PoS(HEASA2017)001 https://pos.sissa.it/ ∗ [email protected] Speaker. We review LIGO and VIRGO detectionalso events review reported the so prospects far, for asfrom pulsar gravitational well timing as wave arrays, future measurements, and prospects. from but the in We space lower system LISA. frequency bands, ∗ Copyright owned by the author(s) under the terms of the Creative Commons c Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). 5th Annual Conference on High Energy4-6 Astrophysics October, in 2017 Southern Africa University of the Witwatersrand (Wits), South Africa Nigel T. Bishop Current status of observations Rhodes University E-mail: PoS(HEASA2017)001 (2.1) 4 about the plane but at , / z π 0). They con- Nigel T. Bishop , y = dydz 2 ) x x ∂ , t − ( is required. As will be 2 × t -direction. The functions ) ∂ h x

2 M + 2 30 ]. If, for a given wave, the arms ( 1 dz O )) x , t ( axes are rotated through + ) h y , − x ( 1 + ( 1 2 dy )) x , t ( is measured; whereas if the arms are in the + for a plane GW propagating in the h + ) z + , Lh y 1 , x , + ( t is meaured. If one of the arms is in the direction of propagation of 2 ( × dx Lh + 2 axes, then 2 is zero, and the arms are oriented such that the zero component is measured. dt z × − h are independent and each satisfies the wave equation ( , and laser light travels along each arm and back again. The difference in travel = L ) and or x 2 y , t + ds ( h × h , ) x Advanced LIGO has completed its first observation runs (O1: 2015.09.29 to 2016.01.12, O2: The limited results obtained so far have already had an impact on theoretical astrophysics, This review starts with an introduction to the basic theory of GW generation and propagation; The simplest approach to GWs is to linearize Einstein’s equations, usually about a flat Minkowski , t 4 to the axes, then 2 ( / + -axis, then the two modes are interchanged. A GW detector comprises two perpendicular arms Gravitational wave observations lie along the The current LIGO detectors have arms of length 4km, and for VIRGO the length is 3km. This π the wave, then its length willrelative not change change; of but length the will otherThe be arm one half will case that be in of transverse which the to nothat the case effect either wave, where will and both be the arms recorded are is transverse if to the the wave wave. is linearly polarized, which means 2016.11.30 to 2017.08.25); Virgo2017.07.28 has also to come 2017.08.25. onhole line To mergers, and date, completed GW150914, there its GW151226,and have first GW170104, one observation been neutron GW170608, run reported star GW170814 5 mergernals and event, that definite LVT151012; with were and an possibly 1 expected, electromagneticsupernovas have counterpart, probable and not cosmic GW170817. black been string observed, interactions, Other including and sig- axisymmetric continuous burst sources events body caused such examples by as the being Galactic rotation theprovide of constraints Crab, a on non- Vela astrophysical and models. other pulsars. Even suchsince non-detections an expalanation for the origin of black holes with mass then summarizes the functioning ofare a described, LIGO as machine; well theprospects, as events in the that terms of have implications the been for currentdetection, detected physics LIGO i.e. to facilities and the date as satellite astrophysics. well system as LISA planned We and ones; pulsar then and timing look other arrays. at plans for future GW 2. GW basics background, and it is found that, to leading order, the GW field takes the form 1. Introduction discussed later, an enormousgravitational increase wave (GW) in observations detections will isastrophysical provide and expected strong physical constraints over processes. on the the next modeling few of various years. Thus using Cartesian coordinates h stitute the two polarization modesz of the GW; if the each of length time between the two armsdiscussion is of measured the as operation an of interference a fringe; LIGO see detector, see Fig. for 1. example [ For a more detailed PoS(HEASA2017)001 , , 3 ω (2.2) (2.3) 8 times = , 3  ] ) Nigel T. Bishop 2 φ 2 − t ω 2 ( sin θ and with angular velocity 0 r , 2cos 2 ) , ) × h φ t 2 ∂ − t + ( 2 ω ) 2 ( + h t 2 ∂ cos ( ) S θ I 2 that of the orbital frequency. The wave magnitude falls π 1 4 cos − + twice in circular orbit of diameter = 1 ( 2 M dt dE × 2 r ) and ω 2 2 0 1 r M 2 M + M 1 1 Illustration of the schematics of the LIGO facilities, adapted from [ M M ] ( 8 5 − ) = Figure 1: × h . Thus, if detection sensitivity increases by a factor 2, a source could be 2 times further, , r + / h ( The generation of GWs by a source is often described by Einstein’s quadrupole formula [ The energy carried away by GWs is ], which is valid for systems where the relative velocities are small. For a binary (see Fig. 2) Gravitational wave observations leads to the detectors being sensitive2000Hz. only to signals in the frequency range approximately 20Hz to indicating that the wave frequency is comprising two masses, 4 off as 1 so the volume searched and consequently the expected event detection rate would be 2 the formula gives [ larger. PoS(HEASA2017)001 to ω (2.6) (2.4) (2.5) . ω and ω t ∂ Nigel T. Bishop . The key point by f 0 π r . = 2 ].) The rate of energy 2 0 and M 6 , ω r [ 1 0 4 5 r / M t 3 0 ∂ r  t ) et al. ∂ f , t rotating at angular velocity 0 = ∂ 0 r ( 0 t r 3 r  ∂ / 2 3 2 11 0 M r − − 1 ) to replace f 4 3 M = / 2.4 8 − − ω t  ω π ∂ d dt 5 3 96 , . (Actually, the energy emitted has historically been =  2 ) in Eq. ( r ) M = 3 2 0 r 2.5 + 5 M 8 / 5 1 using Newtonian orbit theory (Kepler’s law), we find 1 / + ) 3 0 M 2 in circular orbit diameter 2 ) 1 r 5 0 2 2 r M M s 5 ( M M + 2 1 2 and = 1 M M ω and ω ( 2 1 M ( 1 M M ≡ − = are determinable entirely from LIGO data, and thus so is the . M f t dt dE ∂ . We can now use Eq. ( 3 / 2 Two masses, and − f ω ∝ is the wave frequency, related to the orbital angular velocity by is a spherical shell at large, constant 0 r f S Figure 2: Gravitational wave observations where is that both where a controversial issue, which was resolved only in 1962 by Bondi Applying the relation between so that emission of GWs is equated to the rate of loss of Newtonian orbital energy, yielding obtain the chirp mass formula PoS(HEASA2017)001 (2.7) (2.8) (2.9) (2.10) , Nigel T. Bishop Hz 20Hz, but if the

≈ M M in f ,  12000 ), we find that the signal is θ of the source as seen by the = θ 2.6 ω 2cos , , ), and find the wave amplitude ) θ ms 2.2 2 ,

, 3 M / M cos 8 . Thus, once the chirp mass is determined in f + )) 3 / in Eq. ( 1 ; if the signal is detectable at 20Hz, then it 056 φ . 5 ( 2

f 0 5000Hz 06s. On the other hand, a system comprising 2 5 M 3 − M .

= / 3 t 3 4 by µ 0 . 2 / . In the absence of an optical counterpart and direct τ M ) 8 1 ω ) 0 f z r = 2 representing merger. For example, GW150914 has π ( ≈ ≈ π t + ( r ∞ 1 3 sin 256 / ( M 5 where × / max → f = A f , t , M ) has ) 2 t φ π ) to replace

2 , and this gives a correction that source frame masses are smaller ω 2 ) ( M ) = z − 2.5 is the reduced mass of the system Thus LIGO would not see events 5 t × . + ) A cos ω 2 1 , ) 2 ( ( M + t τ A 50Hz; this leads to + since throughout the inspiral and merger the wave frequency would be too − ( cos ( 1 ] can be used to estimate the post-merger waveform. This gives ≈

+ 7 M of the source causes frequencies in the source frame to be higher than in the A M ( in exp f z / 2 ∼ 500 M ) ) = ( is the total mass of the system. 1 and be the frequency when the signal is first “in band”; normally, × ' × h M h 2 M

, in , f + M = + M , h h 1 ( µ ( 30 M Qualitatively, the system evolves with the orbital diameter slowly decreasing, and the wave The The amplitude and frequency increase until they reach maxima at merger. The maximum We use Kepler’s law Eq. ( ) is not negligible compared to the velocity of light, then alternative methods need to be ≈ ω 0 Gravitational wave observations magnitude, wave frequency and orbitalr velocity slowly increasing. Onceused the for orbital accurate velocity waveform (i.e. estimation;therefore this more is detectable, essential, in since thisMinkowskian the regime. and wave These self-force), amplitude methods is or may largest, they bebut may and analytic they require (post-Newtonian, are numerical post- not evolution discussed ofthat further the are accurate in Einstein as equations; this order review. ofpossibly, magnitude However, in estimates the the even quadrupole outside case formula itsperturbation of gives domain theory two of results [ applicability. neutron Except, stars, the end product will be a black hole, and black hole measurement, the redshift is estimated from the distance using Hubble’s law. and if the polarization modes candetector be is separated constrained, so then that the the inclination above result may be used to constraindetector the frame distance by to a the factor source. than detector frame masses by a factor 1 signal is weak it could be larger. Integrating the chirp formula Eq. ( where the integration is taken until M frequency is of order low. Let where and where where with viewed in the detector for a time neutron stars each of mass 1 would be in-band for 138s,an so impending in merger such before a it case happens. it may be possible to alert other observatories about Observed changes to the orbitalhave provided frequency confirmation of of the this Hulse-Taylor formula binary since the pulsar, 1970s. and other systems, PoS(HEASA2017)001 (2.12) (2.11) ]. GWs are 8 dependency. [ Nigel T. Bishop θ 12 / 19 0 r ∝ e black holes 25 million km apart. A ,

) . 4 modes have different 2 ) M 2 M = M ` + 1 + 3 0 1 r M 4 0 5 ( M r 2 2 and ( 5 2 5 M = 1 M . In practice, the template bank is constructed using ` 1 2 M S S 4 S M , 1 − 16 S S S = = 0 c r t t ∂ and the spins 2 black holes 10 million km apart, will take 0.9 billion years until merger; M

/ 1 M M ) we have can be constrained since the decays much faster than the orbital diameter, specifically = e θ 2.4 q may be constrained if the two polarization modes can be disentangled, which means that the θ In order to determine the astrophysical parameters of an event, the starting point is a template The angle of incidence of a GW onto the plane of a detector is completely unknown if there is From Eq. ( We have considered only the waveform generated by circular orbits, but in general orbits are between the direction of wave propagation and the orbital axis of the source is unknown, leading Gravitational wave observations bank of merger waveforms over thethe parameter phase space and the to magnitude be of explored. thethe GW For mass scale black ratio with hole the mergers, total both mass, so the parameter space comprises detectable only in the finalto stages have of become the circularized. inspiral, by Onelarger which system exception stage that could the injects be orbits eccentricity if to are the the therefore binary, binary expected but is such a not scenario isolated is but unlikely. is3. part GW of events a that have been detected analytic methods, with various parameters in the expansionsto fixed the by fitting output the of resultant waveforms numerical relatvity simulations. The mismatch between the calculated waveform and approximately the same time would be taken by two 50 only one detector, is constrained toif lie there on are a 3 cone if orimportant). there more are detectors two With (which detectors, only is and one why istranslates detector, concurrent well-constrained to observations the uncertainty by magnitude by several of a detectors theθ factor are wave of so is 2 in uncertain estimating by theto a distance an factor to 2, uncertainty the which event. indegeneracies Further, the may the be angle magnitude broken of in athe the number event is wave of seen ways. and in multiple The thus detectors, sourceangle or of may if be an the electromagnetic well-localized distance counterpart on is the tosignal identified. sky must Further the if be the source. seen insignal multiple These is detectors not with solely the quadrupolar, arms butthe in involves higher different GW order relative signal harmonics orientations. is at The higher atestimated, time-frequencies. GW large then If enough SNR (Signal to Noise Ratio) for a higher order harmonic to be For example, two 10 larger separation would not lead towhy merger black in hole less mergers than are a observed,the Hubble we black time. need holes to Thus close understand in the enough order astrophysical for to processes GW understand that damping drive to be effective at driving the inspiral. elliptical with varying degreesconsequently of so eccentricity. also are The GW orbital emissionthe and eccentricity velocity the is effective highest radiation back at reaction. periastron, The and result is that which may easily be integrated to derive the time to coalescence PoS(HEASA2017)001 (3.1) 1 and O ]; but this 17 07 04 05 . . . . 8 9 13 . . is minimized 7 2 5 1 0 0 0 0 ] 140 140 2 2 4 0 ) + − + − Nigel T. Bishop − + + − + − + − + − c 12 7 h 19 12 85 69 ( 18 . . 340 0 0 M 4 7 2 5 2 8 0 7 05 07 ...... 4 3 . . . . 3 2 3 2 4 2 3 5 0 0 ][ 130 210 0 0 − + − + + − + − − + + − + − 9 2 3 5 11 . Then 7 . . . . . 2 70 . 2 h 540 55 53 25 30 0 × 9 0 7 6 3 9 0 4 20 09 ...... 6 7 . . . . 5 5 5 4 5 5 6 8 0 0 ][ 450 390 0 0 + − + + − + − − − + + − + − 7 7 4 2 i 10 0 . . . . o . constant 64 h . 2 880 50 48 19 31 , 0 = c h 1 . Searches for an electromagnetic counterpart h 2 9 7 1 7 3 7 06 06 ...... 3 3 2 1 − h ...... 5 1 6 1 8 3 0 0 180 190 2 2 0 0 1 ][ 6 + + − + − − − + + − 1 iff − + + − 9 8 8 2 5 0 . . . . . 74 = ) = . 1 7 440 21 20 14 c 0 h ( . 4 1 4 7 8 2 1 7 06 05 ...... 4 5 M . . . .

3 3 4 3 4 3 3 5 0 0 180 150 0 0 1, being ][ − − + + − + − + . Thus, an important question is the formation channel of black − + ) is defined as M − + − + 2 3 3 1 2 o

0 . . . . 2 observation run was the first detection of a binary . h 68 i ≤ . M 3 O 2 65 62 29 36 0 to 40 h , 1

]; see Fig. 3. The Fermi Gamma-ray Burst Monitor observed a burst h M 2 15 c mergers detected as GW events 2015 – 2017.

≤ h , ) 230 850 1200 60 860

2 M

M 14 / 2 / M

/ E m

1 M M / Table 1: m f / f M M a The detection events show clearly the existence of a population of intermediate mass black The consistency between the observed and predicted black hole merger waveforms amounts An exciting result from the The properties of the binary black hole merger events detected to date during the ) and the observed waveform ( c 2 observation runs are summarized in Table 1. 0.4s after GW150914 the Fermi Gamma-ray Total mass Secondary mass Primary mass GW EventReference 150914 151226 [ 170104 170814 170608 Luminosity distance (Mpc) 420 Sky localization (deg Radiated energy Final spin Final mass h Gravitational wave observations where the inner product is normally evaluatedfunction; in and the satisfies Fourier 0 domain; includes the detector response ( over the template bank. O Burst Monitor observed a weak gamma-ray transient (GBM150914) of duration 1s [ about 1.7s after the mergerThe time GW in signal the was GW seensignal data; was in of this both low signal LIGO amplitude, was detectors, andposition also and (but is not observed not had other discernible by a source in INTEGRAL. duration parameters)yielded Fig.3; of to results it 31deg about about contributed to 11 100s. constraining hoursthe the The after galaxy sky the Virgo NGC GW 4993 event, at identifying a distance a of bright about optical 40Mpc. transient This source observation in was confirmed by a number holes. This is interesting becausehole more the massive end-state than about of 10 a star had not been expected to lead to a black observation was not confirmed by otherobserved. detectors. No No counterparts optical, to radio other orcounterparts BBH neutrino is GW counterparts not events were surprising, were because reported.system they comprising would The black require lack holes the of only presence multi-messenger would of not some produce form any of signal other matter: than a GWs. holes in the mass range 20 to a test of general relativitythe in possibility the of strong the field merging regime.holes, objects On nor being the does some other it form hand, exclude some of the alternative data exotic theories do compact of not object gravity. exclude other than black merger GW170817 [ PoS(HEASA2017)001 c 15 − 10 × 3 − Nigel T. Bishop ] and references therein. 15 . In classical , GWs

M 01 04 . . 0 0 − + 74 . 7 2 = 2 M + 1 M , and the constraints obtained on the individual and total masses ,

). The time difference between the arrival of the GW signal and the

c M M -rays by ground based facilities including H.E.S.S. were also unsuc- γ 002 004 60 . . . 0 0 1 − + < 2 188 . M 1 , 1 = M M < Time-frequency plot of the LIGO-Hanford, LIGO-Livingston and Virgo data for GW170817;

M ]. 17 14 . Taking account of all the observations, there is strong evidence that GW170817 was a neutron -process nuclei in the ejecta. The mass of the system, in the form of the chirp mass, is well r Gravitational wave observations propagate at the ( GRB lead to a constraint on the difference between the two speeds to be in the range are 1 star merger followed by aof short gamma ray burstconstrained, (sGRB) and a kilonova with radioactive decay Figure 3: from [ In summary, signals were observed in(after the X-ray a (after delay a of delay of aboutand 9 16 subsequent days), days, searches UV, optical, for and IR including andcessful. radio MeerKAT) bands. No neutrino flux was recorded, of observatories around the world,multi-messenger including search SALT for in additional South signals. Africa. The There details are followed given a in worldwide [ PoS(HEASA2017)001 ]. Λ 1 . The 1 Mpc 180 deg, i.e. 1 ], the precise − < Nigel T. Bishop 17 θ km s < 0 . 0 . 12 8 + − 0 . . The results provide an upper limit on Λ 8 detector, INDIGO, in the right panel. Adapted from [ th ), is constrained since the 3 detectors are at different orienta- Mpc, which is consistent with astronomical estimates of the 9 9 . . 6 2 2.2 − + 8 . ] that the Hubble constant can be measured using GW data, since, in in Eq. ( 16 θ . The internal structure of a neutron star has no effect on the GW signal at early c 16 − Illustration of expected sky localization with 3 detectors (LIGO-Hanford, LIGO-Livingston and 10 × 8 times larger, and so to an expected increase in the event detection rate by a factor of 8. In 7 The advantage of multiple detectors is improved sky localization (see Fig. 4), as well being Currently, there are 3 detectors: 2 in the USA, LIGO Hanford and LIGO Livingston, and Virgo It has been proposed [ = + 3 Gravitational wave observations able to measure thedetectors polarization observing modes, the with GW increases. the Auled constraints further maintenance, becoming and advantage even is tighter during lock-time: observation as runs the the they detectors are have number in sched- lock, of i.e. taking data, only for about Virgo) in the left panel, and with the addition of a 4 Figure 4: in Italy. The detectors are notsensitivity operating by at a design factor sensitivity, of and the 2.2 goal This is translates to into improve being the able current to search for sources in a volume that is distance to NGC 4993. The valueuncertainty obtained in for the the value Hubble should constantthe be is value reduced 70 obtained in is future consistent as withsupernovae more (SN1a), estimates events and from are does both not discovered. cosmic shed microwave At light background this on data stage, the and tension between these4. values. Future prospects addition, detectors are under construction: KAGRA,2020; Japan, and with INDIGO, observations scheduled India, to withthe start observations Einstein in scheduled Telescope have to been start discussed, in but 2022. are still Further at systems the planning such and as fund-raising stages. principle, GW data alone is sufficient to determine the distance to the source. Here [ tions and so provide information about the polarization. It is found that 144deg times where only the masses areThe significant, GW data but constrains it the does tidal have deformation an parameter effect at later times and at merger. to which is about the same asmodels that that obtained have smaller previously radii, by and other thus means. softer The equations results of somewhat state. favour sky localization means that there isGW no propagation uncertainty vector. in The the uncertainty orientation insight of the to the orientation the detectors of detector, relative the i.e. to source the with respect to the linethe of source was viewed nearly face-onthe with the distance rotation to being the clockwise. source The is estimate obtained 43 for PoS(HEASA2017)001 Nigel T. Bishop Hz. Consequently, the 4 − Hz to 10 1 black hole binaries are visible in the black holes, with orbital periods that − ) )

M M 10 10 ( ( O O 9 ]. The design allows the two polarization modes to be measured. 18 Illustration of the orbits of the LISA satellites following the Earth around the Sun. Figure 5: LISA pathfinder was launched on 3 December 2015, and tested technology to achieve freely- Gravitational wave observations merger events relevant to LISA aresupermassive black those hole involving or supermassive with black a holes, stellarGalaxy mass either comprising object. with white There another dwarfs, are neutron aproduce stars number GWs or of in known binaries the in LISA our frequencydoubtless band; LISA these will can discover be more used such as systems. verification signals for LISA, and Sky localization will not be possibleat for least a several burst event, weeks, butallow the if the GWs change from source in a to orientation sourcein be are of a detected localized. the over different system frequency The as band arm it compared length moves to means around LIGO, that the about LISA Sun 10 will will be sensitive to sources falling test masses from 1more March accurate 2016; than the mission target,Consequently, ended the and 18 European meet July Space Agency the 2017. project LISAcomprise The has 3 scheduled design results satellites LISA are specifications in for 5 heliocentric launch to times with orbit, in within following 2034. sides the a of It Earth, will 2.5 factor and million ofwill forming km; an make 1.25. see equilateral a triangle Fig. complete rotation 5.and once interferometer, per The as year. triangle well Each as will satellitefrom a not will external reference be have influences test fixed a and mass relative laser will inside to transmitter,given follow, the the in very receiver, satellite. the precisely, Earth, a mission The but spacetime proposal test [ geodesic. mass is Further protected details are LISA band weeks or months before merger, so permitting a merger event to be predicted so that 75% of the time. Morewill detectors be increases in the lock. probability thatblack Taking hole at all merger a events these given of time factors order at 1 into least per account, day 2 the will detectors be expectation observed. is that by the early 2020s PoS(HEASA2017)001 Nigel T. Bishop Hz. Pulsar timing projects 3 10 ]. Pulsar timing is sensitive to GWs at very low frequencies (about 19 between their directions, then the signals should be correlated according to θ Hz), which, for example, are generated by super-massive black hole binaries well 9 − LIGO, LISA and Pulsar Timing frequency ranges and expected astrophysical events for detection Hz to 10 Millisecond pulsars are highly accurate clocks, beating at about 10 7 − Gravitational wave observations before merger. Pulsar timingbeen data able to collection set has upper limits; beenrequire probably ongoing the several since detction more of 2004, years. an but event Thenates or to International of the date the Pulsar efforts stochastic has Timing of background Array only radio will with is astronomers time. a in collaboration a In that number South coordi- ofence Africa, goals countries, MeerKAT of participates with the in more SKA. pulsar facilities Further details participating timing, on and pulsar it timing projects is may also be one found, of for example, the in sci- the 10 Figure 6: record accurately the time ofexpected arrival time of of each arrival pulse assuming from aas a the uniform timing particular time residual. between pulsar, (In and the practice,averaged pulses. compare the over it process This a is to difference rather number the is more recorded of complicated,glitches because pulses in it that pulse is emission; only uniform when thus pulsethe the emission Earth, data then occurs, analysis the and is timing further statisticaltinuous residuals there will rather signal may oscillate than the according be direct). residuals to will Ifspecific the source, a form oscillate pulsar of GW sinusoidally. timing the passes GW, can In e.g.two be addition pulsars used for to at to a angle searching search con- for forthe a GWs Hellings-Downs stochastic formula from background. [ a Given data from optical telescopes can be pointed towards it when it occurs. PoS(HEASA2017)001 19 Phys. , Nigel T. Bishop ]. Sitzungsberichte der , Living Rev. Relativ. , (1962) 21–52. (1916) 688–696. A269 1602.03837 ,[ 1916 Gravitational waves in general relativity VII. Observation of Gravitational Waves from a Prospects for Observing and Localizing (2016) 061102 11 Proc. R. Soc. London 116 , Sitzungsberichte der Königlich Preussischen Akademie der , (1918) 154–167. Gravitational radiation from point masses in a Keplerian orbit Phys. Rev. Lett. , 1918 collaboration, B. P. Abbott et. al., collaboration, B. P. Abbott et. al., IRGO IRGO Über Gravitationswellen Naherungsweise Integration der Feldgleichungen der Gravitation V V (1963) 435–440. . AND AND ]. 131 20 Waves from axi-symmetric isolated systems Rev. Wissenschaften (Berlin) Königlich Preussischen Akademie der Wissenschaften (Berlin) Binary Black Hole Merger Gravitational-Wave Transients with Advanced LIGO and(2016) Advanced 1 Virgo The frequency ranges of the various detection systems, as well as the astrophysical events that We are now in the age of gravitational wave astronomy. Within a few years, there should As we open a new window onto the Universe, we may observe something interesting and I thank the National Research Foundation, South Africa, for financial support; and Sabanci [6] H. Bondi, M. G. J. van der Burg and A. W. K. Metzner, [4] A. Einstein, [5] P. C. Peters and J. Mathews, [2]LIGO [3] A. Einstein, [1]LIGO Gravitational wave observations are expected to be detected, are summarized in Fig. 6. 5. Conclusion be 5 detectors operating atbinary design neutron sensitivity. star and We can (blackthe expect hole, population to neutron density observe star) of mergers. many these binary objects, Thiscan so black will also providing hole, lead expect guidance to good concerning strong constraints formationstate. constraints on channels. on the We We deformability will of be neutronconstraints stars able on and to general thus test relativity of and gravitation the alternatives.to theory By equation produce in the of positive mid-2020s, the results, pulsar dynamical so timing providing strong-fieldtheir can information be formation regime, about expected channels. leading supermassive black to And holes,provide by and a the perhaps wealth mid-2030s, of LISA information will for precision be astrophysics operating and and cosmology. can beunexpected. expected Perhaps to there will bewhich guidance has towards been unifying unresolved gravitation for and the quantum last mechanics, 85 years. Acknowledgments University, Turkey, for hospitalitycaptions, while use writing material this from the article.making source their cited. publications Some open I figures, access thank under as the the authors indicated creative commons concerned in license. for the permitting thisReferences by review [ PoS(HEASA2017)001 ]. , , , ]. 323 (1964) Nigel T. Bishop Nature 1706.01812 ]. , 136 ,[ (2016) 241103 (2017) 141101 1710.05832 ]. ]. ,[ National Science 116 , 119 Phys. Rev. , 1711.05578 (2017) 221101 ,[ 1710.05833 1710.05835 (2017) 161101 118 A gravitational-wave standard siren Multi-messenger Observations of a ,[ ,[ Phys. Rev. Lett. Phys. Rev. Lett. , 119 . Oxford University Press, Oxford, , (2017) L35 GW151226: Observation of Gravitational GW170104: Observation of a 50-Solar-Mass GW170608: Observation of a 19 Solar-mass GW170817: Observation of Gravitational GW170814: A Three-Detector Observation of (2017) L12 851 ]. (2017) 85–88 Phys. Rev. Lett. 848 , 12 (1983) 39. 551 Phys. Rev. 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