The Gravitational Universe

8/13/2019 The Gravitational Universe

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The Gravitational UniverseA science theme addressed by theeLISA mission observing the entire Universe

Te last century has seen enormous progress in our understanding othe Universe. We know the li e cycles o stars, the structure o galaxies,the remnants o the big bang, and have a general understandingo how the Universe evolved. We have come remarkably ar usingelectromagnetic radiation as our tool or observing the Universe.However, gravity is the engine behind many o the processes in theUniverse, and much o its action is dark. Opening a gravitationalwindow on the Universe will let us go urther than any alternative.Gravity has its own messenger: Gravitational waves, ripples in the abric o spacetime. Tey travel essentially undisturbed and let us peer

deep into the ormation o the rst seed black holes, exploring redshifsas large as z ~ 20, prior to the epoch o cosmic re-ionisation. Exquisiteand unprecedented measurements o black hole masses and spins willmake it possible to trace the history o black holes across all stageso galaxy evolution, and at the same time constrain any deviation rom the Kerr metric o General Relativity. eLISA will be the rst evermission to study the entire Universe with gravitational waves. eLISAis an all-sky monitor and will offer a wide view o a dynamic cosmosusing gravitational waves as new and unique messengers to unveilTe Gravitational Universe. It provides the closest ever view o theearly processes at eV energies, has guaranteed sources in the ormo verication binaries in the Milky Way, and can probe the entireUniverse, rom its smallest scales around singularities and blackholes, all the way to cosmological dimensions.

Prof. Dr. Karsten DanzmannAlbert Einstein Institute Hannover

MPI or Gravitational Physics andLeibniz Universitt HannoverCallinstr. 3830167 HannoverGermany [email protected]

el.: +49 511 762 2229Fax: +49 511 762 2784

Detailed in ormation athttp://elisascience.org/whitepaper


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2 Te Gravitational Universe Authors, Contributors, Supporters

AUTHORS

Pau Amaro Seoane, Soane Aoudia, Gerard Auger, Stanislav Babak, Enrico Barausse, Massimo Bassan, Vmann, Pierre Bintruy, Johanna Bogenstahl, Camille Bonvin, Daniele Bortoluzzi, Julien Brossard, Iouri BykCaprini, Antonella Cavalleri, Monica Colpi, Giuseppe Congedo, Karsten Danzmann, Luciano Di Fiore, Marcilo, Ingo Diepholz, Rita Dolesi, Massimo Dotti, Carlos F. Sopuerta, Luigi Ferraioli, Valerio Ferroni, Noemi FFitzsimons, Jonathan Gair, Domenico Giardini, Ferran Gibert, Catia Grimani, Paul Groot, Hubert Halloin,

Heinzel, Martin Hewitson, Allan Hornstrup, David Hoyland, Mauro Hueller, Philippe Jetzer, Nikolaos Karnetian Killow, Andrzej Krolak, Ivan Lloro, Davor Mance, Tomas Marsh, Ignacio Mateos, Lucio Mayer, Joseph MGijs Nelemans, Miquel No rarias, Frank Ohme, Michael Perreur-Lloyd, Antoine Petiteau, Eric Plagnol, EdwPierre Prat, Jens Reiche, David Robertson, Elena Maria Rossi, Stephan Rosswog, Ashley Ruiter, B.S. Sathyanard Schutz, Alberto Sesana, Benjamin Sheard, Ruggero Stanga, im Sumner, akamitsu anaka, Michael r

u, Daniele Vetrugno, Ste ano Vitale, Marta Volonteri, Gudrun Wanner, Henry Ward, Peter Wass, William JosePeter Zwei el

Affiliations can be ound in a detailed list o authors athttp://elisascience.org/authors

CONTRIBUTORS

Heather Audley, John Baker, Simon Barke, Matthew Benacquista, Peter L. Bender, Emanuele Berti, Nils CBrause, Sasha Buchman, Jordan Camp, Massimo Cerdonio, Giacomo Ciani, John Conklin, Neil Cornish, GlenDan DeBra, Marc Dewi Freitag, Germn Fernndez Barranco, Filippo Galeazzi, Antonio Garcia, Oliver GLluis Gesa, Felipe Guzman Cervantes, Zoltan Haiman, Craig Hogan, Daniel Holz, C.D. Hoyle, Scott Hugheslogera, Mukremin Kilic, William Klipstein, Evgenia Kochkina, Natalia Korsakova, Shane Larson, Maike LLittenberg, Jeffrey Livas, Piero Madau, Peiman Maghami, Christoph Mahrdt, David McClelland, Kirk McKMcWilliams, Stephen Merkowitz, Cole Miller, Shawn Mitryk, Soumya Mohanty, Anneke Monsky, Guido MMller, Daniele Nicolodi, Samaya Nissanke, Kenji Numata, Markus Otto, E. Sterl Phinney, Scott Pollack, ATomas Prince, Douglas Richstone, Louis Rubbo, Josep Sanjuan, Stephan Schlamminger, Daniel Schtze, Dadock, Sweta Shah, Aaron Spector, Robert Spero, Robin Stebbins, Gunnar Stede, Frank Steier, Ke-Xun Sun, A

ton, David anner, Ira Torpe, Massimo into, Michele Vallisneri, Vinzenz Wand, Yan Wang, Brent Ware, YiNicolas Yunes Affiliations can be ound in a detailed list o contributors athttp://elisascience.org/contributors

SUPPORTERS

Among the, roughly, 1000 scientic supporters o the Gravitational Universe science theme, areG H Utrecht University (Netherlands) , B B Caltech (United States), C C -T -

College de France (France), N G NASA Goddard Space Flight Center (United States), G G -LIGO Scientic Collaboration Spokesperson, LSU (United States), D G Institute o Astronomy, Univer-sity o Cambridge (United Kingdom), S H University o Cambridge, DAM P (United Kingdom), S

K Stan ord University/SLAC National Accelerator Laboratory (United States), M K Stan ord Univer-sity, Physics Dept. (United States), M K Max-Planck-Institut uer Radioastronomie (Germany), AL Harvard University (United States) , P M University o Cali ornia, Santa Cruz (United States), LM Universit di Roma La Sapienza (Italy) , J M NASA Goddard Space Flight Center (United States), D -

M Rochester Institute o echnology (United States), V M LMU Mnchen (Germany) , G P Universita di Roma la Sapienza (Italy) , S S University o Illinois at Urbana-Champaign(United States), G S Universite Paris Diderot (France) , S T Cornell University (United States), K T Cali ornia Institute o echnology (United States), G V Collge de (France) (France), J -Y V Virgo Collaboration Spokesperson, OCA Nice (France), R W MI (United States), C

W University o Florida (United States), E W Institute or Advanced Study, Princeton (United States), A - W Durham University (United Kingdom) , and S -T Y Harvard University (United States) .

A complete list o supporters can be ound athttp://elisascience.org/supporters


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Te Gravitational Universe Introduction 3

INTRODUCTION

In the early years o this millennium, our view o the Uni- verse has been com ortably consolidated in some aspects,but also pro oundly changed in others. In 2003, the doublepulsar PSR J0737-3039 was discovered[12], and GeneralRelativity passed all o the most stringent precision tests in

the weak-eld limit.In 2013, the toughest test on General Relativity was per-ormed through the observation o PSR J0348+0432, a

tightly-orbiting pair o a newly discovered pulsar and itswhite-dwar companion. Given the extreme conditions othis system, some scientists thought that Einsteins equa-tions might not accurately predict the amount o gravi-tational radiation emitted, but General Relativity passedwith ying colours[3]. However, no test o General Rela-tivity could be considered complete without probing thestrong-eld regime o gravitational physics, where mass

motions are close to the speed o light,c, and gravitationalpotentials close to c2: Around a black hole, a central sin-gularity protected by an event horizon, relativistic gravityis extreme. Exploring the physics o the inspiral o a smallcompact object skimming the event horizon o a largeblack hole, or the physics o a black hole-black hole colli-sion, is probing gravity in the relativistic strong-eld limit.2013 marked an important date: ESAs Planck mission con-rmed the -CDM paradigm o cosmology at an unprec-edented level o accuracy, offering the most precise all-skyimage o the distribution o dark matter across the entire

history o the Universe[4], urther conrming that the mi-nuscule quantum uctuations which ormed at the epocho ination were able to grow hierarchically rom the smallto the large scale under the effect o the gravity o darkmatter, and eventually evolved into the galaxies we observetoday with their billions o stars and central black holes[5].In the -CDM model, the rst black holes, called seeds,

ormed in dark matter halos rom the dissipative collapseo baryons[67], and as halos clustered and merged, so didtheir embedded black holes[89]. As a consequence, bina-ry black holes invariably orm, driven by galaxy collisionsand mergers, and trace the evolution o cosmic structures.In 2003 an ongoing merger between two hard X-ray ga-lactic nuclei, with the characteristics o an Active Galac-tic Nucleus (AGN), was discovered just 150 Mpc away inthe ultra-luminous in rared galaxy NGC 6240 (see Fig-ure 1)[10]. Recent optical surveys have shown evidence odual AGN[11] in todays Universe; even Andromeda andour Milky Way are due to collide and both house a centralblack hole!Galaxy mergers were even more requent in the past. Teensuing coalescence o massive black holes offers a new

and unique tool, not only to test theories o gravity andthe black hole hypothesis itsel , but to explore the Universerom the onset o the cosmic dawn to the present.

In 2000, we discovered that dormant black holes are ubiq-

uitous in nearby galaxies and that there are relationshipbetween the black hole mass and the stellar mass o thhost galaxy[1213]. Tis gave rise to the concept thatblack holes and galaxies evolve jointly. Black holes tragalaxies and affect their evolution; likewise, galaxies trablack holes and affect their growth. Coalescing binarblack holes pinpoint the places and times where galaxiemerge, revealing physical details o their aggregation.

Tis new millennium also witnessed the discovery o numerous ultra-compact binary systems containing whitedwar s and/or neutron stars in close orbit, in the MilkWay[14]. Tese are excellent laboratories or exploringthe extremes o stellar evolution in binary systems. Tewill trans orm into ype Ia supernovae or into merginbinaries which will soon be detected by the ground-basegravitational wave detectors LIGO and VIRGO.Stellar mass black holes with a pulsar as companion remain elusive, as they are very rare systems. racing thealmost dark ultra-compact binaries o all avours witgravitational waves all over the Milky Way will reveal hobinary stars ormed and evolved in the disc and halo othe Galaxy. Some o these are already known through eletromagnetic observations and serve as guaranteed verication sources.All o these advances were possible using only our sense or observing the Universe, electromagnetic radtion, tracing electromagnetic interactions o baryonic mater in the Universe. However, almost all o the Univerremains electromagnetically dark. On astronomical scale

gravitation is the real engine o the Universe. By listenito gravity we will be able to see urther than ever be orWe can listen to the Universe by directly observing gratational waves, ripples in the abric o spacetime travling at the speed o light, which only weakly interact wmatter and travel largely undisturbed over cosmologicadistances. Teir signature is a ractional squeezing o spcetime perpendicuar to the direction o propagation, witan amplitudeh = L/L on the order o 1020. Laser inter er-ometry is a standard tool or such measurements and habeen under development or over 30 years now[15].

Electromagnetic observations o the Universe, plus theretical modelling, suggest that the richest part o the gravtational wave spectrum alls into the requency range acessible to a space inter erometer, rom about 0.1 mHz100 mHz. In this band, important rst-hand in ormationcan be gathered to tell us how binary stars ormed in ouMilky Way, and to test the history o the Universe out tredshifs o orderz~ 20, probing gravity in the dynami-cal strong-eld regime and on the eV energy scale o tearly Universe[16]. eLISA will be the rst ever mission tosurvey the entire Universe with gravitational waves, addressing the science theme Te Gravitational Universe . TeNext Gravitational Observatory (NGO) mission concepstudied by ESA or the L1 mission selection is used astrawman mission concept oreLISA [15].


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4 Te Gravitational Universe Astrophysical Black Holes

In Sections I, II, and III, we will describe the eLISA obser-vational capabilities in terms o an observatory sensitivitythat is shown in Figure 12 as a U-shaped curve. Te curveis based upon the sensitivity model o the strawman mis-sion concept presented in Section IV. For the purpose othe discussion in Sections I, II, and III, this is just a base-line requirement. Te question o how this requirementcan be met, or exceeded, in an actual mission is addressedin Section IV.

I. ASTROPHYSICAL BLACK HOLESAs we will discuss in more detail in this section,eLISA ob-servations will probe massive black holes over a wide, al-most unexplored, range o redshif and mass, covering es-sentially all important epochs o their evolutionary history.eLISA probes are coalescing massive binary black holes,which are among the loudest sources o gravitational wavesin the Universe. Tey are expected to appear at the cosmicdawn, around a redshif oz ~ 11 or more, when the rstgalaxies started to orm. Coalescing binary black holes ata redshif as remote asz ~ 20 can be detected witheLISA,i they exist.eLISA will also explore black holes throughcosmic high noon (a term introduced by[17]), at redshifso z ~ 3 toz ~ 1.5, when the star ormation rate in the Uni- verse and the activity o Quasi Stellar Objects (QSOs) andAGNs was highest. In the late cosmos, atz < 1,eLISA willcontinue to trace binary mergers, but it will also detectnew sources, the Extreme Mass Ratio Inspirals (EMRIs),i.e., the slow inspiral and merger o stellar mass black holes

into large, massive black holes at the centres o galaxies.Tese are excellent probes or investigating galactic nucleiduring the AGN decline.

I.I Massive binary black holesTe exploration o the sky across the electromagneticspectrum has progressively revealed the Universe at thetime o the cosmic dawn. Te most distant star collapsinginto a stellar mass black hole, the Gamma Ray Burst GRB090429B, exploded when the Universe was 520 Myr old (ata redshif o z ~ 9.4), conrming that massive stars wereborn and died very early on in the li e o the Universe[18].Te most distant known galaxy, MACS0647-JD, at a red-shif o z ~ 10.7, was already in place when the Universewas about 420 Myr old[19], and ULAS J1120+0641 holdsthe record or being the most distant known QSO, thus themost distant supermassive (~ 109 M 9 ) accreting black holeat redshifz ~ 7.08, about 770 Myr afer the big bang[20].Such observations clearly show that stars, black holes, andgalaxies, the key, ubiquitous components o the Universe,were present be ore the end o the reionisation phasearoundz ~ 6[21]. Tese are the brightest sources, prob-ing only the peak o an underlying distribution o smallerobjects: the less luminous pre-galactic discs, and the lessmassive stars and black holes, o which little is known.Even the brightest QSOs ade away in the optical regime

due to the presence o neutral hydrogen in the intergalactmedium (the Gunn-Peterson trough,[22]), and the search

or the deepest sources using X-rays may be hindered bintrinsic obscuration, con usion due to crowding and thunresolved background light[23].Te entire zoo o objects, which in the past ormed thsmall building blocks o the largest ones we see today,so ar pretty much unexplored. Tese primitive objectsstarted to orm at the onset o the cosmic dawn, arounz ~ 20 30, according to current cosmological models[24].In act, simulations indicate that the very early pre-galatic, gas-rich discs had low masses, small luminosities anwere very metal-poor. At an epoch oz ~ 20 to 30, the earli-est stars may have had masses exceeding 100 M 9 , endingtheir lives as comparable stellar mass black holes, proviing the seeds that would later grow into supermassive blacholes[6, 25]. However, as larger, more massive and metalenriched galactic discs progressivly ormed, other path

or black hole seed ormation became viable (see[26] or areview). Global gravitational instabilities in gaseous dismay have led to the ormation o quasi-stars o 103 104 M 9 that later collapsed into seed black holes[7]. Further alter-natives arise in the orm o the collapse o massive s

ormed in run-away stellar collisions in young, dense stclusters[27] or the collapse o unstable sel -gravitating gaclouds in the nuclei o gas-rich galaxy mergers at later eochs[28]. Tus, the initial mass o seed black holes remainsone o the largest uncertainties in the present theory oblack hole ormation, as the mechanism is still unknowand the electromagnetic horizon too small or the direcdetection o the ormation o individual seeds.

Most o the investigations o galaxies and AGNs in telectromagnetic Universe, in terms o richness o soues, ocus on a later epoch: the cosmic high noon, a perod aroundz ~ 1.5 3. Tis epoch eatures several critical

Figure 1: Merging galaxy NGC 6240 and a representative waveform ofthe expected gravitational waves from the coalesence of two super-massive black holes. Observations with NASAs Chandra X-ray observa-tory have disclosed two giant black holes inside NGC 6240. They will drifttoward one another and eventually merge into a larger black hole. Credit:NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).


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trans ormations in galaxy evolution, since around thattime both the luminous QSOs and the star ormation ratewere at their peak[2930]. Galaxy mergers and accretionalong laments during cosmic high noon were likely to bethe driving orce behind the processes o star ormation,black hole ueling, and galaxy growth. Tis turned star-

orming discs into larger discs or quenched spheroidal sys-tems hosting supermassive black holes o billions o solar

masses[3133]. In this ramework, massive black hole bi-naries inevitably orm in large numbers, over a variety omass scales, driven by requent galaxy mergers[89, 34].Signs o galaxy mergers with dual black holes at wide sepa-rations (on the order o kpc) come rom observations odual AGNs in optical and X-ray surveys, while observa-tions o binary black holes with sub-pc scale separationsremain uncertain and only candidates exist at present[35].Studies o the dynamics o black holes in merging galaxieshave shown that black hole coalescences trace the mergero the dense baryonic cores better than the mergers o dark

halos, as their dynamics are sensitive to gas and star con-tent and eed-back[3637].Teoretical models developed in the context o the -CDMparadigm[3841] have been success ul in reproducingproperties o the observed evolution o galaxies and AGNs,such as the colour distribution o galaxies, the local massdensity and mass unction o supermassive black holes,and the QSO luminosity unction at several wavelengthsout toz ~ 6. In ormation about the underlying popula-tion o inactive, less massive and intrinsically ainter blackholes, which grew through accretion and mergers acrossall cosmic epochs, is still lacking and difficult to gather.Te Gravitational Universe proposes a unique, new way toprobe both cosmic dawn and high noon, to address a num-ber o unanswered questions:

When did the rst black holes orm in pre-galactic halos,and what is their initial mass and spin?

What is the mechanism o black hole ormation in ga-lactic nuclei, and how do black holes evolve over cosmictime due to accretion and mergers?

What is the role o black hole mergers in galaxy orma-tion?

eLISA will study the evolution o merging massive blackholes across cosmic ages, measuring their mass, spin andredshif over a wide, as yet unexplored, range. Black holeswith masses between 104 M 9 and 107 M 9 will be detected byeLISA, exploring or the rst time the low-mass end o themassive black hole population, at cosmic times as early asz ~ 10, and beyond.

eLISA discovery domain

Coalescing black hole binaries enter theeLISA sensitiv-ity band rom the low requency end, sweeping to higherrequencies as the inspiral gets aster and aster, as shown

in Figure 13. Eventually they merge, with the ormation

o a common event horizon, ollowed by the ringdowphase during which residual de ormation is radiated awaand a rotating (Kerr) black hole remnant is ormed. Te

wave orm detected byeLISA is a measure o the ampli-tude o the strain in space as a unction o time in the rrame o the detector. Tis wave orm carries in ormati

about the masses and spins o the two black holes prioto coalescence, the inclination o the binary plane reltive to the line o sight, the luminosity distance and sklocation, among other parameters[42]. Complete wave-

orms have been designed by combining Post Newtoniaexpansion wave orms or the early inspiral phase with analytical description o the merger and ringdown phascalibrated against highly accurate, ully general relativisnumerical simulations o black hole coalescence[4344].Te rst gure o merit o theeLISA per ormance is thesignal-to-noise ratio (SNR) o a massive black hole binacoalescence with parameters in the relevant astrophysicrange. Figure 2 showseLISA SNRs or equal mass, non-spinning coalescing binaries. Here we compute the SNRas a unction o the total mass, M , and o the redshif,z ,averaging over all possible source sky locations and wapolarisations, assuming two-year observations. Te plothighlights the extraordinary capabilities o the instrument in covering almost all o the mass-redshif parameter space needed to trace black hole evolution. Binariewith 104 M 9 < M < 107 M 9 can be detected out toz ~ 20 withan SNR 10, i they exist. Figure 2 shows that virtuaall massive black holes in the Universe were loudeLISA sources at some point in their evolution.

Figure 2: Constant-contour levels of the sky and polarisation angle-averaged SNR for eLISA, for equal mass non-spinning binaries as afunction of their total rest frame mass, M, and cosmological redshift, z .The tracks represent the mass-redshift evolution of selected supermas-

sive black holes: two possible evolutionary paths for a black hole power-ing a z ~ 6 QSO (starting from a massive seed, blue curve, or from a PopIII seed from a collapsed metal-free star, yellow curve); a typical 10 9 M 9 black hole in a giant elliptical galaxy (red curve); and a Milky Way-like blackhole (green curve). Circles mark black hole-black hole mergers occurringalong the way. These were obtained using state of the art semi-analyticalmerger tree models [65]. The grey transparent area in the bottom rightcorner roughly identi es the parameter space for which massive blackholes might power phenomena that will likely be observable by futureelectromagnetic probes.

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6 Te Gravitational Universe Astrophysical Black Holes

Figure 3 shows error distributions in the source parameterestimation or events collected and extracted rom a meta-catalogue o ~ 1500 simulated sources. Te catalogue isconstructed by combining predicted merger distributions

rom a number o cosmological models encompassing abroad range o plausible massive black hole evolution sce-narios[45]. Uncertainties are evaluated using the FisherIn ormation Matrix approximation, which gives an esti-

mate o the errors on the in erred parameters. Figure 3 il-lustrates that individual redshifed masses can be measuredwith unprecedented precision, with an error o 0.1 % 1 %(we recall that observations can only determine the red-shifed source masses, i.e., the product o mass and (1 +z )).Te spin o the primary black hole can be measured with very high accuracy, with 0.01 0.1 absolute uncertainty.Current theoretical models predict coalescence rates inthe range 10 100 per year[4648]. For more than 10 % othese, mostly occurring at a redshif oz < 5, the distancecan be determined to better than a ew percent and the skylocation determined to better than a ew degrees, whichmakes these sources suitable targets or coincident search-es o electromagnetic counterparts (see Section V).

Astrophysical impact

eLISA will allow us to survey the vast majority o all coa-lescing massive black hole binaries throughout the wholeUniverse. Tis will expose an unseen population o objectswhich will potentially carry precious in ormation aboutthe entire black hole population. It will provide both thewidest and deepest survey o the sky ever, since gravita-tional wave detectors are essentially omni-directional bynature, and thus act as ull-sky monitors. As highlighted inFigure 2, the range o black hole redshifs and masses thatwill be explored is complementary to the space explored by

electromagnetic observations (see Figure 2).eLISA will create the rst catalogue o merging black holewhich will enable us to investigate the link between thgrowing seed population and the rich population o activsupermassive black holes evolving during cosmic dawand high noon. In doing this, we will probe the light eno the mass unction at the largest redshifs and investigathe role o early black holes in cosmic re-ionisation and theating o the intergalactic medium[49].Black hole coalescence events will illuminate the physicprocess o black hole eeding. While the mass distributcarries in ormation about the seeds, the spin distributiocharts the properties o the accretion ows, whether theare chaotic or coherent[50]. Gravitational wave observa-tions alone will be able to distinguish between the differeevolution scenarios[46].

I.II Extreme Mass Ratio Inspirals (EMRIs)Te present Universe is in a phase in which both the star

ormation rate and AGN activity are declining. In this lacosmos we observe quiescent massive black holes at thcentres o galaxies within a volume o about 0.02 Gpc3[51].Te current census comprises about 75 massive black holeout toz < 0.03. Te black hole o 4 106 M 9 at the GalacticCentre is the most spectacular example[5253]. Tanks toits proximity, a young stellar population has been revealeprecisely where no young stars were predicted to orm, star- orming clouds are expected to be tidally disruptethere[54]. Tis indicates our lack o understanding aboutthe origin o stellar populations around black holes, anin particular stellar dynamics, even in our own Galaxy. Bprobing the dynamics o intrinsically dark, relic stars in tnearest environs o a massive black hole,eLISA will offerthe deepest view o galactic nuclei, exploring phenomeinaccessible to electromagnetic observations[5556]. Teprobes used are the so-called EMRIs: a compact star (ether a neutron star or a stellar mass black hole) captured

Figure 4: An artists impression of the spacetime of an extreme-mass-ratio inspiral and a representative waveform of the expected gravita-tional waves. A smaller black hole orbits around a supermassive blackhole. Credit: NASA.

Figure 3: eLISA parameter estimation accuracy for massive black holebinaries probability density functions. Left panels show errors on theredshifted mass, and right panels on the spins. Top panels refer to theprimary black hole, bottom panels to the secondary black hole.

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in a highly relativistic orbit around the massive black holeand spiralling through the strongest eld regions a ewSchwarzschild radii rom the event horizon be ore plung-ing into it (Figure 4).Key questions can be addressed in the study o galactic nu-clei with EMRIs:

What is the mass distribution o stellar remnants at the

galactic centres and what is the role o mass segregationand relaxation in determining the nature o the stellar populations around the nuclear black holes in galaxies?

Are massive black holes as light as ~105 M 9 inhabitingthe cores o low mass galaxies? Are they seed black holerelics? What are their properties?

eLISA will observe EMRI events, exploring the deepest re-gions o galactic nuclei, those near the horizons o blackholes with masses close to the mass o the black hole at ourGalactic Centre, out to redshifs as large asz ~ 0.7.Stellar mass black holes are expected to dominate theobserved EMRI rate oreLISA, as mass segregation bytwo-body relaxation tends to concentrate the heavi-est stars near the central, massive black hole[16, 5758],and a stellar mass black hole inspiral has a higher SNR,so it can be detected out to arther distances. EMRIs canbe tracked around a central black hole or up to 104 105 cycles on complex relativistic orbits (see Figure 5). As aconsequence, the wave orm carries an enormous amount

o in ormation[59]. Trough observations o dark com-ponents alone,eLISA will detect EMRIs with an SNR > 20in the mass interval or the central black hole betwee104 M 9 < M < 5 106 M 9 out to redshifz ~ 0.7 (see Fig-ure 13), covering a co-moving volume o 70 Gpc3, a muchlarger volume than observations o dormant galactic nuclei today. Te estimated detection rates, based on the bestavailable models o the black hole population and o tEMRI rate per individual galaxy[60], are about 50 events

or a 2 year mission, with a actor o 2 uncertainty the wave orm modelling and lack o knowledge about system parameters, and an additional uncertainty o aleast an order o magnitude stemming rom the uncertadynamics o dense stellar nuclei[6162]. As shown in Fig-ure 6, the masses o both black holes are, in most casemeasured to better than one part in 104, the eccentricity atplunge is determined to a 104 accuracy, and the spin o theprimary black hole to better than 103. Te deviation o thequadrupole moment o the massive black hole with respeto the Kerr metric value is determined to better than 0.01enabling unprecedented tests o General Relativity to per ormed (see Section II).

Astrophysical impact

In Te Gravitational Universe , EMRIs are exquisite probesor testing stellar mass black hole populations in galac

nuclei. WitheLISA we will learn about the mass spectrumo stellar mass black holes, which is largely unconstrainboth theoretically and observationally. Te measuremento even a ew EMRIs will give astrophysicists a totallyway o probing dense stellar systems, allowing us to detmine the mechanisms that govern stellar dynamics in thegalactic nuclei[58].

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Figure 6: eLISA parameter estimation accuracy for EMRIs. Top left pan-el: estimation of the redshifted masses ( lled: massive black hole; solidline: stellar black hole); top right panel: spin of the massive black hole;bottom left panel: eccentricity at plunge; bottom right panel: minimummeasurable deviation of the quadrupole moment of the massive blackhole from the Kerr value.

Figure 5: EMRI orbit and signal. In the top panel we see the geometricalshape of the ornate relativistic EMRI orbit. The lower panel shows the cor-responding gravitational wave amplitude as a function of time.

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l d i r e c t i o n

horizontal direction

-10 108-8 -6 6-4 -2 2 40

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8 Te Gravitational Universe Te Laws of Nature

Measurements o a hand ul o events will suffice to con-strain the low end o the massive black hole mass unc-tion, in an interval o masses where electromagnetic ob-servations are poor, incomplete or even missing[63]. By2028 the Large Synoptic Survey elescope (LSS ) will haveobserved a large number o tidal disruption events[64],which will also teach us a lot about black holes and stellarpopulations in galactic centres. However, these events will

typically be on the higher end o the black holes mass unc-tion, and will not reveal masses with the same precision aseLISA, whose observations will give us in ormation aboutthe nature and the occupation raction o massive blackholes in low mass galaxies (as yet unconstrained)[65]. Tiswill provide additional in ormation about the origin omassive black holes, complementary to that gathered viaobservations o highz massive black hole mergers. EMRIswill also provide the most precise measurements o MilkyWay type massive black hole spins, and make it possible

or us to investigate the spin distribution o single, massive

black holes up toz ~ 0.7. EMRIs can occur around blackholes in all galaxies, regardless o their nature, i.e., whetherthey are active or not. As such, spin measurements will notbe affected by observational uncertainties in the spectra othe AGN.Detection rates will tell us about the density o stellar massblack holes in the vicinity o the central black hole, con-straining the effectiveness o the mass segregation process-es[66]. Te measured eccentricity and orbital inclinationdistributions can be linked directly to the pre erred chan-nel o EMRI ormation, giving us important clues aboutthe efficiency o dynamical relaxation, and the requencyo binary ormation and breakup in dense nuclei[67].

II. THE LAWS OF NATURETis section covers tests o strong gravity and cosmology,two regimes whereTe Gravitational Universe will offermajor advances.

II.I High precision measurements of stronggravityEinsteins General Relativity is one o the pillars o moderncosmology. Te beauty o General Relativity is that it is a

alsiable theory: once the underlying mass distributionis identied to be a black hole binary system with xedmasses and spins, the theory has no urther adjustableparameters. Tus even a single experiment incompatiblewith a prediction o the theory would lead to its invalida-tion, at least in the physical regime o applicability o theexperiment.Te Gravitational Universe will explore relativistic gravityin the strong-eld, non-linear regime. It seems unlikelythat any other methods will achieve the sensitivity oeLISA to deviations o strong-eld gravity by 2028 (see SectionV). Unlike the ground-based instruments,eLISA will have

sufficient sensitivity to observe even small corrections Einstein gravity.Te strong-eld realm o gravity theories can be probednear the event horizon o Kerr black holes or in othelarge-curvature environments (e.g., in the early UniverseGravity can be thought o as strong in the sense that gravtational potentials are a signicant raction oc2 or in thesense that the curvature tensor (or tidal orce) is o velarge magnitude. esting strong gravity takes on differenmeanings depending upon which notion o strong gravitis being used. In any case, both are very important to tewith high precision. Te measurement o the anisotropyo the Cosmic Microwave Background (CMB) by ESAPlanck satellite directly constrains properties o the quatum uctuations during ination which are thought to bethe seeds o structure ormation. Tis connection betweephysics on the smallest and largest scales is strong mot vation or testing General Relativity to the highest posble accuracy, and in particular in the strong-eld regimewhere deviations could hint at a ormulation o a quantutheory o gravity.Te nature o gravity in the strong-eld limit is, so alargely unconstrained, leaving open several questions:

Does gravity travel at the speed o light ? Does the graviton have mass? How does gravitational in ormation propagate: Are

there more than two transverse modes o propagation? Does gravity couple to other dynamical elds, such as,

massless or massive scalars? What is the structure o spacetime just outside astrophysical black holes? Do their spacetimes have horizons?

Are astrophysical black holes ully described by the Kerrmetric, as predicted by General Relativity?

An outstanding way to answer these questions and learnabout the undamental nature o gravity is by observithe vibrations o the abric o spacetime itsel , or wcoalescing binary black holes and EMRIs are ideal probe

Exploring relativistic gravity with binary black holemergers in the strong- eld, dynamical sector

Te coalescence o a massive black hole binary with maratio above one tenth generates a gravitational wave signal strong enough to allow detection o tiny deviation

rom the predictions o General Relativity. Te signal comprises three partsinspiral, merger and ringdowneacho which probes strong-eld gravity. Te inspiral phase iwell understood theoretically: It can last several monthin-band, and it could be observed with an SNR o tens hundreds byeLISA. Te non-linear structure o GeneralRelativity, and possible deviations rom it, are encoded the phase and amplitude o the gravitational waves. Aneffect that leads to a cumulative dephasing o a signica

raction o a wave cycle over the inspiral phase can be


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Te Gravitational Universe Te Laws of Nature 9

tected through matched ltering. Tousands o inspiralwave cycles will be observed, making it possible to detecteven very small deviations in the inspiral rate predictedby General Relativity[6871]. Te propagation o gravita-tional waves can be probed through the dispersion o theinspiral signal. In General Relativity, gravitational wavestravel with the speed o light (the graviton is massless)and interact very weakly with matter. Alternative theories

with a massive graviton predict an additional requencydependent phase shif o the observed wave orm due todispersion that depends on the gravitons mass,mg , and thedistance to the binary. AneLISA-like detector could set abound aroundmg < 4 1030eV[72], improving current So-lar System bounds on the graviton mass,mg < 4 1022 eV,by several orders o magnitude. Statistical analysis o anensemble o observations o black hole coalescences couldalso be used to place stringent constraints on theories withan evolving gravitational constant[73] and theories withLorentz-violating modications to General Relativity[74].

Te inspiral is ollowed by a dynamical coalescencethat produces a burst o gravitational waves. Tis is abrie event, comprising a ew cycles and lasting about5 103 sec ( M /106 M 9 ), yet it is very energetic, releasing1059 ( M /106 M 9 ) ergs o energy, corresponding to 1022 timesthe power o the Sun.Afer the merger, the asymmetric remnant black hole set-tles down to a stationary and axisymmetric state throughthe emission o quasi-normal mode (QNM) radiation. InGeneral Relativity, astrophysical black holes are expected

to be described by the Kerr metric and characterised byonly two parameters: mass and spin (the no-hair theo-rem). Each QNM is an exponentially damped sinusoidwith a characteristic requency and damping time that de-pends only on these two parameters[7577]. A measure-ment o two QNMs will there ore provide a strong-eld verication that the nal massive object is consistent withbeing a Kerr black hole[7879]. Te QNM spectrum o ablack hole also has unique eatures which allow it to be dis-tinguished rom other (exotic) compact objects[8083].eLISA will observe ringdown signals with sufficient SNRto carry out these tests[8486].

Exploring relativistic gravity with EMRIs in the strong-eld, stationary, non-linear regime

EMRIs will provide a precise tool to probe the structure ospacetime surrounding massive black holes. Te inspiral-ling compact object can generate hundreds o thousands ogravitational wave cycles while it is within ten Schwarzs-child radii o the central black hole. Tese wave orm cyclestrace the orbit that the object ollows, which in turn mapsout details o the underlying spacetime structure, in a way

similar to how stellar orbits have been used to preciselycharacterise the supermassive black hole at the centre othe Milky Way[8788].As seen in Figure 6, EMRI observations will not only pro-

vide very precise measurements o the standard parameters o the system, but will provide strong constraints odepartures o the central massive object rom the Keblack hole o General Relativity[89]. Te no-hair theoremtells us that the stationary axisymmetric spacetime arounit should be completely determined by its mass and spiparameter. Te gravitational wave signal rom an EMRoccurring in a bumpy black hole spacetime in which th

multipole moments differ rom their Kerr values woushow distinctive, detectable signatures[8994]. Figure 6shows that 10 % deviations in the mass quadrupole moment,Q, rom the Kerr value would be detectable or anEMRI observed with an SNR greater than 20. For typicsystems, 0.1 % deviations will be detectable, and or tbest systems, 0.01 % deviations will be detectable[59].An observed inconsistency with the Kerr multipole structure might indicate a surprisingly strong environmentalperturbation, the discovery o a new type o exotic copact object consistent with General Relativity, or a ailuin General Relativity itsel , but these possibilities will observationally distinguishable ( or a review o differhypotheses see[9597]). Tese deviations could exhibitthemselves in the ollowing ways: For a boson star, tEMRI signal would not shut off afer the last stable orbit[98]. For horizonless objects such as gravastars, the or-biting body would resonantly excite the modes o the (ptative) membrane replacing the black hole horizon[99],and or certain non-Kerr axisymmetric geometries, orbicould become ergodic[100] or experience extended reso-nances[101].

Alternatives that will be testable witheLISA observationsinclude the dynamical Chern-Simons theory[102105],scalar-tensor theories (with observable effects in neutron star-black hole systems where the Neutron Star (NScarries scalar charge[106]), Randall-Sundrum-inspiredbraneworld models[107108] and theories with axionsthat give rise to oating orbits[109]. Generic alternativescould also be constrained using phenomenologically parametrised models[110].

Cosmography

Te Gravitational Universe will use black hole binarymergers as standard sirens to extract in ormation on thexpansion o the Universe, by measuring the expansiohistory with completely different techniques to electromagnetic probes. Te term standard siren or gravitationawave sources, re ers to a source that has its absolute lumnosity encoded in its signal shape, analogous to a standard candle (like a ype Ia supernova) or electromagnetsources. Black hole coalescences could serve as standasirens or cosmography[111112] by providing abso-lute and direct measurements o the luminosity distancDL(z ). When coupled with independent measurements oredshif,z , ( or example, rom associated transient electromagnetic sources ), these standard siren sources put pointon the distance vs. redshif curve, and directly constrain


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10 Te Gravitational Universe Te Laws of Nature

the evolution history o the Universe. Several mechanismshave been proposed that will provide an electromagneticcounterpart to massive black hole coalescences detectablebyeLISA [113], however, condent identication might be viable only or low redshif events. Although rare, eventsat redshifz 1 2 are so loud that the baselineeLISA mission has the capability o localising them to within 10square degrees, perhaps even 1 square degree, when in-

ormation rom the late merger o the black holes is in-cluded in the measurement model. Tis pins down theseevents on the sky well enough to allow searches or elec-tromagnetic counterparts to the merger using wide areasurveys such as LSS that will be active in 2028. With anassociated counterpart,eLISA observations will allow 1 %measurements oDL(z ) or 60 % o the sources, offering theprospect o ultra-precise determination o points on thedistance-redshif curve that are completely independent oall existing constraints rom ype Ia supernova, the CMB,etc. It is to be emphasised that there is no distance lad-

der in these measurements, since the luminosity distanceis measured directly. Tis is possible because these sourcesare undamentally understood, and General Relativity cal-ibrates the distances. Weaker statistical constraints couldalso be derived in the absence o electromagnetic coun-terparts, using mergers at low redshif (z < 2)[114] or EM-RIs[115116]. EMRI observations could provide an inde-pendent measurement oH 0 to a precision o a ew percent.

Impact on science

Te Gravitational Universe will permit unprecedented

measurements o General Relativity in the strong-eld re-gime.eLISA will map the spacetime around astrophysicalblack holes, yielding a battery o precision tests o GeneralRelativity in an entirely new regime. Tese have the poten-tial to uncover hints about the nature o quantum gravity,as well as enabling measurements o the properties o theUniverse on the largest scales.

II.II Cosmology on the TeV energy scale afossil background of gravitational wavesSeveral processes occurring at very high energies in theprimordial Universe can produce a stochastic backgroundo gravitational waves. Te detection o this relic radiationwould have a pro ound impact both on cosmology and onhigh energy physics. Any ossil radiation o gravitationalwaves, i not washed away by ination and later phase tran-sitions, would have decoupled rom matter and energy atthe Planck scale. It can there ore directly probe cosmologi-cal epochs be ore the decoupling o the cosmic microwavebackground, currently our closest view o the big bang(Figure 7). Te characteristic requency o the gravitation-al waves is set by the horizon scale and there ore by thetemperature o the Universe at the time o production. TeeLISA requency band o 0.1 mHz to 100 mHz correspondsto 0.1 to 100 eV energy scales in the early Universe, atwhich new physics is expected to become visible. Te

Large Hadron Collider (LHC) has been built to investigathe physics operating at this energy scale, and in 2012 thexperiment produced with the remarkable discovery othe Higgs boson which has completed the particle spectrum o the Standard Model. It is the nal conrmatiothat spontaneous symmetry breaking is the mechanismat play in electroweak physics, and is the rst example a undamental scalar eld playing a role in a phase transition that took place in the very early Universe. Tesendings urther motivate the search or a cosmic bacground o gravitational waves.eLISA would have the sen-sitivity to detect a relic background created by new phyics active at eV energies i more than a modest ractiGW ~ 105 o the energy density o the Universe is, con verted to gravitational radiation at the time o productioA gravitational wave detector in space has the potentiato revolutionise our understanding o the physics o thin ant Universe by exploring the microphysical behavioo matter and energy through the direct detection o gratational waves produced at this epoch, rather than by observing collisions o elementary particles.

Discovery space

Abundant evidence suggests that the physical vacuum hanot always been in its current state, and in many theoriebeyond the Standard Model, the conversion between vacuum states corresponds to a rst-order phase transitionAs the Universe expands and its temperature drops belowthe critical temperature, bubbles o a new phase orm, epand, and collide, generating relativistic bulk ows, whoenergy then dissipates in a turbulent cascade. Te cor-responding acceleration o matter radiates gravitationawaves on a scale not ar below the horizon scale[117120].eLISA could detect these gravitational waves, thus probinthe Higgs eld sel couplings and potential, and the posble presence o supersymmetry, or o con ormal dynamat eV scales. In general, since the Hubble length at th

eV scale is about 1 mm, the current threshold at whic

Figure 7: Evolution timeline (big bang and the early Universe) and a typi-cal random waveform of the expected gravitational waves. Gravitationalwaves are the only way to see beyond the cosmic microwave background.Credit: NASA / WMAP science team.


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Te Gravitational Universe Ultra-compact binaries in the Milky Way 11

the effects o extra dimensions might appear happens to beabout the same or experimental gravity in the laboratoryand or the cosmological regime accessible toeLISA, thusallowingeLISA to probe the dynamics o warped sub-mil-limetre extra dimensions, present in the context o somestring theory scenarios[121122].In some braneworld scenarios the Planck scale itsel is not

ar above the eV scale. Consequently, the reheating tem-perature would be in the eV range andeLISA could probeinationary reheating. Afer ination the internal poten-tial energy o the inaton is converted into a thermal mixo relativistic particles, which can generate gravitationalwaves with an energy o about 103 or more o the totalenergy density [123125]. eLISA could also probe gravi-tational waves produced directly by the amplication oquantum vacuum uctuations during ination, in the con-text o some unconventional inationary models, such aspre-big bang or bouncing brane scenarios[126128].

Phase transitions ofen lead to the ormation o one-di-mensional topological de ects known as cosmic strings.Fundamental strings also arise as objects in string theoryand, although ormed on submicroscopic scales, it has beenrealised that these strings could be stretched to astronomi-cal size by cosmic expansion[129130]. Cosmic strings in-teract and orm loops which decay into gravitationl waves;eLISA will be the most sensitive probe or these objects,offering the possibility o detecting direct evidence o un-damental strings. Te spectrum rom cosmic strings is dis-tinguishably different rom that o phase transitions or anyother predicted source[130]: It has nearly constant energyper logarithmic requency interval over many decades athigh requencies, offering the possibility o simultaneousdetection byeLISA and ground-based inter erometers.Moreover, i strings are not too light, occasional distinc-tive gravitational wave bursts might be observed romkinks or cusps on string loops. I detected, these individualbursts will provide irre utable evidence or a cosmic stringsource.

III. ULTRA-COMPACT BINARIES INTHE MILKY WAY

Only a minority o the stars in the Universe are compan-ionless, the majority being part o binary or multiple starsystems (Figure 8). About hal o the binaries orm withsufficiently small orbital separations to interact and evolveinto compact systems, ofen with white dwar s, neutronstars, or possibly stellar mass black holes as components.Te shortest period systems, known as ultra-compact bi-naries, are important sources o gravitational waves in themHz requency range[131]. Tese binaries are the out-come o one or more common-envelope phases that occurwhen one star evolves to the giant or supergiant stage. Un-stable mass exchange leads to a short-lived phase in which

the stellar core and the companion spiral towards eachother, trans erring angular momentum to the extendedenvelope o the giant that is blown away, leaving a tight nary system behind. Tis a necessary step in the ormationo X-ray binaries, binary pulsars and double white dwabinaries, observed in a variety o states and congurations[132]. Altough the Milky Way is ull o these sourceonly a tiny raction are currently observed and studied indepth through observations rom radio to X-rays.Afer the common-envelope phase, the compact stars inthe binary are well separated, but evolve with shorter anshorter orbital periods due to the angular momentum los via gravitational waves, until eventually the stars underganother mass exchange once they reach orbital periodo minutes or less. Depending on the nature o the compact objects, they either merge or survive. Double neutrostar binaries ultimately merge in bursts o high- requengravitational waves, potentially emitting a burst o eletromagnetic radiation in the orm o a short Gamma-RaBurst. Although predicted to exist, no neutron star-stellamass black hole binary or black hole-black hole binary hyet been detected. For binaries with a white dwar compnent, mass loss is delicately balanced against loss o anglar momentum, the outcome o which is unclear.Almost always, the loss o energy through gravitationwaves is so strong that it causes the system to merge anpossibly to end in a ype Ia or sub-luminous supernovaexplosion[133134] or in (rapidly spinning) neutron starsthat may have millisecond radio pulsar or magnetar properties[135]. In the remaining small raction o cases, thmass trans er stabilises the system and long-lived interacing binaries are ormed (as in the AM CVn systems i companion is a white dwar , or ultra-compact X-ray binries i it is a neutron star). Te physics o this evolutiona junction is rich and diverse, it involves tides, mass trans

er, highly super-Eddington accretion, and mass ejectioeLISA will provide the data necessary or us to quanti y troles that these various physical processes play.

Figure 8: Illustration of a compact binary star system and a representa-tive waveform of the expected gravitational waves. Two stars orbitingeach other in a death grip are destined to merge all the while floodingspace with gravitational waves. Credit: GSFC/D.Berry.


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12 Te Gravitational Universe Ultra-compact binaries in the Milky Way

Currently ewer than 50 ultra-compact binaries areknown, only two o which have periods less than 10 min-utes[14]; eLISA will discover several thousand o these.Tese systems are relatively short-lived and electromag-netically aint, but several known verication binaries arestrong enough gravitational wave sources that they will bedetected within weeks byeLISA. Te discovery o manynew ultra-compact binaries is one o the main objectives oeLISA, as it will provide a quantitative and homogeneousstudy o their populations and the astrophysics govern-ing their ormation. It also adds an additional acet to theknowledge o the Milky Ways structure: Te distributiono sources in the thin disc, thick disc, halo and its globu-lar clusters (known breeding grounds or the ormation ocompact binaries via dynamical exchanges)[136]. Tesedetections will enable us to address a number o key ques-tions:

How many ultra-compact binaries exist in the MilkyWay?

What is the merger rate o white dwar s, neutron starsand stellar mass black holes in the Milky Way (thus bet-ter constraining the rate o the explosive events associ-ated with these sources)?

What does that imply or, or how does that compare to,their merger rates in the Universe?

What happens at the moment a white dwar starts massexchange with another white dwar or neutron star, andwhat does it tell us about the explosion mechanism otype Ia supernovae?

What is the spatial distribution o ultra-compact bina-ries, and what can we learn about the structure o the Milky Way as a whole?o answer these questionseLISA will observe thousands o

individual sources and or the rst time capture the signalrom a oreground o sources, thesound o millions o tight

binaries.

Discovery spaceTe vast majority o ultra-compact binaries will orm an

unresolved oreground signal ineLISA [137] as shown inFigure 13. Its average level is comparable to the instrumentnoise, but due to its strong modulation during the year (bymore than a actor o two) it can be detected. Te overallstrength can be used to learn about the distribution o thesources in the Galaxy, as the different Galactic components(thin disc, thick disc, halo) contribute differently to themodulation[138]. Teir relative amplitudes can be used toset upper limits on the, as yet completely unknown, halopopulation[139].For a two-yeareLISA mission, several thousand binaries are

expected to be detected individually with an SNR > 7[140] and their periods (below one hour and typically 5 10 min)determined rom the periodicity o the gravitational wavesignal. For many systems it will be possible to measure the

rst time derivative o the requency, and thus determithe chirp mass (a combination o the masses o the twstars that can be used to distinguish white dwar , neutrostar and black hole binaries) and the distance to the sourceFor more than 100 sources concentrated around the innerGalaxy, we expect distance estimates with accuracies betthan 1 %, enabling us to make a direct measurement o tdistance to the Galactic Centre.

Te number o ultra-compact binaries with neutron staror black hole components is still highly uncertain[141].WitheLISA operating as an all sky monitor, these systemscan be observed throughout the Milky Way, providing acomplete sample o binaries at the shortest periods (belo30 minutes), including ones containing stellar mass blacholes, i they exist. Te number o sources detected at mH

requencies is directly related, via the gravitational waorbital decay time scale, both to the number o system

ormed at larger separations, and to the number o merers. Tere ore, the thousands oeLISA detections will alsoprobe the ormation o these binaries and their coalescenrates. UsingeLISA, the sky position o about feen hun-dred sources will be determined to better than ten squardegrees and more than hal will have their distance detemined to better than 20 %[142]. Several hundred o thesemay be ound with current and uture wide-eld instruments such as the Visible and In rared Survey elescop

or Astronomy (VIRCAM) and LSS . A ew dozen ohighest SNR sources will have error boxes that are signicantly smaller and can be realistically ound with smeld o view (several arcmin) cameras such as the Muladaptive Optics Imaging Camera or Deep Observation(MICADO) on the European Extremely Large elescop(E-EL ) or the James Webb Space elescope (JWS ).

Astrophysical impactTe large number o ultra-compact binaries discoveredand the act that the sample is complete at the shortest priods, will help determine the total number o systems all types as well as their merger rates. Te systems con-taining neutron stars and/or black holes will be observeabout a million years prior to coalescence, a phase that still unexplored.Te highest SNR systems will allow a study o the complphysics o white dwar mergers or o how systems suras interacting binaries. Recent detailed simulations[143] have cast doubt on the theory that the actual merger woulbe a truly dynamical process taking only one or two orbits, and instead show that the merger would take placover many orbits, possibly allowingeLISA to observe somemergers directly.eLISA will detect ultra-compact binaries beyond the Ga-lactic Centre and in the Milky Ways halo using observations which are unaffected by dust obscuration, providinan independent probe o the components and ormatiohistory o the Milky Way.


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Te Gravitational Universe Te eLISA Space Gravitational Wave Observatory 13

IV. A STRAWMAN MISSION FORTHE eLISA SPACE GRAVITATIONALWAVE OBSERVATORY

All o the above scientic objectives can be addressed by asingle L-class mission consisting o 3 drag- ree spacecraf

orming a triangular constellation with arm lengths o onemillion km and laser inter erometry between ree- allingtest masses. Te inter erometers measure the variations inlight travel time along the arms due to the tidal de orma-tion o spacetime by gravitational waves. Compared to theEarth-based gravitational wave observatories like LIGOand VIRGO,eLISA addresses the much richer requencyrange between 0.1 mHz and 1 Hz, which is inaccessible onEarth due to arm length limitations and terrestrial gravitygradient noise.Te Next Gravitational wave Observatory (NGO) mission

studied or the L1 selection[15] is aneLISA strawman mis-sion concept. It enables the ambitious science program de-scribed here, and has been evaluated by ESA as both tech-nically easible and compatible with the L2 cost target. Its

oundation is mature and solid, based on decades o devel-opment or LISA, including a mission ormulation study,and the extensive heritage o ight hardware and groundpreparation or the upcoming LISA Pathnder geodesicexplorer mission, which will directly test most o theeLI-SA per ormance and validate theeLISA instrumental noisemodel[144145].

Mission designTe NGO mission has three spacecraf, one mother at the vertex and two daughters at the ends, which orm a singleMichelson inter erometer conguration (Figure 9). Tespacecraf ollow independent heliocentric orbits withoutany station-keeping and orm a nearly equilateral trianglein a plane that is inclined by 60 to the ecliptic. Te con-stellation ollows the Earth at a distance between 10 and

30, as shown in Figure 10. Celestial mechanics causes ttriangle to rotate almost rigidly about its centre as it orbiaround the sun, with variations o arm length and openinangle at the percent level.Te payload consists o our identical units, two on thmother spacecraf and one on each daughter spacecraf(Figure 11). Each unit contains a Gravitational Re erenSensor (GRS) with an embedded ree- alling test mass tacts both as the end point o the optical length measurement, and as a geodesic re erence test particle. A telescowith 20 cm diameter transmits light rom a 2 W laser 1064 nm along the arm and also receives a small ractio the light sent rom the ar spacecraf. Laser inter eroetry is per ormed on an optical bench placed between thtelescope and the GRS.On the optical bench, the received light rom the distanspacecraf is inter ered with the local laser source to pro

Figure 9: eLISA confguration (not to scale). One mother and two daugh-ter spacecraft exchanging laser light form a two-arm Michelson interfer-ometer. There are four identical payloads, one at the end of each arm, asshown in Figure 11.

Figure 10: eLISA Orbits. The three eLISA-NGO spacecraft follow the Earth

as an almost stiff triangle, purely due to celestial mechanics.

Figure 11: eLISA payload. Each payload unit contains a 20 cm telescope,the test mass enclosed inside the Gravitational Reference Sensor (GRS)and an optical bench hosting the interferometers. (Auxiliary reference in -terferometer omitted for clarity, see [15] for details.)

GRS

OpticalBench

Telescope TM/SC

SC/SC

TM

Laser source

Phase Metermeasurementsand laser control

Reference

Modulator

Drag-freecontrol

Thrusters

from 2ndlaser

source

Earth

Sun

1 AU (150 million km)

10 3060

1 m i l l io n km

1 AUSun

DaughterS/C

DaughterS/C

Mother S/C

1 million km1 million km60


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14 Te Gravitational Universe Te eLISA Space Gravitational Wave Observatory

duce a heterodyne beat note signal between 5 and 25 MHz,which is detected by a quadrant photodiode. Te phaseo that beat note is measured with cycle/ Hz precisionby an electronic phasemeter. Its time evolution reectsthe laser light Doppler shif rom the relative motion othe spacecraf, and contains both the macroscopic armlength variations on times cales o months to years, andthe small uctuations with periods between seconds and

hours that represent the gravitational wave science signal.Te measurement o relative spacecraf motion is thensummed with a similar local inter erometer measuremento the displacement between test mass and spacecraf. Tisyields the desired science measurement between distant

ree- alling test masses, removing the much larger motiono the spacecraf, which contains both thruster and solarradiation pressure noise.

Drag-free controlTe spacecraf are actively controlled to remain centredon the test masses along the inter erometric axes, withoutapplying orces on the test masses along these axes. Tisdrag- ree control around the shielded geodesic re erencetest masses uses the local inter erometry measurement asa control signal or an array o micro-Newton spacecrafthrusters, with the residual spacecraf jitter reaching thenm/ Hz level. Tese thrusters also control the spacecrafangular alignment to the distant spacecraf by detectingthe laser beam wave ront with differential wave ront sens-ing with nrad/ Hz precision. Other degrees o reedomare controlled with electrostatic test mass suspensions. Teonly remaining degree o reedom is then the opening an-gle between the arms at the master spacecraf, which variessmoothly by roughly 1.5 over the year, and can be com-pensated or either by moving the two optical assembliesagainst each other or by a steering mirror on the opticalbench.Te test masses are 46 mm cubes, made rom a dense non-magnetic Au-Pt alloy and shielded by the GRS. Te GRScore is a housing o electrodes, at several mm separation

rom the test mass, used or nm/ Hz precision capaci-tive sensing and nN-level electrostatic orce actuation onall non-inter erometric degrees o reedom. Te GRS alsoincludes bres or UV light injection or photoelectric dis-charge o the test mass, and a caging mechanism or pro-tecting the test mass during launch and then releasing itin orbit. Te GRS technology is direct heritage rom LISAPathnder.

SensitivityTe strain sensitivity (shown in Figure 12) corresponds tothe noise spectrum o the instrument.

At low requencies, it is dominated by residual accelerationnoise o 3 m s2/ Hz per test mass. Above about 5 mHz,arm length measurement noise dominates, or which12 pm/ Hz are allocated, out o which 7.4 pm/ Hz are

quantum mechanical photon shot noise. At the highestrequencies, the sensitivity decreases again since multip

wavelengths o the gravitational wave t into the armcausing partial cancellation o the signal.A unique eature o theeLISA inter erometry is the virtualelimination o the effects o laser requency noise. Stlisation to a re erence cavity built into the payload is nenough to suppress it completely. Te remaining noiseis removed by ime-Delay Inter erometry ( DI)[146],which synthesises a virtual balanced arm length inter eometer in postprocessing. Tis requires knowledge o thabsolute arm lengths to roughly 1 m accuracy, measure via an auxiliary ranging phase modulation imposed on

Figure 12: Time, sky and polarisation averaged eLISA sensitivity. Thenoise spectrum (strain sensitivity) is plotted as a linear spectral density.

Figure 13: Examples of gravitational wave astrophysical sources in thefrequency range of eLISA, compared with the sensitivity curve of eLISA. The data is plotted in terms of unitless characteristic strain amplitude.That is the strain resolvable in a single cycle for a broadband source, orthe linear spectral density of Figure 12 times frequency , or times ofnumber cycles spent by the source at a given frequency during the obser-vation for monochromatic sources. The tracks of two massive black holebinaries, located at z = 3 with equal masses (10 7 and 10 6 M 9 ), are shown.The source frequency (and SNR) increases with time, and the remain -ing time before the plunge is indicated on the tracks. An equivalent plotis shown for an EMRI source at 200 Mpc, with 5 harmonic frequenciesevolving simultaneously. Several thousand galactic binaries, with SNRsabove 7, will be resolved after 1 year of observation. Some binary systemsare already known, and will serve as veri cation signals. Millions of otherbinaries result in a confusion noise that varies over the year. The average

level is represented as grey shaded area.

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10 -19

10-18

10 -17

10 -16

10 -5 10 -4 10 -3 10 -2 10 -1 1

C h a r a c t e r i s t i c s t r a

i n a m p

l i t u

d e

Frequency (Hz)

resolved

vericationconfusion

BH binaryEMRI

1 month1 day 1 hour

1 year

1 month

1 day 1 hour

1 minute

1 month

1 day 1 hour

M = 10 6 M 9tot

M = 10 7 M 9tot

1 year

M = 10 5 M + 10 M 99

Inspirals Galactic binaries

1 100Signal-to-noise ratio (SNR) 10

10 -20

10 -15

10 -19

10 -18

10 -17

10 -16

10 -5 10 -4 10 -3 10 -2 10 -1 1

Frequency (Hz)

S t r a i n

l i n e a r s p e c t r a

l d e n s

i t y

( 1 / H Z

)

Acceleration noise (1 test mass)3x 10 -15 m s -1/ Hz

Interferometry(incl. shot noise)12x 10 -12 m/ Hz

Armlength response109 m (0.15 Hz cut-off)


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Te Gravitational Universe Te eLISA Space Gravitational Wave Observatory 15

the laser beams. A second modulation is used to measureand remove noise caused by timing jitter o the Analogueto Digital Convertor sampling clocks in the phaseme-ters[147].

Data Analysishe data analysis algorithms or extracting the gravitation-

al wave signals rom the data and estimating their param-eters were developed within the Mock LISA data challengeprogram. his program was success ully conducted rom2006 until 2011 and a new round oeLISA data challengesis currently being prepared. During the our rounds o theprevious data challenges methods were developed or de-tecting gravitational wave signals rom spinning massiveblack hole binaries, EMRIs, the population o galactic bina-ries, cusps ormed on cosmic strings, as well as stochasticgravitational wave signals. he summary o each challengecan be ound in[148150]. he results show that we canalways success ully detect and resolve gravitational wavesignals, disentangling multiple sources o the same kindand o di erent kinds rom the tens o millions o sourcessimultaneously present in the simulated data. In addition,the recovered parameters are always consistent with thetrue values to within the expected statistical uncertainty.

Technology statusAll critical technologies oreLISA have been under intensedevelopment or more than 15 years, and today all areavailable in Europe, including the phasemeter. Te inter-

erometry with million km arms cannot be directly testedon ground, but it is being studied by scaled experimentsand simulations. For theeLISA GRS, local inter erometry,and the core o the drag- ree and test mass control, LISAPathnder has allowed early identication and resolutiono both technological development challenges and per-

ormance questions (see Figure 14). Te GRS orce noisebudget has been largely veried at the level o LISA Path-nder, and in some respects oreLISA, by torsion pendu-lum testing on the ground[151]. Additionally, tests withthe LISA Pathnder inter erometer have allowed ground

verication o the local displacement measurement across

Figure 14: LISA Pathfnder. Much of the flight hardware for LISA Path-nder is already tested and integrated on the spacecraft. In the last cou -

ple of years, the integrated system has been put through various testcampaigns, from vibration and shock tests, to system level closed-loopcommunication and interface tests. The image shows the LISA Path-

nder satellite mounted on its propulsion module and situated in a spacesimulator in preparation for the Transit Orbit Thermal Test campaign. Thespace simulator aims to reproduce the thermal and vacuum environmentin space. The bright light source at the back of the large vacuum tanksimulates the sun allowing the spacecraft to be powered via its solar

panel during the tests. The spacecraft has also undergone another testcampaign in the same space simulator during which a Thermal OpticalQuali cation Model of the LTP (the science payload) was integrated. Thissecond test campaign aimed to simulate the environment expected at La-grange point 1, with the temperature in the space simulator being cycledbetween the minimum and maximum predicted values. This On-StationThermal campaign included some performance tests of the optical me-trology system, the results of which showed that the performance of thesystem is well within the requirements.

Science ObjectivesTrough the detection and observation o gravitational waves:

Event Rates and Event NumbersFrequency band 1 104 Hz to 1 Hz, (3 105 Hz to 1 Hz as a goal)Massive black hole mergers 10 yr1 to 100 yr1Extreme mass ratio inspirals 5 yr1 to 50 yr1Galactic Binaries ~ 3000 resolvable out o a total o ~ 30 106 in theeLISA band

the LISA Pathnder band[152]. Te 2015 Pathnder ightrepresents a nal verication and in-orbit commissioningo these systems and much o theeLISA metrology capa-bility.Te ESA evaluation in the L1 process showed the NGOmission concept to be both technically easible and compatible with the L2 cost target. It ts, with margin, into aAriane V launch vehicle, though other launch scenario(like the one studied or L1) are possible. Tere is stroninterest rom international partners, such as the US andChina, to participate and contribute toeLISA, in whichcase a generous budget margin and/or enhanced scienccapability would be available. With or without international partners, Europe has the chance to take the lead in thi

revolutionary new science.

est General Relativity with observations Probe new physics and cosmology Survey compact stellar-mass binaries and study the

structure o the Galaxy

race the ormation, growth, and merger history omassive black holes

Explore stellar populations and dynamics in galacticnuclei


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16 Te Gravitational Universe Scientic Landscape of 2028

V. SCIENTIFIC LANDSCAPE OF 2028

Te science capabilities o theeLISA mission have been de-scribed in earlier sections.eLISA will pioneer gravitation-al wave observations in the rich requency band around1 mHz. In this section we examine this science return inthe likely context o the L2 launch date o 2028. Given the

predicted state o knowledge in 2028, we ask what uniquecontributionseLISA will make to our likely understandingo undamental physics and astronomy at that time.Naturally, science is not predictable, and the most interest-ing discoveries between now and 2028 will be the ones wecannot predict! But planned projects already hint at wherethe rontiers o science will be wheneLISA operates. Forexample, massive progress can be expected in transientastronomy. elescopes like LSS and the Square Kilome-tre Array (SKA)[153] are likely to identi y new systemsthat are up irregularly or only once, and there is a good

chance that some o these will be associated with gravita-tional wave signals. As another example, extremely largetelescopes (EEL , M , GM ) and large space telescopes(JWS ) will be observing (proto-)galaxies at unprecedent-edly high redshifs, at whicheLISA will simultaneouslyobserve individual merging black hole systems. As well asproviding a wealth o in ormation that will make it easierto identi y the gravitational wave sources, the expectedprogress in all kinds o electromagnetic astronomy willsharpen the need or complementary gravitational waveobservations o the unseen Universe.

Gravitational wave science by 2028By 2028, gravitational wave astronomy will be well-estab-lished through ground-based observations operating at 10Hz and above, and pulsar timing arrays (P As) at nHz re-quencies. Te huge requency gap between them will becompletely unexplored untileLISA is launched (see Fig-ure 15).Te ground-based network o advanced inter erometricdetectors (three LIGO detectors, VIRGO[154], and theKamioka Gravitational wave Detector, KAGRA[155]) willhave observed inspiralling binaries up to around 100 M 9 and measured the population statistics. Some, or all, othese detectors will have been urther enhanced in sensi-tivity. It is possible that the third-generation Einstein el-escope (E ) will have come into operation by 2028[156],

urther extending the volume o space in which these sig-nals can be detected. At the other end o the mass spectrum,P As[157] will have detected a stochastic background dueto many overlapping signals rom supermassive black holebinaries with masses over 109 M 9 , and they may have iden-tied a ew individual merger events. Te background willhelp determine the mass unction o supermassive blackholes at the high-mass end, but it will not constrain themass unction or the much more common 106 M 9 blackholes that inhabit the centres o typical galaxies and are ac-

cessible toeLISA. Ground-based gravitational wave observations are unlikely to constrain the existence and population statistics o the so- ar elusive intermediate-mass blholes, although optical and X-ray observations might havdone so by 2028. Besides making high-sensitivity obse vations o individual systems,eLISA will characterise thepopulation statistics o black holes in the centres o galies, o intermediate mass black holes, and o the early blholes that eventually grew into the supermassive holes wsee today.By 2028, theoretical advances and predictable improvements in computer power will have made it possible tcompute the complex wave orms expected rom EMRand supermassive black hole binaries with high precisionTis will allow searches ineLISA data to approach the opti-mum sensitivity o matched ltering, and it will make teo General Relativity using these signals optimally sentive.

eLISA and fundamental science in 2028One o the signature goals oeLISA is to test gravitationtheory, and it seems unlikely that any other method wilachieve the sensitivity oeLISA to deviations o strong-eld

Figure 15: Measurement capabilities of ET, eLISA, and PTAs. The hori-zontal axis is the total mass of the binary system. The vertical axis is thefrequency of the gravitational waves. The left side shows the frequencybands of the different instruments. The grey shaded region at the top isinaccessible because no system of a given mass can radiate at such highfrequencies. The shaded region at the bottom is less interesting, becausethe chirp mass cannot be measured in an observation lasting less than10 years, so the mass and distance of the source cannot be independent -ly determined. Sloping dotted lines show the three-year, one-day, and one-minute time-to-merger lines. Sloping dashed lines are relevant dynamicalfrequencies: last stable orbit and the frequencies of ringdown modes ofthe merged black hole. Vertical lines indicate evolutionary tracks of sys-tems of various masses as their orbits shrink and they move to higherfrequencies. There is some overlap in the mass-range of sources thatcan be studied between ground and space detectors, but whether thistranslates into real science depends on the instruments sensitivities andthe source populations. Among the most interesting, would be the ob -servation of an intermediate-mass binary black hole (IMBBH) system (ifsuch systems exist) of around 1000 M 9 during inspiral and coalescencewith both eLISA and ET. This track is shown in the diagram as (IMBBH).With such a source at redshift 0.5, both instruments might have an SNRaround 20 (see Figure 16), but most of the science-including the direc -

tion to the source-would come from the eLISA measurement. Addition-ally, tracks for binary neutron stars (BNS) and super massive binary blackholes (SMBBHs) are shown.

10 0 10 2 10 4 10 6 10 8 10 10 10 12

Mass (solar mass)

10 -8

10 -6

10 -4

10 -2

10 0

10 2

10 4

F r e q u e n c y

( H z

)

G r o u n

d

P T A

S p a c e

BNS

IMBBH

SMBBH

Change infrequency


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Te Gravitational Universe Scientic Landscape of 2028 17

gravity by 2028. Unlike ground-based instruments,eLISA will have sufficient sensitivity to be able to notice smallcorrections to Einstein gravity, and possibly to recogniseunexpected signals that could indicate new phenomena.By observing the long-duration wave orms rom EMRIevents,eLISA will map with exquisite accuracy the geom-etry o supermassive black holes, and will detect or limitextra scalar gravity-type elds. Te MICROSCOPE (Mi-cro-Satellite trane Compense pour lObservation duPrincipe dEquivalence) mission[158] will, by 2028, haveimproved our limits on the violation o the equivalenceprinciple or could, o course, have measured a violation,which would makeeLISAs strong-eld observations evenmore urgently needed. X-ray and other electromagneticobservatories may measure the spins o a number o blackholes, buteLISAs ability to ollow EMRI and merger sig-nals through to the ormation o the nal horizon will nothave been duplicated, nor will its ability to identi y nakedsingularities or other exotic objects (such as boson stars orgravastars), i they exist.By 2028 we will know much more about the large-scaleUniverse: in particular, about the nature o dark energy

rom the upcoming optical surveys dedicated to prob-ing the large-scale structure. However, many questionswill have remained open concerning the early Universe.From Planck and balloon ights o CMB instruments, wemay know how much primordial gravitational radiationwas produced at the end o ination, which will help topin down the actual inationary scenario. However, thereexists no means other than gravitational waves to probethe period in the evolution o the Universe ranging romreheating afer ination until big bang nucleosynthesis.Trough the detection o gravitational waves,eLISA cangather in ormation on the state o the Universe at muchearlier epochs than those directly probed by any other cos-mological observation. Gravitational waves are the nextmessengers to probe the very early Universe. Tey reachbeyond the cosmic microwave background:eLISA has ac-cess to a undamental requency/energy-scale window,that o eV. Tis scale is presently our boundary o knowl-edge in undamental particle physics; new physics is there-

ore expected to emerge around that scale.It is unclear how much progress will have been made by2028 in understanding undamental particle physics. TeLHC will start probing new physics afer its rst upgradein 2015 (reaching the scale o 14 eV); another upgrade bya actor o 10 is expected around 2022, which will take theexperiment through to 2030. It is difficult to oresee whatthe LHC will reveal about the nature o the Higgs, o darkmatter particles, supersymmetry, extra dimensions, and soon. High-sensitivityeLISA observations may be crucial inproviding clues here, since they explore the relevant en-ergy scales in a completely unique way. Te in ormationcontained in gravitational waves rom the early Universe iscomplementary to, and independent o , the one accessibleby particle accelerators. Te presence o a rst order phase

transition at the eV scale, the presence o cosmic (supestrings in the Universe, the properties o low-energy intionary reheating, even the nature o the quantum vacuumstate be ore ination began (which could be different rthe standard Quantum Field Teory nature in loop quan-tum gravity), are some o the undamental issues that wstill be open in 2028, and to whicheLISA might providesome answers.

eLISA and late-time cosmology in 2028By 2028 our understanding o the way cosmologicstructures ormed will have been dramatically improveby high-redshif observations o QSOs and protogalaxie

rom missions like JWS[159], EUCLID[160] and theWide-Field In rared Survey elescope (WFIRS )[161],and by the Atacama Large Millimeter/submillimeter Array (ALMA)[162] on the ground. Tese observations maywell have constrained the supermassive black hole masspectrum rom a ew times 1010 M 9 , or even higher, downto around 107 M 9 , but probably not into the maineLISA range o 104 106 M 9 , especially atz > 2.eLISA observa-tions will ll this gap and also provide a check on selectieffects and other systematics o the electromagnetic obs vations. By being able to measure the mass and spins massive black holes as a unction o redshif out toz = 20,eLISA will allow us to greatly improve models o how spermassive black holes grow so quickly, so as to be in plaat z ~ 7. We will additionally learn what roles accretionand mergers play in the growth o all massive black holeLISA observations o mergers o 104 105 M 9 black holesout toz = 20 (i they exist) can provide a strict test o thamount o growth by merger expected in these models.

eLISA and massive black holes in 2028In the next ew years, eROSI A will study tidal disrutions o stars out to redshifs oz~ 1 and will look or mas-sive black holes[163], although in the high-mass regimecompared toeLISA. eLISA has extraordinary sensitivityto massive black holes in the mass-range characteristio most galactic-core black holes (see Figure 16). Gratational wave detectors likeeLISA are inherently all-skymonitors: always on and having a nearly 4 steradian eo view. Tey naturally complement other surveys andmonitoring instruments operating at the same time, likeLSS [164], SKA, neutrino detectors, gamma-ray and X-ray monitors. Te massive black hole mergers detected byeLISA out to modest redshifs (z = 5 10) could well be vis-ible to SKA and LSS as transients in the same region the sky. Te identication o 5 to 10 counterparts during a2 yeareLISA mission would not be surprising. Tese mightthen be ollowed up by large collecting area telescopes l

M , GM , and EEL , providing an unprecedented vieo the conditions around two merging massive black holeInterestingly, the advent o observing with detectors likaLIGO is leading to the development o networks o otical telescopes or multimessenger astronomy. Tese ar


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18 Te Gravitational Universe Scientic Landscape of 2028

designed to ollow up gravitational wave triggers and ndassociated transient phenomena. Tese systems may use-

ully supplement LSS in picking outeLISA counterparts.eLISA has very good sensitivity to as many as 4 or 5 ring-down requencies o newly ormed black holes (see Fig-ure 16). Tis black hole spectroscopy will alloweLISA toaddress important questions that, by 2028, will probablynot yet have been answered. Most important will be to ndevidence or naked singularities or other exotic objects:Te ringdown requencies make it possible to determinethe mass and spin o the nal black hole, and will be verydifferent or any o the proposed alternatives. In addition,the ringdown modes show in detail how a dynamical blackhole behaves; not even E , i operational at that time, willhave the sensitivity to make this kind o detailed study ostrong gravity.

eLISA and the ast

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