesarevastlymoreabundantthanquasars,especiallyathighredshifts,by~100‐folddowntoacommonfluxlevel.Thisallowsonetoperformspatiallyresolvedkinematicsofindividualforegroundgalaxies.An8‐mto16‐mUVOIRspacetelescopecanuniquelyprobethe1<z<3.5rangewheretheabsorptionlinesaresuperposedonnearlyfeaturelesspartsoftheemitter’srest‐UVspectrum.DarkMatterHaloMapswithStrongLensing.LUVOIRwillimproveindarkmatterhalomaps of clusters and galaxies using high‐resolution images of strong lensingsystems.Near‐infraredimagingwiththeHubbleSpaceTelescopehasalreadytracedtheearliestdetectedseedsofmoderngalaxiestowithinafewhundredmillionyearsof the Big Bang. This process startedwith the Deep Field in 1995 and continuedthrougheachnewgenerationofHubbleinstruments,allowingHubbletopushbacktheredshiftfrontierandidentifythebrighteststar‐forminggalaxiesatredshiftz~10,when theUniversewasonly400millionyearsold.Theon‐goingHubble “FrontierFields” are carrying this search to potentially higher redshifts and lower galaxyluminosities,byusinggravitationallensingtomagnifythemoredistantuniverseinselect regions where the lensing effect is particularly strong. LUVOIR’S higherresolution and greater sensitivity match the typical increase in resolutionand depthofferedbygravitationallensing,andthusLUVOIRcouldeffectively carryoutthesamesearchasthe FrontierFields,butanywherein theUniverseitchoosestopoint.Ultra‐FaintSatelliteDynamic.Dwarfspheroidalgalaxies(dSph)areextraordinarysites to explore the properties of non‐baryonic dark matter (DM). Their mass isdominatedbyDM–theyareobservedtohavemass‐to‐lightratios10to100timeshigherthanthetypicalL*galaxy.Theyareabundantnearby–todate23dSphgalaxieshavebeenfoundintheLocalGroup.Moststriking,isthediscoverythatnearlyalldSphsatellites,coveringmorethanfourordersofmagnitudeinluminosity, inhabitdarkmatterhaloswiththesamemass(∼10)withintheircentral300pc.TheabilityofDMtoclusterinphasespaceislimitedbyintrinsicpropertiessuchasmassandkinetictemperature. ColdDMparticles havenegligible velocity dispersion and very largecentralphase‐spacedensity,resultingincuspydensityprofiles.WarmDMhalos,incontrast,havesmallercentralphase‐spacedensities,sothatdensityprofilessaturatetoformconstantcentralcores.ThemeandensityprofileofdSphgalaxiesis,thus,afundamentalconstraintonthenatureofdarkmatter.TomeasuretheDMprofiles,oneneedstomeasurethepropermotionsandtheradialvelocitiesofstarsindSph.Bothmeasurementsareneededtobreakdegeneracybetweentheinnerslopeoftheprofileandvelocityanisotropiesofstellarorbits.FundamentallynewconstraintsonDMcanbereachedbydeterminingpropermotionsfor~100starspergalaxywithaccuracy<10km/s(<40 as/yratadistanceof60kpc)plus~1000radialvelocities(cf.Bullocket al. 2009, Astro2010White Paper). LUVOIR will have the required astrometricprecision to perform these proper motion measurements owing to its very highwavefrontstabilityanditswide‐fieldimagingcapability(Postmanetal.2010).

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Multi‐messengerastronomy.Oneofthemostrapidlygrowingareasofastrophysicsin the next decadewill bemulti‐messenger time domain astronomy. By themid‐2020s, several powerful ground‐based wide‐field synoptic surveys will be in fulloperation, including the Large Synoptic Survey Telescope (LSST) at opticalwavelengths, the Square Kilometer Array (SKA) at radio wavelengths, and theAdvancedLIGOandVirgodetectors forgravitationalwaves. At theredshift limitsshown for single visits by LSST (one night’s observation), LUVOIR will reachlocalizationprecisionsufficienttoidentifythestellarpopulationsgivingrisetotheexplosivephenomena,andwithitsspectroscopiccapabilities,measurethemetallicitylocaltotheexplosion(animportantdriverofstellarevolutionandmassloss).Therest‐frameUVcapabilityofLUVOIRisparticularlyimportantforprobingyoung(hot)SNe,andcanbeusedasasensitivediagnosticoftheirprogenitormassandradii,andexplosionenergies.TheUVandopticaldiagnosticsfortheageandmetallicityoftheprogenitor star and its host population are more effective than the near‐IRdiagnosticsthatwouldbeusedbyground‐basedtelescopes,especiallyatyoungages.The deep magnitude limits of LUVOIR are not achievable from the ground, andLUVOIR will be the only telescope capable of following the evolution of a faintoptical/near‐IRcounterpart(postulatedtobeakilonova)toalocalizedgravitationalwavesourceforlongperiodsaftertheevent.d. Habitable‐Exoplanet Imaging Mission 

Whileexoplanetsurveysaregenerallynotintendedtofurthertheexplorationofnewlawsofphysicsinthecosmos,wefindthereareseveralscientificthemesthathavesomeconnectionwithapplicationsofPCOSscienceinquestionsofdynamicsandgravitation. TheHABEXdevelopmentmay find it fruitful toaddressastrophysicalissuesofinteresttoPCOSinthecontextofplanetformationandevolution,suchas:

Howdoterrestrialplanetsformgiventhetendencyofcollidingrockybodiesto

bounce or fragment, and for bodies growing in gaseousdisks to spiral inwardtowardthehoststar?

Whatistheroleofphasetransitionsinplanetformation?Howdoplanetsformwithinthesnowlineacquirewaterandothervolatilesnecessaryfortheoriginoflife?

Whatistheroleofgasdynamicsinearlystagesofplanetformation?Howdothegasdynamics(laminarvs. turbulent flow)andchemistryof theprotoplanetarydisk affect the growth and composition of protoplanets that may becomehabitable?

What is the role of external and internal ionization on early stages of planetformation?Howdoterrestrialatmospheresevolveoverlongertimescalesastheyinteractwithradiationandparticlesfromthehoststarandbombardmentfromsolidsintheinterplanetaryenvironment?

How do gravitational dynamics affect later stages of planet formation andplanetary system evolution? How important are resonant orbits and majorgravitational events (e.g. Nice and Grand Tack models of planetary orbitexchange)forforminghabitableplanets? 

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e. Table of Flagship Mission Parameters 

X-Ray Surveyor Far-Infrared Surveyor UV/Optical/IR Surveyor Habitable Exo-Planet Imaging Mission

Primary Science Goals: • Origin & growth of 1st

supermassive black holes • Co-evolution of black holes, galaxies & cosmic structure • Physics of accretion, particle acceleration and cosmic plasmas Measurement Requirements: • Chandra-like (0.5”) angular resolution • Detection sensitivity ~ 3 x 10-19 erg cm-2 s-1 • Spectral resolving power: R>3000 @ 1 keV; R~1200 @ 6 keV Architecture and Orbit: • Eff. area ~3 m2 • Sub-arcsecond angular resolution • High-resolution spectroscopy (R ~ few x 103) over broad band via micro-calorimeter & grating spectrometer instruments • FOV ≳ 5’ • Energy range ~0.1-10 keV • Orbit: L2 likely

Primary Science Goals: • History of energy release in galaxies: formation of stars, and growth of black holes. • Rise of first heavy elements from primordial gas. • Formation of planetary systems and habitable planets. Measurement Requirements: • Spectral-line sensitivity better than 10-20 Wm-2 in the 25-500 m band. (5, 1h) • Imaging spectroscopy at R~500 over tens of square degrees. • R~10,000 imaging spectroscopy of in thousands of z<1 galaxies and protoplanetary disks. • High-spectral-resolution capabilities desired for Galactic star-forming systems and the Galactic Center. Architecture & Orbit: • Complete spectroscopic coverage at R~500 from 25-500 um. • Monolithic telescope cooled to <4 K, diameter ~5 m. • FOV = 1 deg at 500 μm • R~10,000 mode via etalon • Background limited detector arrays with few x 105 pixels, likely at T<0.1 K. • Mission: 5 years+ in L2 halo orbit. • High-resolution (heterodyne) spectroscopy under study, possibly for warm phase.

Primary Science Goals: • Direct imaging of Earthlike planets, search for bio-signatures • Broad range of cosmic origins science Measurement Requirements: Cosmic Origins Science: • HST-like wavelength sensitivity (FUV to Near IR) • Suite of imagers & spectrographs, properties to be determined Exo-Earth Detection: • ~10-10 contrast • Coronagraph (likely), perhaps with a starshade • Optical and near-IR camera for planet detection and characterization. • IFU, R>70 spectrum of 30 mag exoplanet • 1” radius FOV Possible instrument for spectroscopic characterization of transiting planets. Architecture and Orbit: • Aperture: ~8-16m likely • Likely segmented, obscured primary. • Orbit: L2 likely

Primary Science Goals: • Direct imaging of Earthlike planets. • Cosmic origins science enabled by UV capabilities; considered baseline science. Measurement Requirements: Exo-Earth Detection: • ~10-10 contrast • Coronagraph and/or starshade • Optical and near-IR camera for planet detection and characterization • IFU, R>70 spectrum of 30 mag exoplanet • 1” radius FOV Cosmic Origins Science: • UV-capable telescope / instrument suite: properties and wavelength range to be determined. • Enable constraints on the high-energy radiation environment of planets. Possible instrument for spectroscopic characterization of transiting planets. Architecture and Orbit: • Aperture: <~8m likely • Monolithic or segmented primary • Optimized for exoplanet direct imaging, • Orbit: L2 or Earth-trailing likely.

Table1:NotionalMissionParameters. These are the notional parameters of the fourmissions,developed through coordinateddiscussionswith andbetween the threePAGs.We emphasize thattheseparametersarenotional:theyarenotmeanttoprovidedefinitiveorrestrictivespecificationsforrange of possible range of architectures to be studied by the STDTs. We encourage the STDT toconsiderarchitecturesandparametersoutsideof those indicatedhere, inorder toexplore the fullrangeofsciencegoals,andmaximizethescienceachievablebythesemissionsforagivencost,schedule,andtechnologicalreadiness.

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4.  PhysPAG Science for the L3 Gravitational Wave and Inflation Probe Missions a. L3 Gravitational Wave Mission Science Howandwhendid the firstmassiveblackholes form? The GravitationalWaveSurveyor(LISA/eLISA),willtrackthemergerhistoryofmassiveblackholes(MBHs)fromthefirstseedsoutatz~20.Blackholemergerswithmassesintheintervalbetween104Mand107Mwillbeobservedwithsignificantsignal‐to‐noiseratios,allowingtheindividualmasses,spinsandluminositydistancesoftheMBHstobemeasuredwithunprecedentedaccuracy.Therangeofblackholemassesandredshiftssamplediscomplementarytothatofblack‐hole‐poweredphenomenathatcanbeobservedelectromagnetically.Evolutionofgalaxiesand theirsupermassiveblackholes.LISAwill provide thewidestanddeepestsurveyoftheuniverseeverproduced,measuringthevastmajorityofallmergingMBHbinaries.Thiswillexposeanunseenpopulationofobjects,revealingprecisecharacteristicsoftheheartsofgalaxies,tracingtheirmergerhistoryovercosmologicaltime,andexposingthelinksbetweentheblackholeseedpopulationandthebrighttailofearlyEMobservable galaxies.LISA’smeasurementsofMBHbinarymasses, spinsandeccentricitieswillprovideinformationabouttheirgalacticenvironments.Gravitationalwaveobservationsalone will be able to distinguish between the different MBH formation and evolutionscenarios.Measure the spin distribution of supermassive black holes throughout theuniverse.Themassesandspinsof themergingblackholesare faithfullyencoded in thegravitationalwavesignals.Theindividualmassescanbemeasuredtoafractionofapercent,andthespinsmeasuredtoafewpercent,allowingprecisecharacterizationofthesurveyedpopulation.Survey compact stellar‐mass binary systems in our own galaxy and betterunderstandthestructureoftheMilkyWay.Theashesofstellarevolutionaredarkcompact objects. LISA will survey compact binary systems in our galaxy, individuallymeasuring the properties of over 104 binaries. These discoveries will shed light on theoutcomeofthecommonenvelopephase,ontheprogenitorsoftypeIasupernovaeandonbinaryevolutionphysics.Explorethepopulationsofstellar‐masscompactobjectsingalacticnuclei.LISAwillexplorethelargepopulationsofstellar‐massblackholes,neutronstars,andwhitedwarfstars, which are expected to be present in galactic centers. These compact objectsoccasionally scatter into the central MBH to form Extreme Mass‐Ratio Inspiral binaries(EMRIs), that LISA can observe out to z~0.5 to z~1, depending on the detector design,providingpreciseinformationaboutgalacticcenterswithMBHsbetween104Mand106M.

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Multi‐wavelengthstudiesofGWpopulationsandmulti‐messengerstudieswithEMobservations.LISAwill observe the growth ofMBHs through the era of structureformationwhich are the likelyprogenitors for the local populationof supermassiveBHsobservableinthepulsartimingGWband;together,theseobservationswillprovidestrongconstraintsonthemergerpopulationandevolutionaryhistoryofMBHs.AttheotherendoftheGWspectrum,LIGO’sbinarymergersbegintheirhistoryinLISA’sband. LISAsystemshavethepotentialfordirectcounterpartobservationswithfutureEMinstruments,includingguaranteed counterparts such as compact galactic binaries, and possible signatures fromMBHmergers.TeststrongGRinrichdetail.LISAwillprovidethemostprecisetestsofEinstein’stheoryofGeneralRelativityinthehighlydynamicalandnon‐linearregime.TheobservationofMBHmergers will allow us to answer fundamental questions about gravity: does gravitypropagatesatthespeedoflight?doesthegravitonhaveamass?doesgravityviolateLorentzinvariance?dogravitationalwaveshavemorethantwopolarizationstates?aretheblackhole“no‐hairtheorems”violated?.LISAobservationswillalsoallowustosearchfortheunexpected,letting the data determine whether there are anomalies, which in turn could point to asurprisingdeviationfromEinstein’spredictions.Inaddition,manyofLISA’sgalacticbinarysystemswillbepreciselymeasurablethroughbothGWandEMandobservations,allowingend‐to‐endtestsofGWemission,propagationanddetection.Mapthespacetimeofsupermassiveblackholeswithcompactobjectcaptures.ThespiralofsmallcompactobjectsintoMBHswillallowustodynamicallymapthespacetimeoftheMBHtounprecedentedprecision.SuchmapswillrevealwhethertheunseenmassiveobjectsatthecenterofgalaxiesaretheblackholesofGeneralRelativity.Revealingthegravitationaluniverse.WecanonlyspeculateaboutwhatelseLISAmayobserve. Cosmological, dynamical and gravitational lensing observations tell us that themajority ofmatter in the universe is dark and invisible, known only through large‐scalegravitationalinteraction.LISAwillpeerintothisvoid,potentiallymakingdiscoveriesthatwillrevolutionizeourunderstandingoftheUniverse.

b. Inflation Probe Science The Cosmic Microwave Background (CMB) radiation consists of a bath of

photonsemittednearly14billionyearsago,whentheuniversewas in its infancy.Observations of the small intensity variations in the CMB have led to precisemeasurementsoftheage,composition,andcurvatureoftheuniverse,andprovidestrongevidencefortheexistenceofdarkmatteranddarkenergy.Wearenowpoisedforanotherexcitingeraofdiscovery.Thesimplestmodelsofinflationpredictthatastochasticbackgroundofgravitationalwavesshouldexistthatleaveafaintsignatureon the CMB polarization. This signature can be detected and characterized withmodernpolarization‐sensitiveinstruments.

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A convincing detection of this distinctive “B‐mode” signature provides ameasure of the amplitude of gravitational waves emitted during a primordialinflationaryepoch.Suchadetectionandspectralcharacterizationwouldconfirmtheinflationary paradigm and identify the energy scale atwhich inflation took place,revealingfundamentalphysicsatenergyscalesimpossibletoachieveinaterrestriallaboratory.AcompellingcharacterizationoftheB‐modepolarizationsignature,oran absence of detection at cosmologically meaningful levels would thereforerepresent,asdescribedinthe2010NationalAcademyofSciencesDecadalReviewofAstronomy and Astrophysics, "a watershed discovery" that would significantlyadvanceourunderstandingof thephysical conditionsof theearlyuniverse.Theseeffortswere identifiedby thedecadal reviewashighpriority science. In addition,thereismuchtobegainedfromacosmic‐variancelimitedmeasurementoftheentireskyoverawiderangeoffrequenciesandangularscales.Thiswillenablethescientificcommunitytocharacterizeastronomicalforegrounds,toquantifythepropertiesofouruniverseonitslargestscales,includingadefinitivedeterminationoftheepochofreionization and to explore in polarization the temperature anomalies that mayultimatelyleadtothediscoveryofnewphysics.

Rapidscientificandtechnicalprogresshasbeenmadesincethebeginningofthedecade,andfull‐skymapsofCMBpolarizationtofundamentalsensitivitylimitsarenowtechnologicallyachievable.Thetensor‐to‐scalarratio,r,derivedfromthesemeasurements, isagaugeofthepresenceofprimordialgravitationalwaves,andameasure of the energy scale of inflation. Improved measurements of the scalarspectralindexprovideacompellingreasontobelievethatanon‐negligiblevalueofris likely. A current upper limit of r < 0.11 derived from measurements of thetemperatureanisotropyaloneislimitedbycosmicvariance,reflectingtherealitythatweonlyhaveoneuniversetoobserve.Thismeansthatpolarizationmeasurementsare required tomake further advances. Upperlimits frompolarization anisotropymeasurements have reached a notable milestone for the field ‐‐they are nowcomparableandwillsoonprovidethedominantobservationalconstraintoninflation.CombiningalloftheavailableCMBinformation,inflationarygravitationalwavesarecurrentlyconstrainedatr<0.09.

TheInflationProbemissionwillbecriticalinordertomakefull‐sky,uniformlywell‐calibratedandinterconnectedmaps,tominimizesystematicerrors,andtoavoidatmosphericeffectsbothinbandsinwhichmeasurementsareimpossiblefromtheground, and also to maximize sensitivity and foreground rejection across thespectrum.Thedevelopmentofthismissionstandsinthecontextofplansforthenextstage in ground‐based observations for which the CMB community is optimizingmappingspeedandsensitivityatfrequenciesaccessibleacrossavailableatmosphericwindowstoproducemapsoftheCMB.Asatellitemissionisuniqueinitsabilitytostudy large spatial scaleswith complete spectral coverage for removingpolarizedforegrounds.Ashashistoricallybeenthecase,ground‐based,sub‐orbital,andorbitalexperimentsarehighlycomplementary.Overlapinangularandfrequencycoveragewillbeessentialforconsistency incalibrationandsystematiccontrolbetweenthesecomplementaryapproaches,aswillarobustexchangeoftechnologicaldevelopments,analysis techniques,and jointanalysisof finaldataproducts. Inaddition,ground‐

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based and sub‐orbital efforts provide tests of experimental techniques andtechnologydevelopmentandreadiness‐raisingexperienceforspacemissions.

5.  PhysPAG Interest in Probe Missions  There is strong community support for “probe sizedmissions”,which costbetween500M$and1B$,andfillthegapbetweenExplorertypeSMEXandMIDEXmissionsandstrategicflagshipmissionscosting1B$andmore.Atthemeeting“High‐EnergySpaceMissionsinthe2020s”oftheHighEnergyAstrophysicsDivisionoftheAmerican Astronomical Society (June 29‐July 1, 2015, Chicago, IL), a number ofprobe‐sizedmissionconceptswerediscussedwhichoffercompellingsciencelyingattheheartofthescienceprogramdescribedinthe2015NASAAstrophysicsRoadmap.ProbeconceptsbroughtforthtothePhysPAGinclude:•AnX‐rayGratingSpectrometerProbeMissionthatwouldenablestudiesofthewarm‐hotintergalacticmedium(WHIM),collimatedanduncollimatedoutflowsfromaccretingsupermassiveblackholes,andburstingneutronstars. ThesecapabilitiesareaparticularlyattractivecomplimenttoESA’sATHENAmission.• A Large X‐ray Timing Observatory Probe Mission that would enable themeasurement of the equation of state of neutron star matter, and studies of thestructureofblackholeandneutronstaraccretionflows.•ATransientX‐rayAstrophysicsProbeMissionthatincorporatesawidefield‐of‐viewX‐raytelescopewithanearinfraredtelescopetoenableobservationsofhigh‐redshiftgamma‐raybursts,tidaldisruptionevents,supernovashockbreakouts,andelectromagneticcounterpartsofgravitationalwavedetectionsofmergingbinaries.•AHighEnergyX‐rayProbeMissionthatwouldenablemeasurementsofthespinsof a large (O(100)) sample of stellar and supermassive black holes, deepsupermassiveblackholesurveyswhicharenotbiasedagainstobscuredsources,andradio‐nuclear studies of Type Ia supernovae. This mission can build upon thescientificdiscoveriesofNuSTAR.•AnAdvancedGamma‐rayTelescopeProbeMissionoperatinginthekeVtoMeVenergy range for the study of the 511 keV emission from the galactic center, asupernova census in nuclear gamma‐rays, and polarimetric studies of the jets(collimatedplasmaoutflows)fromgamma‐rayburstsandactivegalacticnuclei.• An Advanced Cosmic Ray ProbeMission designed to discover the origin ofultrahighenergycosmicraysbyobservingextremelyenergeticextensiveairshowersfrom space, accumulating significantly more events at the highest energies thanground‐basedobservatories.Themissionwouldincorporateawidefieldofviewwithan ultrafast UV camera that records the extensiveair shower fluorescence and

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backscatteredCherenkovlight.Extremelyenergeticneutrinosmayalsobeobservedaswellasfastatmosphericphenomenaandmeteoroidemission.

Acknowledgements  The PhysPAGwould like to acknowledge the contributions frommembersacrosstheastrophysicscommunitywhocontributedtothedevelopmentofthereport.Whilethosewhocontributedinthewritingarelistedonthecover,manyothersmustbethankedforprovidingcomments,emails,andoralcontributionsattownhallandsplintersessions.ThePhysPAGECwouldalsoliketothanktheNASAPhysicsoftheCosmos office for their help in preparing this report, including arranging manyteleconsandconferencesessionsoverthepastyear.Finallywewouldliketothankthereport leadsfortheCOPAG(KenSembach)andExoPAG(ScottGaudi)fortheireffortsincoordinatingthejointfindingsofthethreePAGs.

 

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Appendix  The process for collecting community input and coordinating the PhysPAGreportinvolvedaseriesofopensessionsatprominentconferences,aswellasfocusedsplintermeetingsorganizedbythePhysPAGSIGstoreachouttoparticularscientificconstituencies.InadditionaseriesofplanningteleconswereheldforthePhysPAGEC,andjoint‐PAGmeetingswereheldtocoordinateourreportfindings.Alistofthesesessionsiscollectedinthefollowingtable.Table2.MeetingsandEventsHeldinResponsetoFlagshipMissionChargeDate(2015) Location Meeting

4January Seattle,WA

PhysPAGandJointPAGmeetingsattheAmericanAstronomicalSocietyconferenceGamma‐rayandX‐raySIGsplintersattheAASGW special session The Centennial of GeneralRelativityatAAS

14‐17January Minneapolis,MN InflationProbeSIGmeetingatPhysicsoftheCMBandItsPolarizationconference

5‐6February Greenbelt,MD Gamma‐raySIGmeetingatFutureofSpace‐BasedGamma‐rayObservatoriesworkshop

19March Baltimore,MD JointPAGExecutiveCommitteemeeting

11‐14April Baltimore,MD

PhysPAG session at American Physical SocietyconferenceMeetings of the Gravitational‐wave, Cosmic‐rayandGamma‐raySIGsatAPS

29April Telecon PhysPAGECmeeting22May Telecon PhysPAGECmeeting8June Telecon PhysPAGEC meeting26June Telecon PhysPAGECmeeting

1July Chicago,ILPaneldiscussionatAASHighEnergyAstrophysicsDivisionmeetingX‐rayandGamma‐raySIG meetingsatHEAD

3July Telecon JointPAGchairplanningmeeting20July Telecon PhysPAGECmeeting

7August Honolulu,HI Joint PAG session at International AstronomicalUnionconference

28August Telecon PhysPAGECmeeting

31August Pasadena,CAJoint PAG presentation at special session of theAmericanInstituteofAeronauticsandAstronautics