Gravity Waves PhysRevLett.116.061102

Observation of Gravitational Waves from a Binary Black Hole Merger B. P. Abbott et al.* (LIGO Scientific Collaboration and Virgo Collaboration) (Received 21 January 2016; published 11 February 2016) On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10−21. It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of theresulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203 000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410 þ160 −180Mpc corresponding to a redshift z¼0.09þ0.03 −0.04. In the source frame, the initial black hole masses are 36þ5 −4M⊙and29þ4 −4M⊙, and the final black hole mass is 62þ4 −4M⊙,w i t h 3.0þ0.5 −0.5M⊙c2radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger. DOI: 10.1103/PhysRevLett.116.061102 I. INTRODUCTION In 1916, the year after the final formulation of the field equations of general relativity, Albert Einstein predictedthe existence of gravitational waves. He found that the linearized weak-field equations had wave solutions: transverse waves of spatial strain that travel at the speed oflight, generated by time variations of the mass quadrupolemoment of the source [1,2] . Einstein understood that gravitational-wave amplitudes would be remarkably small; moreover, until the Chapel Hill conference in1957 there was significant debate about the physicalreality of gravitational waves [3]. Also in 1916, Schwarzschild published a solution for the field equations [4]that was later understood to describe a black hole [5,6], and in 1963 Kerr generalized the solution to rotating black holes [7]. Starting in the 1970s theoretical work led to the understanding of black hole quasinormalmodes [8–10], and in the 1990s higher-order post- Newtonian calculations [11] preceded extensive analytical studies of relativistic two-body dynamics [12,13] . These advances, together with numerical relativity breakthroughsin the past decade [14–16], have enabled modeling of binary black hole mergers and accurate predictions of their gravitational waveforms. While numerous black hole candidates have now been identified through electromag-netic observations [17–19], black hole mergers have not previously been observed.The discovery of the binary pulsar system PSR B 1913 þ16 by Hulse and Taylor [20] and subsequent observations of its energy loss by Taylor and Weisberg [21] demonstrated the existence of gravitational waves. This discovery, along with emerging astrophysical understanding [22], led to the recognition that direct observations of theamplitude and phase of grav itational waves would enable studies of additional relativistic systems and provide newtests of general relativity, especially in the dynamicstrong-field regime. Experiments to detect gravitational waves began with Weber and his resonant mass detectors in the 1960s [23], followed by an international network of cryogenic reso-nant detectors [24]. Interferometric detectors were first suggested in the early 1960s [25] and the 1970s [26].A study of the noise and performance of such detectors [27], and further concepts to improve them [28],l e dt o proposals for long-baseline broadband laser interferome-ters with the potential for significantly increased sensi-tivity [29–32]. By the early 2000s, a set of initial detectors was completed, including TAMA 300 in Japan, GEO 600in Germany, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, and Virgo in Italy. Combinations of these detectors made joint obser-vations from 2002 through 2011, setting upper limits on avariety of gravitational-wave sources while evolving intoa global network. In 2015, Advanced LIGO became thefirst of a significantly more sensitive network of advanced detectors to begin observations [33–36]. A century after the fundamental predictions of Einstein and Schwarzschild, we report the first direct detection ofgravitational waves and the first direct observation of a binary black hole system merging to form a single black hole. Our observations provide unique access to the *Full author list given at the end of the article. Published by the American Physical Society under the terms of theCreative Commons Attribution 3.0 License . Further distri- bution of this work must maintain attribution to the author(s) and the published article ’s title, journal citation, and DOI.PRL 116, 061102 (2016) Selected for a Viewpoint inPhysics PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 0031-9007 =16=116(6) =061102(16) 061102-1 Published by the American Physical Society properties of space-time in the strong-field, high-velocity regime and confirm predictions of general relativity for the nonlinear dynamics of highly disturbed black holes. II. OBSERVATION On September 14, 2015 at 09:50:45 UTC, the LIGO Hanford, WA, and Livingston, LA, observatories detectedthe coincident signal GW150914 shown in Fig. 1. The initial detection was made by low-latency searches for generic gravitational-wave transients [41] and was reported within three minutes of data acquisition [43]. Subsequently, matched-filter analyses that use relativistic models of com- pact binary waveforms [44] recovered GW150914 as the most significant event from each detector for the observa- tions reported here. Occurring within the 10-ms intersite FIG. 1. The gravitational-wave event GW150914 observed by the LIGO Hanford (H1, left column panels) and Livingston (L1, right column panels) detectors. Times are shown relative to September 14, 2015 at 09:50:45 UTC. For visualization, all time series are filteredwith a 35 –350 Hz bandpass filter to suppress large fluctuations outside the detectors ’most sensitive frequency band, and band-reject filters to remove the strong instrumental spectral lines seen in the Fig. 3spectra. Top row, left: H1 strain. Top row, right: L1 strain. GW150914 arrived first at L1 and 6.9 þ0.5 −0.4ms later at H1; for a visual comparison, the H1 data are also shown, shifted in time by this amount and inverted (to account for the detectors ’relative orientations). Second row: Gravitational-wave strain projected onto each detector in the 35 –350 Hz band. Solid lines show a numerical relativity waveform for a system with parameters consistent with those recovered from GW150914 [37,38] confirmed to 99.9% by an independent calculation based on [15]. Shaded areas show 90% credible regions for two independent waveform reconstructions. One (dark gray) models the signal using binary black hole template waveforms[39]. The other (light gray) does not use an astrophysical model, but instead calculates the strain signal as a linear combination of sine-Gaussian wavelets [40,41] . These reconstructions have a 94% overlap, as shown in [39].Third row: Residuals after subtracting the filtered numerical relativity waveform from the filtered detector time series. Bottom row: A time-frequency representation [42] of the strain data, showing the signal frequency increasing over time.PRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-2 propagation time, the events have a combined signal-to- noise ratio (SNR) of 24 [45]. Only the LIGO detectors were observing at the time of GW150914. The Virgo detector was being upgraded,and GEO 600, though not sufficiently sensitive to detect this event, was operating but not in observational mode. With only two detectors the source position is primarily determined by the relative arrival time and localized to an area of approximately 600deg 2(90% credible region) [39,46] . The basic features of GW150914 point to it being produced by the coalescence of two black holes —i.e., their orbital inspiral and merger, and subsequent final black hole ringdown. Over 0.2 s, the signal increases in frequency and amplitude in about 8 cycles from 35 to 150 Hz, where the amplitude reaches a maximum. The most plausible explanation for this evolution is the inspiral of two orbitingmasses, m 1andm2, due to gravitational-wave emission. At the lower frequencies, such evolution is characterized by the chirp mass [11] M¼ðm1m2Þ3=5 ðm1þm2Þ1=5¼c3 G/C205 96π−8=3f−11=3_f/C213=5 ; where fand _fare the observed frequency and its time derivative and Gandcare the gravitational constant and speed of light. Estimating fand _ffrom the data in Fig. 1, we obtain a chirp mass of M≃30M⊙, implying that the total mass M¼m1þm2is≳70M⊙in the detector frame. This bounds the sum of the Schwarzschild radii of the binary components to 2GM=c2≳210km. To reach an orbital frequency of 75 Hz (half the gravitational-wave frequency) the objects must have been very close and very compact; equal Newtonian point masses orbiting at this frequency would be only ≃350km apart. A pair of neutron stars, while compact, would not have the required mass, while a black hole neutron star binary with the deduced chirp mass would have a very large total mass, and would thus merge at much lower frequency. This leaves black holes as the only known objects compact enough to reach an orbital frequency of 75 Hz without contact. Furthermore, the decay of the waveform after it peaks is consistent with the damped oscillations of a blackhole relaxing to a final stationary Kerr configuration. Below, we present a general-relativistic analysis of GW150914; Fig. 2shows the calculated waveform using the resulting source parameters. III. DETECTORS Gravitational-wave astronomy exploits multiple, widely separated detectors to distinguish gravitational waves from local instrumental and environmental noise, to provide source sky localization, and to measure wave polarizations. The LIGO sites each operate a single Advanced LIGOdetector [33], a modified Michelson interferometer (see Fig.3) that measures gravitational-wave strain as a differ- ence in length of its orthogonal arms. Each arm is formedby two mirrors, acting as test masses, separated byL x¼Ly¼L¼4km. A passing gravitational wave effec- tively alters the arm lengths such that the measured difference is ΔLðtÞ¼δLx−δLy¼hðtÞL, where his the gravitational-wave strain amplitude projected onto the detector. This differential length variation alters the phasedifference between the two light fields returning to thebeam splitter, transmitting an optical signal proportional tothe gravitational-wave strain to the output photodetector. To achieve sufficient sensitivity to measure gravitational waves, the detectors include several enhancements to thebasic Michelson interferometer. First, each arm contains aresonant optical cavity, formed by its two test mass mirrors,that multiplies the effect of a gravitational wave on the lightphase by a factor of 300 [48]. Second, a partially trans- missive power-recycling mirror at the input provides addi- tional resonant buildup of the laser light in the interferometer as a whole [49,50] : 20 W of laser input is increased to 700 W incident on the beam splitter, which is further increased to100 kW circulating in each arm cavity. Third, a partiallytransmissive signal-recycling mirror at the output optimizes FIG. 2. Top: Estimated gravitational-wave strain amplitude from GW150914 projected onto H1. This shows the fullbandwidth of the waveforms, without the filtering used for Fig. 1. The inset images show numerical relativity models of the blackhole horizons as the black holes coalesce. Bottom: The Keplerian effective black hole separation in units of Schwarzschild radii(R S¼2GM=c2) and the effective relative velocity given by the post-Newtonian parameter v=c¼ðGMπf=c3Þ1=3, where fis the gravitational-wave frequency calculated with numerical relativityandMis the total mass (value from Table I).PRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-3 the gravitational-wave signal extraction by broadening the bandwidth of the arm cavities [51,52] . The interferometer is illuminated with a 1064-nm wavelength Nd:YAG laser, stabilized in amplitude, frequency, and beam geometry [53,54] . The gravitational-wave signal is extracted at the output port using a homodyne readout [55]. These interferometry techniques are designed to maxi- mize the conversion of strain to optical signal, thereby minimizing the impact of photon shot noise (the principal noise at high frequencies). High strain sensitivity also requires that the test masses have low displacement noise, which is achieved by isolating them from seismic noise (low frequencies) and designing them to have low thermal noise (intermediate frequencies). Each test mass is suspended as the final stage of a quadruple-pendulum system [56], supported by an active seismic isolation platform [57]. These systems collectively provide more than 10 orders of magnitude of isolation from ground motion for frequen- cies above 10 Hz. Thermal noise is minimized by using low-mechanical-loss materials in the test masses and theirsuspensions: the test masses are 40-kg fused silica substrates with low-loss dielectric optical coatings [58,59] ,a n da r e suspended with fused silica fibers from the stage above [60]. To minimize additional noise sources, all components other than the laser source are mounted on vibration isolation stages in ultrahigh vacuum. To reduce optical phase fluctuations caused by Rayleigh scattering, the pressure in the 1.2-m diameter tubes containing the arm- cavity beams is maintained below 1μPa. Servo controls are used to hold the arm cavities on resonance [61]and maintain proper alignment of the optical components [62]. The detector output is calibrated in strain by measuring its response to test mass motion induced by photon pressure from a modulated calibration laser beam [63]. The calibration is established to an uncertainty ( 1σ)o f less than 10% in amplitude and 10 degrees in phase, and is continuously monitored with calibration laser excitations at selected frequencies. Two alternative methods are used to validate the absolute calibration, one referenced to the main laser wavelength and the other to a radio-frequency oscillator (a)(b) FIG. 3. Simplified diagram of an Advanced LIGO detector (not to scale). A gravitational wave propagating orthogonally to the detector plane and linearly polarized parallel to the 4-km optical cavities will have the effect of lengthening one 4-km arm and shorteningthe other during one half-cycle of the wave; these length changes are reversed during the other half-cycle. The output photodetectorrecords these differential cavity length variations. While a detector ’s directional response is maximal for this case, it is still significant for most other angles of incidence or polarizations (gravitational waves propagate freely through the Earth). Inset (a): Location and orientation of the LIGO detectors at Hanford, WA (H1) and Livingston, LA (L1). Inset (b): The instrument noise for each detector near the time of the signal detection; this is an amplitude spectral density, expressed in terms of equivalent gravitational-wave strainamplitude. The sensitivity is limited by photon shot noise at frequencies above 150 Hz, and by a superposition of other noise sources atlower frequencies [47]. Narrow-band features include calibration lines (33 –38, 330, and 1080 Hz), vibrational modes of suspension fibers (500 Hz and harmonics), and 60 Hz electric power grid harmonics.PRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-4 [64]. Additionally, the detector response to gravitational waves is tested by injecting simulated waveforms with the calibration laser. To monitor environmental disturbances and their influ- ence on the detectors, each observatory site is equipped with an array of sensors: seismometers, accelerometers,microphones, magnetometers, radio receivers, weather sensors, ac-power line monitors, and a cosmic-ray detector [65]. Another ∼10 5channels record the interferometer ’s operating point and the state of the control systems. Datacollection is synchronized to Global Positioning System (GPS) time to better than 10μs[66]. Timing accuracy is verified with an atomic clock and a secondary GPS receiverat each observatory site. In their most sensitive band, 100 –300 Hz, the current LIGO detectors are 3 to 5 times more sensitive to strain thaninitial LIGO [67]; at lower frequencies, the improvement is even greater, with more than ten times better sensitivity below 60 Hz. Because the detectors respond proportionallyto gravitational-wave amplitude, at low redshift the volumeof space to which they are sensitive increases as the cube of strain sensitivity. For binary black holes with masses similar to GW150914, the space-time volume surveyed bythe observations reported here surpasses previous obser- vations by an order of magnitude [68]. IV. DETECTOR VALIDATION Both detectors were in steady state operation for several hours around GW150914. All performance measures, inparticular their average sensitivity and transient noise behavior, were typical of the full analysis period [69,70] . Exhaustive investigations of instrumental and environ- mental disturbances were performed, giving no evidence to suggest that GW150914 could be an instrumental artifact [69]. The detectors ’susceptibility to environmental disturb- ances was quantified by measuring their response to spe- cially generated magnetic, radio-frequency, acoustic, and vibration excitations. These tests indicated that any externaldisturbance large enough to have caused the observed signalwould have been clearly recorded by the array of environ- mental sensors. None of the environmental sensors recorded any disturbances that evolved in time and frequency likeGW150914, and all environmental fluctuations during thesecond that contained GW150914 were too small to account for more than 6% of its strain amplitude. Special care was taken to search for long-range correlated disturbances thatmight produce nearly simultaneous signals at the two sites. No significant disturbances were found. The detector strain data exhibit non-Gaussian noise transients that arise from a variety of instrumental mecha- nisms. Many have distinct signatures, visible in auxiliary data channels that are not sensitive to gravitational waves;such instrumental transients are removed from our analyses [69]. Any instrumental transients that remain in the data are accounted for in the estimated detector backgroundsdescribed below. There is no evidence for instrumental transients that are temporally correlated between the two detectors. V. SEARCHES We present the analysis of 16 days of coincident observations between the two LIGO detectors fromSeptember 12 to October 20, 2015. This is a subset of the data from Advanced LIGO ’s first observational period that ended on January 12, 2016. GW150914 is confidently detected by two different types of searches. One aims to recover signals from the coalescence of compact objects, using optimal matchedfiltering with waveforms predicted by general relativity. The other search targets a broad range of generic transient signals, with minimal assumptions about waveforms. Thesesearches use independent methods, and their response todetector noise consists of different, uncorrelated, events. However, strong signals from binary black hole mergers are expected to be detected by both searches. Each search identifies candidate events that are detected at both observatories consistent with the intersite propa-gation time. Events are assigned a detection-statistic valuethat ranks their likelihood of being a gravitational-wave signal. The significance of a candidate event is determined by the search background —the rate at which detector noise produces events with a detection-statistic value equal to orhigher than the candidate event. Estimating this back- ground is challenging for two reasons: the detector noise is nonstationary and non-Gaussian, so its properties mustbe empirically determined; and it is not possible to shieldthe detector from gravitational waves to directly measure a signal-free background. The specific procedure used to estimate the background is slightly different for the twosearches, but both use a time-shift technique: the timestamps of one detector ’s data are artificially shifted by an offset that is large compared to the intersite propagation time, and a new set of events is produced based on thistime-shifted data set. For instrumental noise that is uncor- related between detectors this is an effective way to estimate the background. In this process a gravitational-wave signal in one detector may coincide with time-shiftednoise transients in the other detector, thereby contributing to the background estimate. This leads to an overestimate of the noise background and therefore to a more conservativeassessment of the significance of candidate events. The characteristics of non-Gaussian noise vary between different time-frequency regions. This means that the searchbackgrounds are not uniform across the space of signals being searched. To maximize sensitivity and provide a better estimate of event significance, the searches sort both theirbackground estimates and their event candidates into differ-ent classes according to their time-frequency morphology. The significance of a candidate event is measured against the background of its class. To account for having searchedPRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-5 multiple classes, this significance is decreased by a trials factor equal to the number of classes [71]. A. Generic transient search Designed to operate without a specific waveform model, this search identifies coincident excess power in time-frequency representations of the detector strain data[43,72] , for signal frequencies up to 1 kHz and durations up to a few seconds. The search reconstructs signal waveforms consistent with a common gravitational-wave signal in both detectorsusing a multidetector maximum likelihood method. Each event is ranked according to the detection statistic η c¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2Ec=ð1þEn=EcÞp , where Ecis the dimensionless coherent signal energy obtained by cross-correlating the two reconstructed waveforms, and Enis the dimensionless residual noise energy after the reconstructed signal issubtracted from the data. The statistic η cthus quantifies the SNR of the event and the consistency of the data between the two detectors. Based on their time-frequency morphology, the events are divided into three mutually exclusive search classes, asdescribed in [41]: events with time-frequency morphology of known populations of noise transients (class C1), events with frequency that increases with time (class C3), and allremaining events (class C2).Detected with η c¼20.0, GW150914 is the strongest event of the entire search. Consistent with its coalescence signal signature, it is found in the search class C3 of eventswith increasing time-frequency evolution. Measured on a background equivalent to over 67 400 years of data and including a trials factor of 3 to account for the searchclasses, its false alarm rate is lower than 1 in 22 500 years. This corresponds to a probability <2×10 −6of observing one or more noise events as strong as GW150914 during the analysis time, equivalent to 4.6σ. The left panel of Fig.4shows the C3 class results and background. The selection criteria that define the search class C3 reduce the background by introducing a constraint on thesignal morphology. In order to illustrate the significance ofGW150914 against a background of events with arbitrary shapes, we also show the results of a search that uses the same set of events as the one described above but withoutthis constraint. Specifically, we use only two search classes: the C1 class and the union of C2 and C3 classes (C 2þC3). In this two-class search the GW150914 event is found inthe C 2þC3class. The left panel of Fig. 4shows the C2þC3class results and background. In the background of this class there are four events with η c≥32.1, yielding a false alarm rate for GW150914 of 1 in 8 400 years. This corresponds to a false alarm probability of 5×10−6 equivalent to 4.4σ. FIG. 4. Search results from the generic transient search (left) and the binary coalescence search (right). These histograms show the number of candidate events (orange markers) and the mean number of background events (black lines) in the search class whereGW150914 was found as a function of the search detection statistic and with a bin width of 0.2. The scales on the top give thesignificance of an event in Gaussian standard deviations based on the corresponding noise background. The significance of GW150914is greater than 5.1σand4.6σfor the binary coalescence and the generic transient searches, respectively. Left: Along with the primary search (C3) we also show the results (blue markers) and background (green curve) for an alternative search that treats eventsindependently of their frequency evolution (C 2þC3). The classes C2 and C3 are defined in the text. Right: The tail in the black-line background of the binary coalescence search is due to random coincidences of GW150914 in one detector with noise in the otherdetector. (This type of event is practically absent in the generic transient search background because they do not pass the time-frequencyconsistency requirements used in that search.) The purple curve is the background excluding those coincidences, which is used to assessthe significance of the second strongest event.PRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-6 For robustness and validation, we also use other generic transient search algorithms [41]. A different search [73]and a parameter estimation follow-up [74]detected GW150914 with consistent significance and signal parameters. B. Binary coalescence search This search targets gravitational-wave emission from binary systems with individual masses from 1 to 99M⊙, total mass less than 100M⊙, and dimensionless spins up to 0.99 [44]. To model systems with total mass larger than 4M⊙, we use the effective-one-body formalism [75], which combines results from the post-Newtonian approach [11,76] with results from black hole perturbation theory and numerical relativity. The waveform model [77,78] assumes that the spins of the merging objects are aligned with the orbital angular momentum, but the resulting templates can, nonetheless, effectively recover systemswith misaligned spins in the parameter region of GW150914 [44]. Approximately 250 000 template wave- forms are used to cover this parameter space. The search calculates the matched-filter signal-to-noise ratio ρðtÞfor each template in each detector and identifies maxima of ρðtÞwith respect to the time of arrival of the signal [79–81]. For each maximum we calculate a chi-squared statistic χ 2rto test whether the data in several different frequency bands are consistent with the matching template [82].V a l u e so f χ2rnear unity indicate that the signal is consistent with a coalescence. If χ2ris greater than unity, ρðtÞ is reweighted as ˆρ¼ρ=f½1þðχ2rÞ3/C138=2g1=6[83,84] . The final step enforces coincidence between detectors by selecting event pairs that occur within a 15-ms window and come from the same template. The 15-ms window is determined by the10-ms intersite propagation time plus 5 ms for uncertainty in arrival time of weak signals. We rank coincident events based on the quadrature sum ˆρ cof the ˆρfrom both detectors [45]. To produce background data for this search the SNR maxima of one detector are time shifted and a new set ofcoincident events is computed. Repeating this procedure ∼10 7times produces a noise background analysis time equivalent to 608 000 years. To account for the search background noise varying across the target signal space, candidate and background events are divided into three search classes based on template length.The right panel of Fig. 4shows the background for the search class of GW150914. The GW150914 detection- statistic value of ˆρ c¼23.6is larger than any background event, so only an upper bound can be placed on its false alarm rate. Across the three search classes this bound is 1 in 203 000 years. This translates to a false alarm probability <2×10−7, corresponding to 5.1σ. A second, independent matched-filter analysis that uses a different method for estimating the significance of itsevents [85,86] , also detected GW150914 with identical signal parameters and consistent significance.When an event is confidently identified as a real gravitational-wave signal, as for GW150914, the back-ground used to determine the significance of other events isreestimated without the contribution of this event. This is the background distribution shown as a purple line in the right panel of Fig. 4. Based on this, the second most significant event has a false alarm rate of 1 per 2.3 years and corresponding Poissonian false alarm probability of 0.02. Waveform analysis of this event indicates that if it isastrophysical in origin it is also a binary black holemerger [44]. VI. SOURCE DISCUSSION The matched-filter search is optimized for detecting signals, but it provides only approximate estimates of the source parameters. To refine them we use generalrelativity-based models [77,78,87,88] , some of which include spin precession, and for each model perform a coherent Bayesian analysis to derive posterior distributions of the source parameters [89]. The initial and final masses, final spin, distance, and redshift of the source are shown in Table I. The spin of the primary black hole is constrained to be <0.7(90% credible interval) indicating it is not maximally spinning, while the spin of the secondary is only weakly constrained. These source parameters are discussed in detail in [39]. The parameter uncertainties include statistical errors and systematic errors from averaging theresults of different waveform models. Using the fits to numerical simulations of binary black hole mergers in [92,93] , we provide estimates of the mass and spin of the final black hole, the total energy radiated in gravitational waves, and the peak gravitational-waveluminosity [39]. The estimated total energy radiated in gravitational waves is 3.0 þ0.5 −0.5M⊙c2. The system reached a peak gravitational-wave luminosity of 3.6þ0.5 −0.4×1056erg=s, equivalent to 200þ30 −20M⊙c2=s. Several analyses have been performed to determine whether or not GW150914 is consistent with a binaryTABLE I. Source parameters for GW150914. We report median values with 90% credible intervals that include statistical errors, and systematic errors from averaging the results ofdifferent waveform models. Masses are given in the sourceframe; to convert to the detector frame multiply by ( 1þz) [90]. The source redshift assumes standard cosmology [91]. Primary black hole mass 36 þ5 −4M⊙ Secondary black hole mass 29þ4 −4M⊙ Final black hole mass 62þ4 −4M⊙ Final black hole spin 0.67þ0.05 −0.07 Luminosity distance 410þ160 −180Mpc Source redshift z 0.09þ0.03 −0.04PRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-7 black hole system in general relativity [94]. A first consistency check involves the mass and spin of the finalblack hole. In general relativity, the end product of a black hole binary coalescence is a Kerr black hole, which is fully described by its mass and spin. For quasicircular inspirals,these are predicted uniquely by Einstein ’s equations as a function of the masses and spins of the two progenitorblack holes. Using fitting formulas calibrated to numericalrelativity simulations [92], we verified that the remnant mass and spin deduced from the early stage of thecoalescence and those inferred independently from the latestage are consistent with each other, with no evidence fordisagreement from general relativity. Within the post-Newtonian formalism, the phase of the gravitational waveform during the inspiral can be expressedas a power series in f 1=3. The coefficients of this expansion can be computed in general relativity. Thus, we can test forconsistency with general relativity [95,96] by allowing the coefficients to deviate from the nominal values, and seeingif the resulting waveform is consistent with the data. In thissecond check [94]we place constraints on these deviations, finding no evidence for violations of general relativity. Finally, assuming a modified dispersion relation for gravitational waves [97], our observations constrain the Compton wavelength of the graviton to be λ g>1013km, which could be interpreted as a bound on the graviton mass mg<1.2×10−22eV=c2. This improves on Solar System and binary pulsar bounds [98,99] by factors of a few and a thousand, respectively, but does not improve on the model-dependent bounds derived from the dynamics of Galaxyclusters [100] and weak lensing observations [101] .I n summary, all three tests are consistent with the predictionsof general relativity in the strong-field regime of gravity. GW150914 demonstrates the existence of stellar-mass black holes more massive than ≃25M ⊙, and establishes that binary black holes can form in nature and merge within aHubble time. Binary black holes have been predicted to formboth in isolated binaries [102–104] and in dense environ- ments by dynamical interactions [105–107]. The formation of such massive black holes from stellar evolution requires weak massive-star winds, which are possible in stellar environments with metallicity lower than ≃1=2the solar value [108,109] . Further astrophysical implications of this binary black hole discovery are discussed in [110] . These observational results constrain the rate of stellar- mass binary black hole mergers in the local universe. Using several different models of the underlying binary black holemass distribution, we obtain rate estimates ranging from 2–400Gpc −3yr−1in the comoving frame [111–113]. This is consistent with a broad range of rate predictions asreviewed in [114] , with only the lowest event rates being excluded. Binary black hole systems at larger distances contribute to a stochastic background of gravitational waves from thesuperposition of unresolved systems. Predictions for such abackground are presented in [115] . If the signal from such a population were detected, it would provide informationabout the evolution of such binary systems over the historyof the universe. VII. OUTLOOK Further details about these results and associated data releases are available at [116] . Analysis results for the entire first observational period will be reported in futurepublications. Efforts are under way to enhance significantlythe global gravitational-wave detector network [117] . These include further commissioning of the AdvancedLIGO detectors to reach design sensitivity, which willallow detection of binaries like GW150914 with 3 timeshigher SNR. Additionally, Advanced Virgo, KAGRA, anda possible third LIGO detector in India [118] will extend the network and significantly improve the positionreconstruction and parameter estimation of sources. VIII. CONCLUSION The LIGO detectors have observed gravitational waves from the merger of two stellar-mass black holes. Thedetected waveform matches the predictions of generalrelativity for the inspiral and merger of a pair of blackholes and the ringdown of the resulting single black hole.These observations demonstrate the existence of binarystellar-mass black hole systems. This is the first directdetection of gravitational waves and the first observation ofa binary black hole merger. ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the United States National Science Foundation (NSF) forthe construction and operation of the LIGO Laboratoryand Advanced LIGO as well as the Science andTechnology Facilities Council (STFC) of the UnitedKingdom, the Max-Planck Society (MPS), and the Stateof Niedersachsen, Germany, for support of the constructionof Advanced LIGO and construction and operation of theGEO 600 detector. Additional support for Advanced LIGOwas provided by the Australian Research Council. Theauthors gratefully acknowledge the Italian IstitutoNazionale di Fisica Nucleare (INFN), the French CentreNational de la Recherche Scientifique (CNRS), and theFoundation for Fundamental Research on Matter supportedby the Netherlands Organisation for Scientific Research,for the construction and operation of the Virgo detector, andfor the creation and support of the EGO consortium. Theauthors also gratefully acknowledge research support fromthese agencies as well as by the Council of Scientific andIndustrial Research of India, Department of Science andPRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-8 Technology, India, Science & Engineering Research Board (SERB), India, Ministry of Human Resource Development,India, the Spanish Ministerio de Economía y Competitividad, the Conselleria d ’Economia i Competitivitat and Conselleria d ’Educació, Cultura i Universitats of the Govern de les Illes Balears, the National Science Centre of Poland, the European Commission, the Royal Society, the Scottish FundingCouncil, the Scottish Universities Physics Alliance, theHungarian Scientific Research Fund (OTKA), the Lyon Institute of Origins (LIO), the National Research Foundation of Korea, Industry Canada and the Provinceof Ontario through the Ministry of Economic Development and Innovation, the Natural Sciences and Engineering Research Council of Canada, Canadian Institute forAdvanced Research, the Brazilian Ministry of Science, Technology, and Innovation, Russian Foundation for Basic Research, the Leverhulme Trust, the Research Corporation,Ministry of Science and Technology (MOST), Taiwan, and the Kavli Foundation. 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Kang,77J. B. Kanner,1S. Karki,59M. Kasprzack,2,23,34E. Katsavounidis,10 W. Katzman,6S. Kaufer,17T. Kaur,51K. Kawabe,37F. Kawazoe,8,17F. Kéfélian,53M. S. Kehl,69D. Keitel,8,66D. B. Kelley,35 W. Kells,1R. Kennedy,86D. G. Keppel,8J. S. Key,83A. Khalaidovski,8F. Y. Khalili,49I. Khan,12S. Khan,91Z. Khan,95 E. A. Khazanov,109N. Kijbunchoo,37C. Kim,77J. Kim,110K. Kim,111Nam-Gyu Kim,77Namjun Kim,40Y.-M. Kim,110 E. J. King,104P. J. King,37D. L. Kinzel,6J. S. Kissel,37L. Kleybolte,27S. Klimenko,5S. M. Koehlenbeck,8K. Kokeyama,2 S. Koley,9V. Kondrashov,1A. Kontos,10S. Koranda,16M. Korobko,27W. Z. Korth,1I. Kowalska,44D. B. Kozak,1 V. Kringel,8B. Krishnan,8A. Królak,112,113C. Krueger,17G. Kuehn,8P. Kumar,69R. Kumar,36L. Kuo,73A. Kutynia,112 P. Kwee,8B. D. Lackey,35M. Landry,37J. Lange,102B. Lantz,40P. D. Lasky,114A. Lazzarini,1C. Lazzaro,63,42P. Leaci,29,79,28 S. Leavey,36E. O. Lebigot,30,70C. H. Lee,110H. K. Lee,111H. M. Lee,115K. Lee,36A. Lenon,35M. Leonardi,89,90 J. R. Leong,8N. Leroy,23N. Letendre,7Y. Levin,114B. M. Levine,37T. G. F. Li,1A. Libson,10T. B. Littenberg,116 N. A. Lockerbie,107J. Logue,36A. L. Lombardi,103L. T. London,91J. E. Lord,35M. Lorenzini,12,13V. Loriette,117 M. Lormand,6G. Losurdo,58J. D. Lough,8,17C. O. Lousto,102G. Lovelace,22H. Lück,17,8A. P. Lundgren,8J. Luo,78 R. Lynch,10Y. Ma,51T. MacDonald,40B. Machenschalk,8M. MacInnis,10D. M. Macleod,2F. Magaña-Sandoval,35 R. M. Magee,56M. Mageswaran,1E. Majorana,28I. Maksimovic,117V. Malvezzi,25,13N. Man,53I. Mandel,45V. Mandic,84 V. Mangano,36G. L. Mansell,20M. Manske,16M. Mantovani,34F. Marchesoni,118,33F. Marion,7S. Márka,39Z. Márka,39 A. S. Markosyan,40E. Maros,1F. Martelli,57,58L. Martellini,53I. W. Martin,36R. M. Martin,5D. V . Martynov,1J. N. Marx,1 K. Mason,10A. Masserot,7T. J. Massinger,35M. Masso-Reid,36F. Matichard,10L. Matone,39N. Mavalvala,10 N. Mazumder,56G. Mazzolo,8R. McCarthy,37D. E. McClelland,20S. McCormick,6S. C. McGuire,119G. McIntyre,1 J. McIver,1D. J. McManus,20S. T. McWilliams,105D. Meacher,72G. D. Meadors,29,8J. Meidam,9A. Melatos,85 G. Mendell,37D. Mendoza-Gandara,8R. A. Mercer,16E. Merilh,37M. Merzougui,53S. Meshkov,1C. Messenger,36 C. Messick,72P. M. Meyers,84F. Mezzani,28,79H. Miao,45C. Michel,65H. Middleton,45E. E. Mikhailov,120L. Milano,67,4 J. Miller,10M. Millhouse,31Y. Minenkov,13J. Ming,29,8S. Mirshekari,121C. Mishra,15S. Mitra,14V. P. Mitrofanov,49 G. Mitselmakher,5R. Mittleman,10A. Moggi,19M. Mohan,34S. R. P. Mohapatra,10M. Montani,57,58B. C. Moore,88 C. J. Moore,122D. Moraru,37G. Moreno,37S. R. Morriss,83K. Mossavi,8B. Mours,7C. M. Mow-Lowry,45C. L. Mueller,5 G. Mueller,5A. W. Muir,91Arunava Mukherjee,15D. Mukherjee,16S. Mukherjee,83N. Mukund,14A. Mullavey,6 J. Munch,104D. J. Murphy,39P. G. Murray,36A. Mytidis,5I. Nardecchia,25,13L. Naticchioni,79,28R. K. Nayak,123V. Necula,5 K. Nedkova,103G. Nelemans,52,9M. Neri,46,47A. Neunzert,98G. Newton,36T. T. Nguyen,20A. B. Nielsen,8S. Nissanke,52,9 A. Nitz,8F. Nocera,34D. Nolting,6M. E. N. Normandin,83L. K. 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Sturani,121A. L. Stuver,6T. Z. Summerscales,128L. Sun,85P. J. Sutton,91B. L. Swinkels,34M. J. Szczepa ńczyk,97 M. Tacca,30D. Talukder,59D. B. Tanner,5M. Tápai,96S. P. Tarabrin,8A. Taracchini,29R. Taylor,1T. Theeg,8 M. P. Thirugnanasambandam,1E. G. Thomas,45M. Thomas,6P. Thomas,37K. A. Thorne,6K. S. Thorne,76E. Thrane,114 S. Tiwari,12V. Tiwari,91K. V . Tokmakov,107C. Tomlinson,86M. Tonelli,18,19C. V. Torres,83,cC. I. Torrie,1D. Töyrä,45 F. Travasso,32,33G. Traylor,6D. Trifirò,21M. C. Tringali,89,90L. Trozzo,129,19M. Tse,10M. Turconi,53D. Tuyenbayev,83 D. Ugolini,130C. S. Unnikrishnan,99A. L. Urban,16S. A. Usman,35H. Vahlbruch,17G. Vajente,1G. Valdes,83 M. Vallisneri,76N. van Bakel,9M. van Beuzekom,9J. F. J. van den Brand,61,9C. Van Den Broeck,9D. C. Vander-Hyde,35,22 L. van der Schaaf,9J. V . van Heijningen,9A. A. van Veggel,36M. Vardaro,41,42S. Vass,1M. Vasúth,38R. Vaulin,10 A. Vecchio,45G. Vedovato,42J. Veitch,45P. J. Veitch,104K. Venkateswara,131D. Verkindt,7F. Vetrano,57,58A. Viceré,57,58 S. Vinciguerra,45D. J. Vine,50J.-Y. Vinet,53S. Vitale,10T. Vo,35H. Vocca,32,33C. Vorvick,37D. Voss,5W. D. Vousden,45 S. P. Vyatchanin,49A. R. Wade,20L. E. Wade,132M. Wade,132S. J. Waldman,10M. Walker,2L. Wallace,1S. Walsh,16,8,29 G. Wang,12H. Wang,45M. Wang,45X. Wang,70Y . Wang,51H. Ward,36R. L. Ward,20J. Warner,37M. Was,7B. Weaver,37 L.-W. Wei,53M. Weinert,8A. J. Weinstein,1R. Weiss,10T. Welborn,6L. Wen,51P. Weßels,8T. Westphal,8K. Wette,8 J. T. Whelan,102,8S. E. Whitcomb,1D. J. White,86B. F. Whiting,5K. Wiesner,8C. Wilkinson,37P. A. Willems,1L. Williams,5 R. D. Williams,1A. R. Williamson,91J. L. Willis,133B. Willke,17,8M. H. Wimmer,8,17L. Winkelmann,8W. Winkler,8 C. C. Wipf,1A. G. Wiseman,16H. Wittel,8,17G. Woan,36J. Worden,37J. L. Wright,36G. Wu,6J. Yablon,82I. Yakushin,6 W. Yam,10H. Yamamoto,1C. C. Yancey,62M. J. Yap,20H. Yu,10M. Yvert,7A. Zadro żny,112L. Zangrando,42M. Zanolin,97 J.-P. Zendri,42M. Zevin,82F. Zhang,10L. Zhang,1M. Zhang,120Y. Zhang,102C. Zhao,51M. Zhou,82Z. Zhou,82X. J. Zhu,51 M. E. Zucker,1,10S. E. Zuraw,103and J. Zweizig1 (LIGO Scientific Collaboration and Virgo Collaboration) 1LIGO, California Institute of Technology, Pasadena, California 91125, USA 2Louisiana State University, Baton Rouge, Louisiana 70803, USA 3Università di Salerno, Fisciano, I-84084 Salerno, Italy 4INFN, Sezione di Napoli, Complesso Universitario di Monte S. Angelo, I-80126 Napoli, Italy 5University of Florida, Gainesville, Florida 32611, USA 6LIGO Livingston Observatory, Livingston, Louisiana 70754, USA 7Laboratoire d ’Annecy-le-Vieux de Physique des Particules (LAPP), Université Savoie Mont Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France 8Albert-Einstein-Institut, Max-Planck-Institut für Gravitationsphysik, D-30167 Hannover, Germany 9Nikhef, Science Park, 1098 XG Amsterdam, Netherlands 10LIGO, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USAPRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-13 11Instituto Nacional de Pesquisas Espaciais, 12227-010 São José dos Campos, São Paulo, Brazil 12INFN, Gran Sasso Science Institute, I-67100 L ’Aquila, Italy 13INFN, Sezione di Roma Tor Vergata, I-00133 Roma, Italy 14Inter-University Centre for Astronomy and Astrophysics, Pune 411007, India 15International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560012, India 16University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, USA 17Leibniz Universität Hannover, D-30167 Hannover, Germany 18Università di Pisa, I-56127 Pisa, Italy 19INFN, Sezione di Pisa, I-56127 Pisa, Italy 20Australian National University, Canberra, Australian Capital Territory 0200, Australia 21The University of Mississippi, University, Mississippi 38677, USA 22California State University Fullerton, Fullerton, California 92831, USA 23LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France 24Chennai Mathematical Institute, Chennai, India 603103 25Università di Roma Tor Vergata, I-00133 Roma, Italy 26University of Southampton, Southampton SO17 1BJ, United Kingdom 27Universität Hamburg, D-22761 Hamburg, Germany 28INFN, Sezione di Roma, I-00185 Roma, Italy 29Albert-Einstein-Institut, Max-Planck-Institut für Gravitationsphysik, D-14476 Potsdam-Golm, Germany 30APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, F-75205 Paris Cedex 13, France 31Montana State University, Bozeman, Montana 59717, USA 32Università di Perugia, I-06123 Perugia, Italy 33INFN, Sezione di Perugia, I-06123 Perugia, Italy 34European Gravitational Observatory (EGO), I-56021 Cascina, Pisa, Italy 35Syracuse University, Syracuse, New York 13244, USA 36SUPA, University of Glasgow, Glasgow G12 8QQ, United Kingdom 37LIGO Hanford Observatory, Richland, Washington 99352, USA 38Wigner RCP, RMKI, H-1121 Budapest, Konkoly Thege Miklós út 29-33, Hungary 39Columbia University, New York, New York 10027, USA 40Stanford University, Stanford, California 94305, USA 41Università di Padova, Dipartimento di Fisica e Astronomia, I-35131 Padova, Italy 42INFN, Sezione di Padova, I-35131 Padova, Italy 43CAMK-PAN, 00-716 Warsaw, Poland 44Astronomical Observatory Warsaw University, 00-478 Warsaw, Poland 45University of Birmingham, Birmingham B15 2TT, United Kingdom 46Università degli Studi di Genova, I-16146 Genova, Italy 47INFN, Sezione di Genova, I-16146 Genova, Italy 48RRCAT, Indore MP 452013, India 49Faculty of Physics, Lomonosov Moscow State University, Moscow 119991, Russia 50SUPA, University of the West of Scotland, Paisley PA1 2BE, United Kingdom 51University of Western Australia, Crawley, Western Australia 6009, Australia 52Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, Netherlands 53Artemis, Université Côte d ’Azur, CNRS, Observatoire Côte d ’Azur, CS 34229, Nice cedex 4, France 54MTA Eötvös University, “Lendulet ”Astrophysics Research Group, Budapest 1117, Hungary 55Institut de Physique de Rennes, CNRS, Université de Rennes 1, F-35042 Rennes, France 56Washington State University, Pullman, Washington 99164, USA 57Università degli Studi di Urbino “Carlo Bo, ”I-61029 Urbino, Italy 58INFN, Sezione di Firenze, I-50019 Sesto Fiorentino, Firenze, Italy 59University of Oregon, Eugene, Oregon 97403, USA 60Laboratoire Kastler Brossel, UPMC-Sorbonne Universités, CNRS, ENS-PSL Research University, Collège de France, F-75005 Paris, France 61VU University Amsterdam, 1081 HV Amsterdam, Netherlands 62University of Maryland, College Park, Maryland 20742, USA 63Center for Relativistic Astrophysics and School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA 64Institut Lumière Matière, Université de Lyon, Université Claude Bernard Lyon 1, UMR CNRS 5306, 69622 Villeurbanne, France 65Laboratoire des Matériaux Avancés (LMA), IN2P3/CNRS, Université de Lyon, F-69622 Villeurbanne, Lyon, France 66Universitat de les Illes Balears, IAC3 —IEEC, E-07122 Palma de Mallorca, Spain 67Università di Napoli “Federico II, ”Complesso Universitario di Monte S. Angelo, I-80126 Napoli, Italy 68NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USAPRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-14 69Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Ontario M5S 3H8, Canada 70Tsinghua University, Beijing 100084, China 71Texas Tech University, Lubbock, Texas 79409, USA 72The Pennsylvania State University, University Park, Pennsylvania 16802, USA 73National Tsing Hua University, Hsinchu City, 30013 Taiwan, Republic of China 74Charles Sturt University, Wagga Wagga, New South Wales 2678, Australia 75University of Chicago, Chicago, Illinois 60637, USA 76Caltech CaRT, Pasadena, California 91125, USA 77Korea Institute of Science and Technology Information, Daejeon 305-806, Korea 78Carleton College, Northfield, Minnesota 55057, USA 79Università di Roma “La Sapienza, ”I-00185 Roma, Italy 80University of Brussels, Brussels 1050, Belgium 81Sonoma State University, Rohnert Park, California 94928, USA 82Northwestern University, Evanston, Illinois 60208, USA 83The University of Texas Rio Grande Valley, Brownsville, Texas 78520, USA 84University of Minnesota, Minneapolis, Minnesota 55455, USA 85The University of Melbourne, Parkville, Victoria 3010, Australia 86The University of Sheffield, Sheffield S10 2TN, United Kingdom 87University of Sannio at Benevento, I-82100 Benevento, Italy and INFN, Sezione di Napoli, I-80100 Napoli, Italy 88Montclair State University, Montclair, New Jersey 07043, USA 89Università di Trento, Dipartimento di Fisica, I-38123 Povo, Trento, Italy 90INFN, Trento Institute for Fundamental Physics and Applications, I-38123 Povo, Trento, Italy 91Cardiff University, Cardiff CF24 3AA, United Kingdom 92National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 93School of Mathematics, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom 94Indian Institute of Technology, Gandhinagar Ahmedabad Gujarat 382424, India 95Institute for Plasma Research, Bhat, Gandhinagar 382428, India 96University of Szeged, Dóm tér 9, Szeged 6720, Hungary 97Embry-Riddle Aeronautical University, Prescott, Arizona 86301, USA 98University of Michigan, Ann Arbor, Michigan 48109, USA 99Tata Institute of Fundamental Research, Mumbai 400005, India 100Rutherford Appleton Laboratory, HSIC, Chilton, Didcot, Oxon OX11 0QX, United Kingdom 101American University, Washington, D.C. 20016, USA 102Rochester Institute of Technology, Rochester, New York 14623, USA 103University of Massachusetts-Amherst, Amherst, Massachusetts 01003, USA 104University of Adelaide, Adelaide, South Australia 5005, Australia 105West Virginia University, Morgantown, West Virginia 26506, USA 106University of Bia łystok, 15-424 Bia łystok, Poland 107SUPA, University of Strathclyde, Glasgow G1 1XQ, United Kingdom 108IISER-TVM, CET Campus, Trivandrum Kerala 695016, India 109Institute of Applied Physics, Nizhny Novgorod, 603950, Russia 110Pusan National University, Busan 609-735, Korea 111Hanyang University, Seoul 133-791, Korea 112NCBJ, 05-400 Świerk-Otwock, Poland 113IM-PAN, 00-956 Warsaw, Poland 114Monash University, Victoria 3800, Australia 115Seoul National University, Seoul 151-742, Korea 116University of Alabama in Huntsville, Huntsville, Alabama 35899, USA 117ESPCI, CNRS, F-75005 Paris, France 118Università di Camerino, Dipartimento di Fisica, I-62032 Camerino, Italy 119Southern University and A&M College, Baton Rouge, Louisiana 70813, USA 120College of William and Mary, Williamsburg, Virginia 23187, USA 121Instituto de Física Teórica, University Estadual Paulista/ICTP South American Institute for Fundamental Research, São Paulo SP 01140-070, Brazil 122University of Cambridge, Cambridge CB2 1TN, United Kingdom 123IISER-Kolkata, Mohanpur, West Bengal 741252, India 124Whitman College, 345 Boyer Avenue, Walla Walla, Washington 99362 USA 125National Institute for Mathematical Sciences, Daejeon 305-390, Korea 126Hobart and William Smith Colleges, Geneva, New York 14456, USA 127Janusz Gil Institute of Astronomy, University of Zielona Góra, 65-265 Zielona Góra, PolandPRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-15 128Andrews University, Berrien Springs, Michigan 49104, USA 129Università di Siena, I-53100 Siena, Italy 130Trinity University, San Antonio, Texas 78212, USA 131University of Washington, Seattle, Washington 98195, USA 132Kenyon College, Gambier, Ohio 43022, USA 133Abilene Christian University, Abilene, Texas 79699, USA aDeceased, April 2012. bDeceased, May 2015. cDeceased, March 2015.PRL 116, 061102 (2016)PHYSICAL REVIEW LETTERSweek ending 12 FEBRUARY 2016 061102-16