FundamentalPhysicswithAstronomicalObservations:TestingGravityandExtremeStatesofMatter

MichaelKramer,LucianoRezzolla,andNorbertWexonbehalfofalargercommunity

ExecutiveSummaryThe main themes/questions in the field of fundament physics, especially in testing gravity andextremestatesofmatterfortheupcoming10-15yearsare:

• Gravity and matter under extreme conditions: relativistic velocities, highly dynamical andstrongcurvature

• Gravitational-wavedetection:thebloomingofgravitational-waveastronomy• Relativisticcomputationalastrophysics:modellingofsupernovaeandgamma-raybursts• Physicsandastrophysicsofcompact-objects:blackholesandneutronstars• Nuclearastrophysics:nucleosynthesisinsupernovaeandbinaryneutronstarsmergers• Cosmological-scalegravity:natureofdarkmatteranddarkenergy.

Currently, there isastrongvisibilityand involvementof theGermancommunity inall the relevantareas(testsofgravity,gravitationalwavesdetectionandmodellingneutron-starphysicsandnuclearastrophysics)boththeoreticallyandexperimentally.Theseincludepulsarobservations,gravitational-waveresearch,Galacticcentreresearchandtheeffortsininstrumentation,observationsandtheory.When considering all aspects of research in gravity and extreme matter (theory/experiment), noother country has similar involvement apart from USA. At the same time German nuclearastrophysicshasalongtraditionandisreferenceatworldwidelevel.Thisisevidentfromanumberofprominentachievementsandsuccessesoverthepastdecade.Theseinclude:

• Firstdirectdetectionofgravitationalwavesfromabinaryblack-holemerger• Besttestsofgeneralrelativityandalternativetheoriesinvolvingpulsars• Tighterconstraintsonequationofstate(EOS)ofmatteratsupranucleardensities• Stringentlimitsonalow-frequencygravitationalwavebackgroundfromPulsarTimingArrays.• Numericalandanalyticalgeneral-relativisticcalculationsofthedynamicsofcompactobjects• Fullynonlinearsimulationofcatastrophiceventssuchassupernovaeandgamma-rayburst.• ObservationsandstellardynamicsatGCanddiscoveryofamagnetarintheGC.

Access to 3rd generation gravitational-wave detectors, SKA, LISA and HPC facilities are the mostimportant conditions to ensure the continuing access of the German community in this excitingresearchfield

1.IntroductionThefundamentallawsofphysicsdescribetheworldthatwelivein,theUniverse,howitbeganandhowitevolves.Theirunderstandingisthereforealsooffundamentalimportanceandthesubjectofresearch acrossmany disciplines. Astronomy and astrophysics provide insight that is unattainablesomewhere else, especially when it comes to understand gravity and the physics under extremeconditions. It is therefore no surprise that the first question formulated in the ASTRONET ScienceVision is “Do we understand the extremes of the Universe?” While astronomy provides cosmiclaboratories of scales and conditions that cannot be matched or created on Earth, obtaining,interpretingandunderstandingsuchobservationaldataiskeywhenconfrontingthetheoriesderivedon Earth with experimental data. These data are gathered with instruments exploiting differentwindowstotheUniverse;theyarecomparedtotheorywithoften-massivecomputersimulationsin

order to understand selection effects and biases or the physics itself, and they complement eachother. However, astronomicalobservationsdonotonly testpredictionsof theories,but theyalsodrive their development. Deviations from accepted models demand and trigger new theoreticalinsight. Past has shown that this new insight has huge implications onmany other areas or evencreatesnewfields(e.g.relativisticastrophysics).Thesedevelopmentsalsohave impactonpracticalapplications like the International Earth Rotation and Reference systems Service and relativisticgeodesy, and further contribute to our fundamental understanding of space and time. Aswewillargue in the following, pushing the studied physical conditions to an extreme using astronomicalobservationswillcontinuetobeafundamentalpillaranddrivingforce inourunderstandingoftheUniverseandthefundamentalphysicsthatgovernsit.Wewillfirstdiscussaspectsofgravitybeforeweconsiderextremestatesofmatter.

1.1GravityAt the heart of understanding the Universe lays our understanding of nature’s arguably mostfundamentalforce,gravity.Thestudyofgravityhastherebystronglinkswiththefieldofcosmologyanditsaspects,whichistreatedspecificallyelsewhereinthisdocument(seeChapter8and9).Whileit isthereforedifficultor impossibletocleanlyseparatethem,wefocushereontheaspectsofthefundamentalforceitself.Inordertotestandunderstandgravity,weneedtoprobeaverywiderangeofscalesandvelocitiesusingvariousobjectsandphenomena.Asgeneralrelativity(GR)describesgravitybythecurvatureofspacetime,wesummarizetheparameterspacetobeprobedasshowninFig.1,whereweplotthecurvatureofspacetime(whichismeasuredintermsoftheinverseofthecurvatureradiussquared)asafunctionofhow“relativistic”agivenastrophysicalsystemis(whichismeasuredintermsofthedimensionlessvelocityofthesysteminunitsofthespeedoflight).On thebottompartof thediagram,weencounter theUniverse itself and its large-scale structure,directly relating to its unknown aspects such as dark matter, dark energy or inflation – as alsodiscussedinChapters8and9.Experimentsinthesolarsystemareindicatedintheupperhalfofthediagram,atacurvatureofabout~10-24cm-2withdimensionlessvelocities(v/c)2≤10-5.Pulsarsallowhigh-precisiontestsofgravityinthequasi-stationarystrong-fieldandradiativeregimeatcurvaturesoforder10-11cm-2and(v/c)2exceeding10-6.The strongest gravitational fields are related to black holes. Black holes are one of the mostoutstanding predictions of General Relativitywith strange and unexpected properties as horizons,singularities, the possibility to allow time travel, etc. Today there is considerable observationalevidencethatblackholesreallyexist.Oneofthemostimportanttasksinrelativisticastrophysicsistofurther explore and understand the physics of black holes through stellarmotion, accretion, jets,light-distortioneffects,andbygravitationalwaves:Themosttop-rightcornerofFig.1isoccupiedbyobservations enabled by ground-based gravitational wave detectors, where the first directdetectionsofgravitationalwaves frombinaryblack-holemergerswas recentlymade (Abbottetal.2016a).ThisportionofthediagramrepresentsanewwindowontheUniverseandaprecioustooltostudygravityandinitsmostextrememanifestations.Other future detections, such as those possible with eLISA, Pulsar Timing Arrays (PTAs), or directimagingofablackhole,alldescribedbelow,marktheotherentriesinthisdiagram.

Figure1:Parameterspaceofobservationsandtestsofgravity.Onthex-axis,vdenotesthetypicalvelocityofthesystem’scomponentswhileΦdenotesthegravitationalpotentialbeingprobedbyphotonspropagatinginthecorrespondingspacetime.Onthey-axiswehavethemaximumspacetimecurvature(takenatthehorizonforblackholes)inthesystemasameasureofhowmuchthesystemdeviatesfromflatspacetime.Filledareasindicategravitationalwavetests,whilehollowareasstandforquasi-stationarytests,includingaccretionontocompactobjects.(Wex,priv.comm.)

1.2ExtremestatesofmatterA further central fundamental physics question is howmatter behaves under extreme conditions.The nuclear equation-of-state (EOS) is highly uncertain and not known. Constraints at lowdensitycome from nuclearmasses and chiral perturbation theory. This nuclear-physics problem connectsacceleratorexperimentswithastrophysicsandisasoldasneutronstars,butitalmostcertainlymayfindasolutionthroughastronomicalobservationsratherthaninalab.Theformationofneutronstarsasthedensestobjects intheobservableUniversestillneedsstudy.Especially, as supernovaexplosions are importantnotonly for their cosmological implications, butalso for their contributions to chemical evolution. It constitutes a very hard problem thatsimultaneously involves theEOS,neutrinos, general relativity,HighPerformanceComputing (HPC),magnetic fields,allofwhichcontributetotheexplosions.Thebinarymergersofneutronstarsalsocontribute to chemical abundance in the Universe and the emission from radioactive decay ofejectedmatter(kilonova)couldbemissinglinkbetweenSoftGamma-rayBurstsandbinaries.Indeed,a number of observations point to the association between short gamma-ray bursts andmergingbinaryneutronstars.Nevertheless,despitedecadesofobservations, it isnotyet clearwhatdrivestheseexplosions.Alloftheabovetopicsarecloselyinterlinked,andresults–boththeoreticallyandobservationally-inoneareaclearlyhaveanimpactintheotherareasaswell.

2.KeyQuestionsfortheUpcomingDecadeThepreviousdecadeshavebeenextremelysuccessful.After the firstevidence for theexistenceofgravitationalwavesusingbinarypulsars,thefirstdirectdetectionwasachievedinSeptember2015.Generalrelativitypassedthisandallotherexperimentaltestswithflyingcolours.However,thealsoestablished field of precision cosmology, with PLANCK fulfilling its promises that the communityformulatedaftertheWMAPresults,supplementedbyotherresults(seeChapter8and9),isstudyingtheimpactandnatureofDarkMatterandDarkEnergy.SomesuggestthatDarkMatterrepresentsadeviation from general relativity (e.g. Famaey & McGaugh 2012), rather than a massive particle,while Dark Energy’s manifestation is even less clear, but it is clearly closely related to gravity.Similarly,understanding the stateof super-densematter relates tonuclear forces,andhence links

astrophysicswithhigh-energy andparticle physics. It is thereforenot surprising that the followingkey questions are essentially those also and already defined in the ASTRONET Science Vision (deZeeuw&Molster2007):

• HowdidtheUniversebegin,howwillitevolve?• Whataredarkmatteranddarkenergy?• Doblackholesreallyexist,andhaveaneventhorizon?• Iftheyexist,doblackholeshostaphysicalsingularity?• Whicheffectsoccurinstrong-fieldgravity?• WhatistheEOSofmatteratnucleardensity?• Howcompactareneutronstars?• Isgeneralrelativityourlastwordinourunderstandingof(macroscopic)gravity?

Inadditiontothesefundamentalquestions,theadventofgravitationalwaveastronomyopensanewwindowtothecosmos,sothatfurtherquestionsare:

• Whatarethepropertiesofthegravitationalwaveskyacrossallfrequencies?• Whatisthepopulationofstellarblackhole-blackholesystems?• Howmanysupermassivebinaryblackholesystemsexistandhowdoesthatrelatetoour

understandingofgalaxyformationandmergers?• Whatisthepopulationofneutronstar-neutronstarsystems?• Whatarethepropertiesofblackholes,andaretheyconsistentwithgeneralrelativity?

3.KeyResultsfromtheGermanCommunityofthePreviousDecadeThelastdecadehasseenatremendousamountofimportantkeyresults,fromhigh-precisiontestsofgeneral relativity using binary pulsars to the ability perform full 3-D simulations of supernovaexplosions, from the discovery of dark energy to the first direct detections of gravitationalwavesfrom a binary black hole merger. We summarize some of these and describe the Germancontributionsbelow.3.1PrecisiontestsofgravitywithpulsarsThefoundationofGeneralRelativityissummarizedintheEquivalencePrinciple.GermanteamsareleadingexperimentsaimedattestingtheLocalLorentzInvariance(Schlippertetal.2014).Germanyisalso strongly contributing to an ongoing space mission aiming at a largely improved test of theuniversalityof freefall (Hagedornetal.2013).TheGermancommunity isalsoworld leading intheexperimentalexplorationoftheinterplaybetweenquantummechanicsandgravity–anissuewhichmightbeof importanceforthe interpretationofastronomicaldata. Thenumericallymostprecisetest of the validity of the Einstein field equations has been so far achieved by studying thepropagation of signals from the Cassini spacecraft in the weak-field gravitational potential of thesolarsystem,achievingaprecisionof10-5(Bertottietal.2003).However, as these tests have only been performed in the weak gravitational field of the Solarsystem, testing the strong-field regime is essential. Testing strong-field effects of strongly self-gravitatingbodieshasbeenpossiblebybinarypulsarobservations,withtheDoublePulsarprovidingthemostprecisetestssofar(Krameretal.inprep.,seeFigure2).TheBonngroupatMPIfRisaworldleader in these tests, conducted not only the best tests for general relativity (e.g. Kramer 2016;Krameretal.inprep.)butalsofortestsofalternativetheoriesofgravity,inparticularscalar-tensorgravity (e.g. Freire et al. 2012, Antoniadis et al. 2013, Wex 2014). Furthermore, importantphenomena like geodetic precession have been first discovered in pulsars with the Effelsbergtelescope (e.g. Kramer 1998, 2012; Desvignes et al. 2016), while other work of the group hasproducednew recipes for the tests of theUniversity of Free Fall (Freire et al. 2012) andprovidedtestsforgravitationalLorentzinvarianceandotherfundamentalsymmetriesofgravity(Shao&Wex2012, Shao et al. 2013, Shao & Wex 2016). This observational effort is matched by an intense

theoreticalresearchcarriedout inTübingen,wheretheimpactofalternativetheoriesofgravityontheequilibriaofneutronstarshasbeenexploredsystematically(Donevaetal.2014,Yazadijevetal.2015).

Figure2:EffectsoftheorbitalperioddecayduetogravitationalwaveemissionasobservedintheDoublePulsar.Theback-reactionofgravitational-waveemissionleadstoacontinuouschangeintheorbitalperiodandconsequentlytoanaccelerationintheorbitalphaseevolution,leadingtoacumulativeadvanceinthetimeofperiastronpassage.Theobservedchangeinperiastronpassage(bluedots)isinperfectagreementwiththetheoreticalpredictionsbyGR’squadrupoleformula(redcurve).(Krameretal.inprep.)

3.2IndirectanddirectgravitationalwavedetectionTheMPIfR leads the exploitation of the Double Pulsar system, a unique system that provides themostprecisetestsforGR’squadrupoleformulaforgravitationalwaveemissiontodate(e.g.Figure2). With this evidence for gravitational waves from binary pulsars, the community had eagerlyexpected the firstdirectdetectionwithground-baseddetectors.Thiswas finallyachieved in2015,when the upgraded Advanced LIGO detectorsmeasuredwith the event GW150914 (Abbott et al.2016a),thesignalofamergeroftwostellar-massblackholes.Germancolleagueswerethefirsttorecognize the signals and continue to play a leading role in the data analysis of LIGO data, bothcomputationallyaswellastheoretically.Indeed,overthelastfewyearsagreatdevelopmentefforthasbeenmadeinGermanytoobtainacomputationalinfrastructurethatisabletoprovideaseam-lessdescriptionoftheevolutionofbinarysystemscontainingcompactobjectssuchasblackholesorneutron stars. These calculations, which have played a key role in the recent detection of thegravitationalwavesignalfromGW150914andtheothersignalsthathavebeendetectedsofar,e.g.GW151226(Abbottetal.2016b).Asexplained furtherbelow, it hasbeen the technologydevelopedby theGermancommunity andsuccessfullydeployedtotheGerman/UKgravitationalwavedetectorGEO600thatmadetheeraofgravitationalwaveastronomyareality. Indeed,thisdiscovery(Figure3)heraldsthebeginofanewera,whichwillnotonlyallowtestsofgravityinthehighlydynamicalstrong-fieldregimethatwasnotaccessiblebeforewithpulsars(seeFigure1),butitalsoopensupanewwindowtotheUniverse.Thiswindowalreadyprovides a first direct viewontoapopulationofblackholes,whosenumbers andproperties were essentially unknown before the LIGO result. Furthermore, as we discuss inmoredetail later on, when the information contained in the gravitational waves from merging binaryneutronstarswillbecombinedwiththeelectromagneticemissionthatisexpectedfromthisprocess,wewillalsoobtainveryimportantcluesonthecentralengineofshortgamma-rayburstsandonthestatusofmatteratnucleardensities.

Whiletheground-baseddetectors likeLIGOandGEOaresensitivetothefrequenciesabove10Hz,onehas tomove to space to access the gravitationalwave sky at lower frequencies. TheGermancommunity,especially thecolleaguesatHannover,are leading theglobalefforts indeveloping thefirst space-based detector. Consequently, they played a crucial role in the success of the LISAPathfinderprobelaunchedin2015(Armanoetal.2016).

Figure3:Firstdirectdetectionofgravitationalwavesfromabinaryblack-holemergerGW150914.(Abbottetal.2016a).

Goingtoevenlowerfrequencies,oneexpectstoseethegravitationalwavesignalsofsupermassivebinaryblackholes.Accordingtothestandardscenarioofgalaxyformation,theyareproducedduringthe merger of galaxies in the early Universe. Emitting signals at nano-Hertz frequencies, thesesystemscanbedetectedusinga“PulsarTimingArray”(PTA)–anetworkoffastrotatingmillisecondpulsars that act as the end points of a Galactic-sized gravitational wave detector. High precisiontimingof thepulsars allows topickup tiny variations in theexpectedarrival timeof thedetectedpulsesasthelocalspacetimeismodifiedbylow-frequencygravitationalwaves,bothatEarthaswellasthepulsars.Thisexperiment isconducted inEuropeastheEuropeanPulsarTimingArray(EPTA;Kramer & Champion 2013), with scientists in Bielefeld, Bonn, Golm and Potsdam co-leading theefforts.Thefirstcompleteofficialdatareleasewiththeprecisiontimingof42millisecondoftheEPTAwasledbytheBonngroup(Desvignesetal.2016).ThedatasetwasusedbyCaballeroetal.(2016)for a sophisticatednoise-analysiswhich is essential to reveal the signal of theexpected stochasticgravitationalwavebackgroundinthepresenceofcompetingred-noiseprocesses,againledinBonn.Alreadyearlier,colleaguesinGolmledanEPTApublication,whichsetsthecurrentlybestlimitonthecontinuousgravitationalwavesfromindividualsupermassiveblackholebinaries,usingtheDesvignesEPTAdata set (Babaket al. 2016). This limitwasderivedusing frequentist andBayesiandetectionalgorithmstosearchbothformonochromaticandfrequency-evolvingsystemsforthefirsttime.Effelsberg, as themost sensitive telescope in Europe, acts as the back-bone of the EPTA and alsoservesasthereferencetelescopeinthe“LargeEuropeanArrayforPulsars”(LEAP,PIKramer;Kramer& Champion 2013), which forms a synthetic 200-m dish for low-frequency gravitational wavedetection.BothEPTAandLEAParethereforecrucialcontributorstothe“InternationalTimingArray”(IPTA)whichcoordinatestheeffortsoftheEPTAwiththeexperimentsinNorth-America(NANOGrav)

properties of space-time in the strong-field, high-velocityregime and confirm predictions of general relativity for thenonlinear dynamics of highly disturbed black holes.

II. OBSERVATION

On September 14, 2015 at 09:50:45 UTC, the LIGOHanford, WA, and Livingston, LA, observatories detected

the coincident signal GW150914 shown in Fig. 1. The initialdetection was made by low-latency searches for genericgravitational-wave transients [41] and was reported withinthree minutes of data acquisition [43]. Subsequently,matched-filter analyses that use relativistic models of com-pact binary waveforms [44] recovered GW150914 as themost significant event from each detector for the observa-tions reported here. Occurring within the 10-ms intersite

FIG. 1. The gravitational-wave event GW150914 observed by the LIGO Hanford (H1, left column panels) and Livingston (L1, rightcolumn panels) detectors. Times are shown relative to September 14, 2015 at 09:50:45 UTC. For visualization, all time series are filteredwith a 35–350 Hz bandpass filter to suppress large fluctuations outside the detectors’ most sensitive frequency band, and band-rejectfilters to remove the strong instrumental spectral lines seen in the Fig. 3 spectra. Top row, left: H1 strain. Top row, right: L1 strain.GW150914 arrived first at L1 and 6.9þ0.5

−0.4 ms later at H1; for a visual comparison, the H1 data are also shown, shifted in time by thisamount and inverted (to account for the detectors’ relative orientations). Second row: Gravitational-wave strain projected onto eachdetector in the 35–350 Hz band. Solid lines show a numerical relativity waveform for a system with parameters consistent with thoserecovered from GW150914 [37,38] confirmed to 99.9% by an independent calculation based on [15]. Shaded areas show 90% credibleregions for two independent waveform reconstructions. One (dark gray) models the signal using binary black hole template waveforms[39]. The other (light gray) does not use an astrophysical model, but instead calculates the strain signal as a linear combination ofsine-Gaussian wavelets [40,41]. These reconstructions have a 94% overlap, as shown in [39]. Third row: Residuals after subtracting thefiltered numerical relativity waveform from the filtered detector time series. Bottom row:A time-frequency representation [42] of thestrain data, showing the signal frequency increasing over time.

PRL 116, 061102 (2016) P HY S I CA L R EV I EW LE T T ER S week ending12 FEBRUARY 2016

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andAustralia(PPTA).TheimportanceoftheGermancommunityifsignalledbythefactthatthefirstofficial IPTAdata is ledby JorisVerbiest at theUniversityofBielefeld (Verbiest et al. 2016),whileKramerchampionedthePTAexperimentasoneoftheKeyScienceProjectsfortheSquareKilometreArray(SKA)formorethanadecade(Krameretal.2004,Kramer&Stappers2015).3.3Super-densematterClearly,oneofthemostimportantresultsofthelastdecadeforthestudyofsuper-densematterwastheprecisionmeasurementofthehighestneutronstarmassinPSRJ0348+0432of2M

¤.Thisworkby

theBonngroup(Antoniadisetal.2013)isalreadythemostcitedGermanradioastronomypaperinthelastdecadewithwellover750citations(asbyADSinJuly2016).Theimportanceoftheresultisgivenbytheimpactofthislargemassforthevalidityofproposedequations-of-stateofsuper-densematter,whichhavetobestiffenoughtosustainsuchalargemass(seeFigure4).Infact,mostofthemassmeasurementsofneutronstarshavecomefromtheBonngroup(seeÖzel&Freire2016foralist with references), and especially the study of the important Double Neutron Star systems isdominatedbyresultsfromBonn(seeMartinezetal.2015andreferencestherein).

Figure4:Radius-massrelationsofneutronstarsfordifferentEOSs(Lattimer&Prakash2001).Highmasspulsarsprovideimportantconstraintsbyrulingoutthoseequationsofstatethatcannotsupporttwosolarmassneutronstars.

4.ParticularRole/StrengthsofResearchGroupsinGermanyMany of the key results obtained in this research field have been obtained under the leadershipand/orcollaborationofGermaninstitutionsandscientists.4.1PulsarandneutronstarobservationsResearchgroupsatBonn(MPIfR)andnowalsoBielefeld(university),haveestablishedthemselvesaswordleadersinthediscoveryandexploitationofpulsarsforstudiesoffundamentalphysics.TheseGermanscientistscombineexpertisefromwideareasinneutronstarandgravitationresearch,frommassmeasurementsandequation-of-statestudiestolow-frequencygravitationalwavedetection,fromradiopulsarsearchingandprecisiontestsofgeneralrelativitytogamma-rayobservations.Anambitiousobservingprogramusingthe100-mEffelsbergtelescopeandotherswithdeepinvolvementsininternationalprojectssuchastheEuropeanPulsarTimingArray,arematchedbyrigorouseffortsinatheoreticalinterpretationofthedata.Indeed,thecombinationofsuchstrongtheoryandobservationaleffortsareunmatchedintheworldofneutronstarandpulsarresearch,positioningthegroupattheforefrontofthisexcitingresearchfield.Mostofthebestconstraintson,ormeasurementsof,parametersdescribingrelativisticgravitationaltheoriesortherotationoffast-

rotatingpulsarshavebeenachievedbyexperimentsledbythesecolleagues.Thisgoesinhandwiththedevelopmentandexploitationofinstrumentationdevelopedforpulsarobservationsinstalledatvarioustelescopesaroundtheworld.ThegroupinBonnisalsoatthecentreoftheSKAgravitationalphysicsKeyScienceProject.WithaccesstotheLOFARstationinEffelsberg,scientistsinBielefeldandBonnarebeingintimatelyinvolvedintheexploitationoftheInternationalLOFARTelescope,andtheearlyengagementandMeerKATscience.Currentprojectsincludeanall-skysurveyofthedynamicradioskyintheNorthernandSouthernhemisphereproducingthedefiningdatasettobestudiedandexploredbeforetheSKAcomeson-lineandtheERC-fundedLargeEuropeanArrayforPulsars(LEAP)thathasagoodchanceindirectlydetectionlow-frequencygravitationalwaves.4.2Gravitational-waveAstronomy4.2.1InstrumentationandDataanalysisAlready in the 1970s, German researches began to develop the technology of interferometers todetect gravitational waves directly, leading eventually to the construction of GEO600 and thedevelopment of innovative technology that was used for critical components (e.g. high-poweredlasers) at the LIGO detectors and its upgrades. Scientists from the AEI have therefore played aleading role developing the instruments that led finally to the successful direct first detection ofgravitationalwaves.TheAEI isalsothedrivingforcebehindthedevelopmentoftechnologyforthespace-baseddetectorLISAandhasbeenakeyplayer inthedevelopmentofLISAPathfinder,whichwaslaunchedonDecember3rd,2015,andisnowinitssciencemission.ThefirstresultspublishedinJuly2016showthatthedesignspecificationsarevastlyexceeded,achievingalreadyaperformancethatisrequiredforLISAitself.These hardware efforts are paired with work on the data analysis side, where AEI scientists areleadingthedevelopmentandimplementationofmathematicalmethods(dataanalysisalgorithms)tosearchforthedifferentexpectedgravitationalwavesignaturesofpossiblecosmicsources.Infact,ithas been colleagues in Hannover who detected the signals of GW150914 for the first time. Inaddition to HPC clusters, a data analysis project known throughout theworld isEinstein@Home.,whichisalsousedtodiscoverradiopulsarsincollaborationwithcolleaguesinBonn.ThescientistsattheAEItakealeadingroleindevelopingandrunningthisglobalproject.4.2.2TheoryAsmentionedabove,thelastdecadehasseenthedevelopmentofnewapproachestothestudyofcompactobjectsthatareeitherbasedonanalyticaltechniquesthatprovidemoreandmoreaccurateapproximationstothedynamicsorfull-scalenonlinearnumericalsimulations.ColleaguesattheAEIin Potsdamare leading experts in the development of accurate analyticalmodels of gravitational-wavemergersthatinvolvepairsofblackholes,whilecolleaguesinJenaandPotsdamhavedevelopedadvancedandaccuratecodes for thenumerical solutionof thisproblemandthe full calculationofthegravitational-wavesignal.Analytical andnumericalmodels for blackholemergers (andother sources) play a crucial role forgravitationalwaveastronomy.Concretely,theworkonpost-NewtonianandnumericalmodelsdoneinGermanyplayedan important role in the interpretationof the first signalsasblackholemergersignals and the derivation of the astrophysical parameters of the observed binaries (Abbott et al.2016c).In the case in which the binary contains at least one neutron star, numerical simulations ofcolleaguesinFrankfurt,JenaandMunichareabletoprovideanaccurateandrealisticdescriptionofthe various stages of the final dynamics of the binary, going from the inspiral stage, over to thecreationofahyper-massiveneutronstar (HMNS)and,ultimately, to the formationofablackholesurrounded by a hot and dense torus (Baiotti, Giacomazzo & Rezzolla 2008). Recently, a veryaccuratedescriptionoftheinspiralandmergerofbinaryneutronstarshasbeenobtainedusinghigh-order finite-difference techniques for the solution of both the Einstein and the relativistichydrodynamicsequations.Anewlydevelopedcodehasbeenshowntoachieveamong thehighestconvergence rates demonstrated for this type of simulation, thus providing the high accuracy

required tomodel even the tiny changes introduced by the tidal deformation of the stars in theevolutionofthegravitational-wavephase(Radice,Rezzolla&Galeazzi,2013). Inaddition,workhasalsobeendonetoincluderadiativelossesvianeutrinos,whichcanbeveryusefultounderstandtherelationshipbetween thematter dynamics and theneutrinoemission,which is essential tomodeltheneutrinosignalsfrommergersofcompact-starbinaries.At the same time, great progress has alsobeenmade in improving thedescriptionof the EOS formergingneutronstars,alsoexploitinginparttheconstraintscomingfromastronomicalobservations.The calculation of the EOS for astrophysical simulations poses significant challenges. In particular,special care has to be taken because of the extremely high neutron densities and the nearlyvanishingelectronfractions.Hence,inadditiontoextendingtheEOStonon-vanishingtemperatures,weak equilibriumand a fixed charge-to-baryon ratio have to be implemented for a large rangeoftemperaturesandbaryondensities.Athermodynamicallyconsistentframeworkhasbeendevelopedrecently fora largevarietyofmodernnuclearEOSs forcore-collapsesupernovasimulationswithinmicroscopicmodels, suchasSkyrme-Hartree-Fock, relativisticmean-fieldmodelsandmoregeneraldensity-functionalapproaches.Inthisway,itwaspossibletoobtainasmoothmatchingbetweentheneutronmatterEOSandthehigh-densitylimitfromperturbativeQCD.

Figure 5Gravitationalwaveformsforequal-massneutronstarbinaries.EachrowreferstoagivenEOS,whileeachcolumnconcentratesonagiveninitialmass.ThedifferentEOSsaredistinguishedbydifferentcolors.

Anexampleofthisabilitytomodelthegravitationaldynamicsofbinaryneutronstarsinfullgeneralrelativity is shown in the figurebelow,which reports thegravitationalwaveforms foranumberofbinariesspanningfiveEOSsandfivedifferentmassranges.GroupsinFrankfurt,Jena,MPIGarching,MPIHannover,andMPIPotsdamhavealongtrackrecordof excellent results in this area and over the last decade have imposed themselves as referencepointsworldwideforthenumericalmodelingofcompactbinariesandtheaccuratecalculationofthegravitational-wave emission that is produced in the process, as well as the development oftechnologiesinuseintheadvancedinterferometricdetectors.Regardingtheanalyticaldescriptionofthemotionofstarsandthepropagationoflightaroundblackholes, new methods have been developed at the Universities of Bremen and Oldenburg. Thesemethodscanbeapplied topulsarandstarmotionaroundSgrA* (Hackmann&Lämmerzahl2008)andalsotothecalculationoflighteffectsinblackholespacetimes,likelensingandshadowsofblackholes (Grenzebach, Perlick & Lämmerzahl 2014), even in the case of black holes surrounded byplasmaasneededfortheEHT.

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4.3RelativisticAstrophysicsSupernovaeandGamma-rayBurstsShortGRBsare flashesofgammaraysreleasingamountsofenergyof theorderof1048–1051erg,whichsuggeststhatthe“centralengines”arehighlyrelativisticobjects.Theseareamongthemostenergeticevents intheUniverseandyet it isnotentirelyclearwhattheircauseandnature is.ThestandardmodelenvisagesshortGRBsastheoutcomeofthemergerofabinaryneutronstarsystemleading to the formation of a HMNS, which eventually collapses to produce a rotating black holesurroundedbyamassiveandhotaccretiontorus.Thisscenarioisalsofavouredbyargumentsbasedon stellar population synthesis and on stellar evolutionarymodels. Given the initialmass functionandtherateofbinaryformationamongmassivestars,itispossibletoestimatethenumberofbinaryneutronstarsthatformfollowingnormalstellarevolutionandhencetherateofsuchsystemsthatareabouttomerge.SuchinformationcanbeexploitedtorelatebinarymergerswithshortGRBs.Itshould be noted that both theHMNS and the black hole-torus systems could supply the requiredenergyvianeutrinoemissionorbyextracting the rotationalenergyof theblackhole viamagneticfields.

Figure 6 Snapshotsatrepresentativetimesoftheevolutionofmagnetisedbinaryofneutronstarsandoftheformationofalarge-scaleorderedmagneticfield(Rezzollaetal.2011)

Over the last few years it has become clear that the corrections to the gravitational wave signalduring the inspiral anddue tomagnetic fields are too small tobedetectedbypresent and futuredetectors, but that magnetic fields do have an impact after the merger. They can influence thestabilityof theHMNS,and they couldbe subject to themagnetorotational instability (MRI),whichleadstotheexponentialgrowthofthemagneticfieldevenfromveryweakseeds.Furthermore,theycan launch a quasi-isotropic, baryon-loaded and mildly relativistic wind, which could be used toexplaintheX-rayafterglowsobservedinalargefractionofshortGRBs.The material ejected during these events can yield to the nucleosynthesis of heavy elements viarapidneutron-captureprocesses(r-processes),butalsotoaradioactivedecaywhichcouldexplainthe infrared excess in the afterglow (kilonovae). Finally,magnetic fields provide the link betweeninspirallingbinariesandthegenerationofalarge-scalejetthatcouldpowershortGRBs.GroupsinFrankfurt,MPIGarching,andTübingenarenowplayinga leadingrole inthemodelingofthese processes that are perfect examples of a novel approach to astronomy based onmultimessengerobservations.4.4NuclearAstrophysics

The Astrophysical Journal Letters, 732:L6 (6pp), 2011 May 1 Rezzolla et al.

Figure 1. Snapshots at representative times of the evolution of the binary and of the formation of a large-scale ordered magnetic field. Shown with a color-code map isthe density, over which the magnetic-field lines are superposed. The panels in the upper row refer to the binary during the merger (t = 7.4 ms) and before the collapseto BH (t = 13.8 ms), while those in the lower row to the evolution after the formation of the BH (t = 15.26 ms, t = 26.5 ms). Green lines sample the magnetic fieldin the torus and on the equatorial plane, while white lines show the magnetic field outside the torus and near the BH spin axis. The inner/outer part of the torus has asize of ∼90/170 km, while the horizon has a diameter of ≃9 km.

(indicated as M1.62-B12 in Giacomazzo et al. 2011). At thisseparation, the binary loses energy and angular momentum viaemission of gravitational waves (GWs), thus rapidly proceedingon tighter orbits as it evolves. After about 8 ms (∼3 orbits), thetwo NSs merge forming a hypermassive NS (HMNS), namely,a rapidly and differentially rotating NS, whose mass, 3.0 M⊙,is above the maximum mass, 2.1 M⊙, allowed with uniformrotation by our ideal-gas EOS8 with an adiabatic index of 2.Being metastable, an HMNS can exist as long as it is ableto resist against collapse via a suitable redistribution of angu-lar momentum (e.g., deforming into a “bar” shape; Shibata &Taniguchi 2006; Baiotti et al. 2008), or through the pressuresupport coming from the large temperature increase producedby the merger. However, because the HMNS is also losing an-gular momentum through GWs, its lifetime is limited to a fewms, after which it collapses to a BH with mass M = 2.91 M⊙and spin J/M2 = 0.81, surrounded by a hot and dense toruswith mass Mtor = 0.063 M⊙ (Giacomazzo et al. 2011).

8 The use of a simplified EOS does not particularly influence our resultsbesides determining the precise time when the HMNS collapses to a BH.

3. DYNAMICS OF MATTER AND MAGNETIC FIELDS

These stages of the evolution can be seen in Figure 1, whichshows snapshots of the density color-coded between 109 and1010 g cm−3, and of the magnetic-field lines (green on theequatorial plane and white outside the torus). Soon after the BHformation the torus reaches a quasi-stationary regime, duringwhich the density has maximum values of ∼1011 g cm−3,while the accretion rate settles to M ≃ 0.2 M⊙ s−1. Usingthe measured values of the torus mass and of the accretion rate,and assuming the latter will not change significantly, such aregime could last for taccr = Mtor/M ≃ 0.3 s, after which thetorus is fully accreted; furthermore, if the two NSs have unequalmasses, tidal tails are produced which provide additional late-time accretion (Rezzolla et al. 2010). This accretion timescaleis close to the typical observed SGRB durations (Kouveliotouet al. 1993; Nakar 2007). It is also long enough for theneutrinos produced in the torus to escape and annihilate in itsneighborhood; estimates of the associated energy deposition raterange from ∼1048 erg s−1 (Dessart et al. 2009) to ∼1050 erg s−1

(Setiawan et al. 2004), thus leading to a total energy deposition

2

We are entering a new era with multi-messenger observations (electromagnetic, neutrinos,gravitational waves) and new nuclear physics facilities. The combined efforts of astrophysicssimulations,observations,nucleosynthesiscalculationsandnuclearphysics(theoryandexperiment)willallowus tounderstand theoriginofheavyelements in theUniverse.Theworkproposedherefocuses on the twomain processes producing heavy elements and their contribution to the SolarSystem abundances: the slow and the rapid neutron capture processes (s- and r-process). The r-process produces half of the heavy elements but it is still a nuclear physics and astrophysicschallenge.Germanyhas a longhistoryofnuclearphysics and the largest experimental facility in Europe is inconstructioninDarmstadt(FacilityforAntiprotonandIonResearch,FAIR).Yet,theextremeneutron-richnucleiinvolvedinastrophysicalconditionssuchasthemergerofbinaryneutronstarscannotbeproduced inthe laboratoryandtheirpropertiesare looselyconstrainedbytheoreticalmodels.Theastrophysical sites involve neutron stars and explosive conditions that allow the rapid capture ofabundantneutrons.Wewill investigatethenucleosynthesis incompactbinarymergersthatareanexcellentcandidatetoexplaintheoriginofheavyelementsintheUniverse.Thisisfurthersupportedby the potential observation of the radioactive decay of r-processmaterial in a kilonova after theshortGRB130603.Eveniftheyarerare,theamountofmatterejectedisenoughtoaccountforther-processelementsinoursolarsystem.

Figure7Finalabundances in theejecta for themergerofbinaryneutronstars.Foreachconfigurationweconsider threedifferentlevelsofmicrophysicaldescription.TheabundancepatternforelementswithA>120isveryrobustandinoverallgoodagreementwiththeSolarr-processabundances

Groups in Bonn, Darmstadt, Frankfurt, Heidelberg and MPI Garching have now built a large androbustframeworkfortheexplorationofthemicrophysicsofnuclearprocessinastrophysics.Suchaframework is built on anovel and collaborative effort inwhich traditionally independent scientificcommunities are brought together to analyse the different phenomena that take place in nuclearastrophysics.4.5ExploringtheGalacticCentreThecentreofourGalaxyisaparticularlyinterestinglocationtoprobegravity.ColleaguesatGarchingand also Cologne have beenworld leaders in the observational efforts to detect and exploit starsorbiting thecentral supermassiveblackhole fora longtime(Gillessenetal.2009).Thisand futureobservations of stars to probe the gravitational field of the black hole are described elsewhere.TheseeffortsarematchedbyeffortsincludingcolleaguesinBonnandFrankfurttoobtainanimageofthe“shadow”oftheeventhorizonusingmm-VLBI,atechniquewheretheGermancommunityistraditionally world-leading (described elsewhere), and to combine these images also with insightfrompulsars observations and theoretical studies. The idea to obtain an image itself originated inBonnandisnowbeingexploredgloballywithintheEventHorizonTelescope(EHT)projectunderco-leadershipofGermancolleagueswithstrongtheoreticalworkbycolleaguesinFrankfurt.

Dynamical Mass Ejection from Binary Neutron Star Mergers 13

50 100 150 200

A

10�5

10�4

10�3

10�2

10�1

Rel

ativ

eab

unda

nces

Solar

HY RP7.5

LK RP7.5

M0 RP7.5

50 100 150 200

A

Solar

HY RP10

LK RP10

M0 RP10

50 100 150 200

A

Solar

HY QC

LK QC

M0 QC

Figure 8. Final abundances in the ejecta for the RP7.5, RP10 and QC configurations. The yields are normalized with the total abundance ofelements with 63 6 A 6 209. For each configuration we consider three di↵erent levels of microphysical description (pure hydrodynamics,HY or leakage with only cooling, LK, or with heating/absorption included, M0). The abundance pattern for elements with A & 120 is veryrobust and in overall good agreement with the Solar r-process abundances taken from Arlandini et al. (1999).

Figure 9. Joint distribution of composition and entropy for theM0 QC simulation. The material to the left of the dashed contourline produces second and third r-process nucleosynthesis, whilematerial to the left of the solid contour line only produces firstpeak r-process nucleosynthesis. The figure also hints at the exis-tence of a correlation between Ye and s as larger proton fractionsare typically found in combination with high entropies.

For the energy production from the radioactive decay of ther-process elements we follow Grossman et al. (2014) andchoose ↵ = 1.3.

As can be inferred from Tab. 1, the macronovatimescales and luminosities show significant variation be-tween our simulations. Simulations such as LK RP5, whichresult in very little ejecta, have macronovae that peak onvery short timescales (less than a day) and that are rela-tively blue compared to those of quasi-circular binaries. TheRP7.5 and RP10 models, which eject significantly more ma-terial than the QC model, have macronova light curves thatpeak on a timescale of one to two weeks. They are also char-acterized by lower e↵ective temperatures and higher intrinsic

luminosities with respect to quasi-circular binaries. We alsofind the basic macronova properties to be rather sensitiveto the level of microphysical description. For instance, thepeak time for the quasi-circular binary goes from 2.8 daysto 1.4 days or 1.1 days when including neutrino cooling orneutrino cooling and heating.

The ejecta are also expected to lead to radio emission asthey transfer their kinetic energy to the surrounding inter-stellar medium and produce non-thermal synchrotron emis-sion (Nakar & Piran 2011). The timescale over which theejecta are decelerated is given by (Nakar & Piran 2011)

tdec = 30

✓Ekin

1049 erg

◆1/3

⇥

⇣ n0

cm�3

⌘�1/3✓hv1ic

◆�5/3

days ,

(12)

where n0 is the density of the interstellar medium. Plasmainstabilities develop at the location of the moving blast wave,generating magnetic field and accelerating the electrons intoa power law spectrum N(E) = E�p with exponent p.

At an observing frequency ⌫obs higher than both theself-absorption and synchrotron peak frequencies the radiofluence (i.e., the flux density per unit frequency) for a sourceat a distance D can be expressed as (Nakar & Piran 2011)

F⌫ = 0.3

✓Ekin

1049 erg

◆⇣ n0

cm�3

⌘ p+14

⇣ ✏B0.1

⌘ p+14 ⇥

⇣ ✏e0.1

⌘p�1✓hv1ic

◆ 5p�72

✓D

1027cm

◆�2

⇥

⇣ ⌫obs1.4 GHz

⌘� p�12

mJy ,

(13)

where ✏B and ✏e are the e�ciencies with which the energyof the blast wave is transfered to the magnetic field and tothe electrons, respectively.

The properties of the radio remnant produced by theejecta from our simulations are reported in Tab. 1. To com-pute tdec and F⌫ , we adopt n0 = 0.1 cm�3 as fiducial valuefor the number-density of the interstellar medium, which is

MNRAS 000, 000–000 (0000)

Figure8:ComputationsbyDexteretal.(2010)oftheunderlyingstructureofanimageprojectoftheGalacticCentreblackholetobetakenbymm-VLBIintheEventHorizonTelescope(left).Theresultsoftheseimagesoftheeventhorizoncanbecombinedwithfutureobservationsofpulsarsandstarsprovidingcomplementaryinformationtodeterminetheblackholepropertieslikemass,spinandquadrupolemoment,providingatestoftheno-hairtheoremwhencombinedwiththetheoreticalinsightasintheblackholeCamproject(right;seePsaltisetal.2016).

5.KeyInfrastructuresNeededandRelevantforResearchersinGermanyIn order to exploit and further enhance the leadership position of theGerman community in thisresearchfield,itisimportantthatGermanscientistsgainaccesstothenumberofkeyinfrastructuresthatarebeingconstructed,designed,inpreparationorplanned.Itisclearthattrueadvanceswillrelyon amulti-messenger approach that includes a number of observationalwindows combinedwithfacilitiesfordataanalysisandinterpretation.Figure8highlightsimportantaspectsofthisapproach.In order to increase the sensitivity of ground-based detectors, cryogenically-cooled undergrounddetectors of a 3rd generation of gravitational wave detectors are needed in the long-term afterAdvancedLIGOandAdvancedVirgo.TheLCGT/KAGRAfacilityisastartinthisdirection,butaccesstoa 10-times longer Einstein Telescope (ET) detector is clearly required for German scientist in thefuture,asthiswouldimproveonexistingdetectorsinsensitivitybyanotherorderofmagnitude.ETwillobservehundredsofbinaryneutronstarsandGammaRayBurstassociationseachyear.TheearlyresultsfromLISAPathfindershowthataspace-baseddetectorLISAcanbebuilt,providingaccesstolowerfrequenciesanddifferentsourceswithasensitivitythatwouldmakeLISAtheperfectfacility for fundamental physics, cosmology and astrophysics at the same time. Questions to beaddressarethenature,populationandphysicsofmassiveblackholes(104to108M☉),thestudyofExtremeMassRatioInspirals(EMRIs,1to10M☉ into104to5x106M☉),theobservationsofultra-compactbinariesintheMilkyWay,andcosmologicalsignalsofstochasticnature(e.g.probingphasetransitions,Higgs-fieldself-coupling,supersymmetry).LISAwillbeperfectlycomplementarytotheSquareKilometreArray(SKA).Astheworld’slargestandmost sensitive radio telescope, the SKAwill improve the sensitivity of Pulsar TimingArrays (PTAs)suchthatnotonly thestochasticbackgroundofmergingsuper-massiveblack-holebinaries inearlygalaxy evolution can be observed, but it will also be able to detect and potentially localize singlesourcesemittingcontinuousgravitationalwaves.SKAwillprobeBHsonmass-scalelargerthanthoseobservedbyLISAand/orstudiesLISAsourcesatdifferentevolutionarytimes.CombinedwithotherEM-observationsatopticalandx-rayfrequencies,thecombinationofSKA,LISAandETwillmarkthebeginning of a “black hole astrophysics” era. SKA observations will also allow to study thepolarizationpropertiesofgravitationalwavesandtoputstringentconstraintsonthegravitonmass.At the same time, the SKA will discover and exploit compact relativistic binaries that will furtherimproveonprecisiontestsofgravity;itwillalsoprovideacompletesampleofGalacticradiopulsars,firmly establishing themass scale of neutron stars, the limits on spin-period, and themoment-of-inertiaofneutronstars–allessentialinformationtoidentifytheEOSofsuper-densematter.AccesstopathfinderinstrumentslikeMeerKATwillpreparetheGermancommunityfortheSKA.

Figure9:Gravitationalwavespectrumdemonstratingtheneedfordifferentcomplementaryinstruments.

Accesstoinstrumentsprobingthecosmicmicrowavebackgroundanditspolarisationwillbeneededto study the properties of gravitational waves from the era of inflation, while space-probes willcomplementtheSKAeffortstostudythepropertiesofDarkEnergy.Thesearedescribedelsewhere.TheobservationaleffortshavetobematchedbyaccesstoadequateHighPerformanceComputing(HPC) facilities,as thedevelopmentand testingofnewcodes is integralpartof thebasic researchactivity. Indeed, inmany respects thedevelopmentof computational facilitiesdedicated toHPC inAstronomy,AstrophysicsandCosmology is as importantas thedevelopmentofnewobservationalfacilities.Wearenowwitnessingafirstmigrationoftheastronomicalcommunityasawholetowardstheuseof complex, large-scale, three-dimensional and fullynonlinear simulations for the interpretationoftheobservations.Thismigrationwillonly increase in the futureandsowill theneed fordedicatedcomputationallyintensivesupercomputingfacilities.Hence,thetimehascomeforaseriousplanningandforthecreationofaninfrastructurethatwillgiveGermanscientiststhemuchneededresourcesthat are needed for self-consistent and informative interpretation of the observations fromastrophysicalobjectsexploringtheextremestatesofmatter.

6.SummaryandConclusionsThe German community is extremely strong and verywell positioned in the area of fundamentalphysics and its study via astronomical techniques. This world-leadership position can only bemaintained if access to theworld’sbest facility is available. Inmany cases, thebest advances andmost important insights areonly guaranteedby engaging in long-termefforts, theparticipation inlarge international projects, and the access to sufficiently powerful computational resources.Reflecting on the important results so far, the German community has succeeded to lead manyeffortsandtobevisibleassuch.Withtheappropriateactionthissuccessstorycancontinue,asitisclearclearerthatagoldenerainfundamentalphysicsresearchhastrulyjustbegun.ReferencesAbbott,B.P.,etal.,Phys.Rev.Lett.,116:061102(2016a)Abbott,B.P.,etal.,Phys.Rev.Lett.,116:241103(2016b)Abbott,B.P.,etal.,Phys.Rev.Lett.116:241102(2016c)Antoniadis,J.,etal.,Science,340:1233232(2013)Armano,M.,etal.,Phys.Rev.Lett.,116:2311101(2016)Babak,S.,etal.,Mon.Not.R.Astron.Soc.455:1665(2016)Baiotti,L.,Giacomazzo,B.,Rezzolla,L.,Phys.Rev.D,78:084033(2008)Bertotti,B.,Iess,L.&Tortora,P.,Nature,425:374(2003)Breton,R.P.,etal.,Science,321:104(2008)

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