First observation of gravitational waves
LIGO measurement of the gravitational waves at the Livingston (right) and Hanford (left) detectors, compared with the theoretical predicted values
|Event type||gravitational waves|
|Detection time||14 September 2015|
|Related media on Wikimedia Commons|
The first observation of gravitational waves on 14 September 2015 was announced by the LIGO and Virgo interferometer collaborations on 11 February 2016. Gravitational waves had previously been observed only indirectly, through their effect on the timing of pulsars in binary systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes and subsequent "ringdown" of the single resulting black hole. The signal was named GW150914 (i.e., "Gravitational Wave 2015-09-14"). It was also the first observation of a binary black hole merger, demonstrating the existence of binary stellar-mass black hole systems, and that such mergers could occur within the current age of the universe.
This first observation was reported around the world as a remarkable accomplishment for many reasons. Efforts to directly prove the existence of such waves had been ongoing for over fifty years, and the waves are so minuscule that Albert Einstein doubted they could ever be detected. The waves given off by the cataclysmic merger of GW150914 reached Earth as a ripple in spacetime that changed the length of a 4-km LIGO arm by a ten thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star by one hair's width. The energy released during the brief climax of the event was immense, with about three solar masses converted to gravitational waves and radiated away at a peak rate of about 3.6×1049 watts – more than the combined power of all light radiated by all the stars in the observable universe.
The observation was heralded as confirming the last remaining unproven prediction of general relativity and validating its predictions of space-time distortion in the context of large scale cosmic events (known as strong field tests), as well as inaugurating a new era of gravitational-wave astronomy, allowing probing of violent astrophysical events unobservable until now.
Gravitational waves were originally predicted in 1916, by Albert Einstein on the basis of his theory of general relativity. The theory interpreted gravity as being a consequence of distortions in space-time, and therefore Einstein also predicted that events in the cosmos would cause "ripples" in space-time – distortions of space itself – which would spread outward, although of such minuscule amount that they would be virtually impossible to detect by any technology foreseen at that time. It was also predicted that objects moving in an orbit would lose energy for this reason (a consequence of the law of conservation of energy), as some energy would be given off as gravitational waves, although so microscopically tiny as to be insignificant in all but the most violent or large-scale cases.
One case where gravitational waves were calculated to be strongest was the final moments before the merging of two compact objects such as neutron stars or black holes. Over a span of millions of years, binary neutron stars, and binary black holes lose energy, largely through weak gravitational waves, and spiral in towards each other. Near the very end of this process, the two objects will reach extreme velocities, and a substantial amount of their mass will be converted to gravitational energy in the final fraction of a second before their merger. This produces gravitational waves within reach of scientific detection.
Observations can be made either indirectly (by observing the effects of waves, and deducing their likely cause) or directly (by actually detecting and observing the waves themselves).
Indirect evidence of gravitational waves was first seen in 1974 through the motion of the double neutron star system PSR B1913+16, in which one of the stars is a pulsar emitting precisely timed radio frequency pulses. Russell Hulse and Joseph Taylor, who discovered the stars, also showed that over time the frequency of pulses shortened, and that the stars were gradually spiralling towards each other with an energy loss that agreed closely with the predicted energy radiated by gravitational waves. For this work, Hulse and Taylor won the 1993 Nobel Prize in Physics. Further observations of this pulsar and others in multiple systems have always agreed with General Relativity.
Direct observation (LIGO)
Direct observation remained frustrated for many decades because of the minuscule effect that would need to be detected and separated from the background of vibration present everywhere on Earth. A technique called interferometry was suggested in the 1960s and eventually technology advanced enough to make its use plausible.
In this approach, a laser beam is split and the two halves allowed to recombine after travelling different paths. Changes to the distance or time taken for the two split beams to reach the point they recombine, are revealed as "beats" and this technique is extremely sensitive to tiny changes in distance or time taken. In theory, an interferometer with arms about 4 km long would be capable of revealing the change of space-time – a tiny fraction of the size of a single atom – as a gravitational wave passed through Earth from elsewhere, although this effect would be imperceptible to anybody and any other instrument on the planet (except for other interferometers of a similar size, such as the Virgo, GEO 600 and planned INDIGO detectors). In practice at least two interferometers would be needed, because any gravitational wave would be detected at both of these but other kinds of disturbance would generally not be present at both, allowing the sought-after signal to be distinguished from other noise. This project was eventually founded in 1992 under the name "LIGO" ("Laser Interferometer Gravitational-Wave Observatory"). The original instruments were upgraded between 2010 and 2015, giving an increase of around 3 times their original sensitivity.
LIGO operates two gravitational-wave observatories in unison: the LIGO Livingston Observatory (30°33′46.42″N 90°46′27.27″W / 30.5628944°N 90.7742417°W) in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site (46°27′18.52″N 119°24′27.56″W / 46.4551444°N 119.4076556°W), located near Richland, Washington. These sites are 3,002 km (1,865 mi) apart. The observatories compare the signals of their laser interferometers. Initial LIGO operations between 2002 and 2010 did not detect any statistically significant events that could be confirmed as gravitational waves. This was followed by a multi-year shut-down while the detectors were replaced by much improved "Advanced LIGO" versions. In February 2015, the two advanced detectors were brought into engineering mode, with formal science observations due to begin on 18 September 2015.
Throughout the development and initial observations by LIGO, several "blind injections" of fake gravitational wave signals took place to test the ability of the researchers to identify such signals. To protect the efficacy of blind injections, only four LIGO scientists knew when such injections occurred, and that information is revealed only after a signal has been thoroughly analyzed by researchers. However no such tests were taking place in September 2015 when GW150914 occurred.
The GW150914 event
GW150914 was detected by the LIGO detectors in Hanford, Washington state, and Livingston, Louisiana, USA, at 09:50:45 UTC on 14 September 2015. The signal probably came from the Southern Celestial Hemisphere, in the rough direction of (but much farther than) the Magellanic Clouds. The chirp signal lasted over 0.2 seconds, and increased in frequency and amplitude in about 8 cycles from 35 Hz to 250 Hz. (The signal has been described as resembling the "chirp" of a bird, and astrophysicists all over the world excitedly imitated the signal on social media upon the announcement of the discovery.)
The trigger indicating a possible detection was reported within three minutes of acquisition of the signal, using low-latency ('online') search methods that provide a quick, initial analysis of the data from the detectors. The pipeline which made the detection was developed by the 'coherent WaveBurst' (cWB) analysis group within LIGO/Virgo. After the initial automatic alert at 09:54 UTC, a sequence of internal emails confirmed that no scheduled or unscheduled injections had been made, and that the data looked clean. After this, the rest of the collaboration was quickly made aware of the tentative detection and its parameters.
More detailed statistical analysis of the signal, and of 16 days of surrounding data from 12 September to 20 October 2015, identified GW150914 as a real event, with a significance of over 5.1 sigma or a confidence level of 99.99994%. Corresponding wave peaks were seen at Livingston seven milliseconds before they arrived at Hanford. Gravitational waves propagate at the speed of light, and the disparity is consistent with the light travel time between the two sites. The waves had traveled at the speed of light for more than a billion years.
At the time of the event, the Virgo gravitational wave detector (near Pisa, Italy) was offline and undergoing an upgrade; had it been online it would likely have been sensitive enough to also detect the signal, which would have greatly improved the positioning of the event. GEO600 (near Hannover, Germany) was not sensitive enough to detect the signal. Consequently, neither of these detectors was able to confirm the signal measured by the LIGO detectors.
The event happened at a luminosity distance of +160
−180 410megaparsecs (determined by the amplitude of the signal), or ±0.6 billion 1.3light years, corresponding to a cosmological redshift of +0.03
−0.04 (90% 0.09credible intervals). Analysis of the signal along with the inferred redshift suggested that it was produced by the merger of two black holes with masses of +5
−4 times and 36±4 times the mass of the 29Sun, resulting in a post-merger black hole of ±4 solar masses. 62 The missing ±0.5 solar masses of energy was radiated away in the form of gravitational waves, in accordance with 3.0mass–energy equivalence.
During the final 20 milliseconds of the merger, the power of the radiated gravitational waves peaked at about 3.6×1049 watts – 50 times greater than the combined power of all light radiated by all the stars in the observable universe.
Across the 0.2-second duration of the detectable signal, the relative tangential (orbiting) velocity of the black holes increased from 30% to 60% of the speed of light. The orbital frequency of 75 Hz (half the gravitational wave frequency) means that the objects were orbiting each other at a distance of only 350 km by the time they merged. The orbital frequency and extreme velocity allowed calculation of the objects' masses and orbital separation (distance apart) when they merged. They show that the objects had to be black holes, because any other kind of known objects with these masses would have been physically larger and therefore merged before that point, or would not have reached such velocities in such a small orbit. The highest observed neutron star mass is two solar masses, with a conservative upper limit for the mass of a stable neutron star of three solar masses, so that a pair of neutron stars would not have sufficient mass to account for the merger (unless exotic alternatives exist, e.g., boson stars), while a black hole-neutron star pair would have merged at a lower frequency.
The decay of the waveform after it peaked was consistent with the damped oscillations of a black hole relaxing to a final merged configuration. Although the inspiral motion can be described well from post-Newtonian calculations, the strong gravitational field merger stage can only be solved in full generality by large-scale numerical relativity simulations.
The post-merger object is thought to be a rotating Kerr black hole with spin parameter 0.67, i.e. one with 2/3 of the maximum possible angular momentum for its mass.
Location in the sky
Gravitational wave instruments are all-sky monitors with little ability to spatially resolve signals. A network of instruments is needed to locate the source on the sky through triangulation. With only the two LIGO instruments in observational mode, GW150914's source location could only be confined to an arc on the sky. This was done via analysis of the +0.5
−0.4 ms time-delay, along with amplitude and phase consistency across both detectors. This analysis produced a credible region of 140 deg2 (50% probability) or 590 deg2 (90% probability) located mainly in the 6.9Southern Celestial Hemisphere.
The search for coincident neutrinos was conducted by the ANTARES telescope and IceCube Neutrino Observatory. The ANTARES telescope detected no neutrino candidates within ±500 seconds of GW150914. The IceCube observatory detected three neutrino candidates within ±500 seconds of GW150914. One event was found in the southern sky and two in the northern sky. This was consistent with the expectation of background detection levels. None of the candidates were compatible with the 90% confidence area of the merger event. Although no neutrinos were detected, the lack of observations provided a limit on neutrino emission from this type of gravitational wave event.
Observations by the Swift Gamma-Ray Burst Mission of nearby galaxies in the region of the detection, two days after the event, did not detect any new X-ray, optical or ultraviolet sources.
The announcement of the detection was made on 11 February 2016 at a news conference in Washington, D.C. by David Reitze, the executive director of LIGO, with a panel comprising Gabriela González, Rainer Weiss and Kip Thorne, of LIGO, and France A. Córdova, the director of NSF.
The Advanced LIGO is predicted to detect five more black hole mergers like GW150914 in its next observing campaign, and then 40 binary star mergers each year, in addition to an unknown number of more exotic gravitational wave sources, some of which may not be anticipated by current theory.
Planned upgrades are expected to double the signal-to-noise ratio, expanding the volume of space in which events like GW150914 can be detected by a factor of ten. Additionally, Advanced Virgo, KAGRA, and a possible third LIGO detector in India will extend the network and significantly improve the position reconstruction and parameter estimation of sources.
Evolved Laser Interferometer Space Antenna (eLISA) is a proposed mission to detect gravitational waves in space. Merging massive binaries like GW150914 would evolve through the proposed sensitivity range of eLISA about 1000 years before they merge, providing for a class of previously unknown sources for this observatory if they exist within about 10 megaparsecs. LISA Pathfinder, eLISA's technology development mission, was launched in December 2015.
Tests of general relativity
The inferred fundamental properties, mass and spin, of the post-merger black hole were consistent with those of the two pre-merger black holes, following the predictions of general relativity. This is the first test of general relativity in the very strong-field regime. No evidence could be established against the predictions of general relativity.
The opportunity was limited in this signal to investigate the more complex general relativity interactions, such as tails produced by interactions between the gravitational wave and curved space-time background. Although a moderately strong signal, it is much smaller than that produced by binary-pulsar systems. It is hoped that in the future stronger signals, in conjunction with more sensitive detector, can be used to explore the intricate interactions of gravitational waves as well as to improve the constraints on deviations from general relativity.
The masses of the two pre-merger black holes provide information about stellar evolution. Both black holes were more massive than previously discovered stellar-mass black holes, which were inferred from X-ray binary observations. This implies that the stellar winds from their progenitor stars must have been relatively weak, and therefore that the metallicity (mass fraction of chemical elements heavier than hydrogen and helium) must have been lower than about half the solar value.
The fact that the pre-merger black holes were present in a binary star system, as well as the fact that the system was compact enough to merge within the age of the universe, constrains either binary star evolution or dynamical formation scenarios, depending on how the black hole binary was formed. A significant number of black holes must receive low natal kicks (the velocity a black hole gains at its formation in a core-collapse supernova event), otherwise binaries in which a black-hole forming supernova takes place would be disrupted, and black holes in globular clusters would exceed the escape velocity of the cluster, and be ejected before being able to form a binary via dynamical interaction. Survival through common envelope phases of high rotation in massive progenitor stars may be necessary. The majority of the latest black hole model predictions comply with these added constraints.
The discovery of the merger event itself increases the lower limit on the rate of such events, and rules out certain theoretical models that predicted very low rates of less than 1 Gpc−3yr−1. Analysis resulted in lowering the previous upper limit rate on events like GW150914 from ~140 Gpc−3yr−1 to +39
−13 Gpc−3yr−1. 17
The graviton is a hypothetical elementary particle associated with gravity, and will be massless if, as it appears, gravitation has an infinite range (the more massive a gauge boson is, the shorter is the range of the associated force, so the infinite range of light is the result of the photon's lack of mass; supposing that the graviton is indeed the gauge boson of a future quantum theory of gravity, the infinite range of gravity implies that a putative graviton would also be expected to be massless). The observations of the inspiral slightly improve (lower) the upper limit on the mass of the graviton to ×10−58 kg (corresponding to 2.16×10−22 eV/c2 or a 1.2Compton wavelength of greater than 1013 km, roughly 1 light-year).
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|Wikimedia Commons has media related to GW150914.|
- LIGO Open Science Center The data associated with the initial detection.
- Video (04:36) – Detecting a gravitational wave, Dennis Overbye, NYT (11 February 2016).
- Video (71:29) – Press Conference announcing discovery: "LIGO detects gravitational waves", National Science Foundation (11 February 2016).