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First observation of gravitational waves

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LIGO measurement of the gravitational waves at the Livingston (right) and Hanford (left) detectors, compared with the theoretical predicted values

On 11 February 2016, the LIGO and Virgo collaborations announced the first direct observation of gravitational waves.[1][2] The waveform, detected on 14 September 2015[3] by Marco Drago, physicist at the Albert Einstein Institute in Hannover, Germany,[4] matched the predictions of general relativity for the inward spiral and merger of a pair of black holes and subsequent "ringdown" of the resulting single black hole. The signal was named GW150914.[1][5] This was 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 age of the universe.

Gravitational waves

Main article: Gravitational wave

Gravitational waves were originally predicted in 1916,[6][7] by Albert Einstein on the basis of his theory of general relativity.[8] Indirect evidence of gravitational waves was seen in 1974 through the motion of the double neutron star system PSR B1913+16, for which Russell Alan Hulse and Joseph Hooton Taylor, Jr. received the 1993 Nobel Prize in Physics.[2] Binary star systems, such as binary black holes, emit gravitational waves. This shrinks their orbit and leads to an inspiral and ultimately, in the case of two black holes, a merger.[1]

Development of LIGO

Main article: LIGO
Northern leg of the LIGO Hanford Observatory

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 / 30.5628944; -90.7742417) 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 / 46.4551444; -119.4076556), located near Richland, Washington. These sites are 3,002 kilometers (1,865 mi) apart. The observatories compare the signals of their laser interferometers. Initial LIGO operations between 2002 and 2010 did not detect any gravitational waves. This was followed by a multi-year shut-down while the detectors were replaced by much improved "Advanced LIGO" versions.[9]  In February 2015, the two advanced detectors were brought into engineering mode,[10] with formal science observations due to begin on 18 September 2015.[citation needed]

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; no such tests were taking place in September 2015.[11]

Event detection

GW150914, which stands for the words "gravitational wave" followed by the date of its detection, 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 came from the Southern Celestial Hemisphere, in the rough direction of (but much farther than) the Magellanic Clouds.[5][2] The signal lasted over 0.2 seconds, and increased in frequency and amplitude in about 8 cycles from 35 to 250 Hz.[1][12] The signal has been described as a "chirp" of a bird.[2] The detection was reported within three minutes of data acquisition of the signal using low-latency search methods that provide a quick, initial analysis of the data from the detectors.[12] It was first seen by Italian post-doctoral researcher Marco Drago who was receiving data from LIGO while in Germany and initially did not think the signal was real;[13] his boss, Bruce Allen, initially thought that it had been an injected signal.[2]

More detailed statistical analysis of the signal, and of 16 days of surrounding data from 12 September to 20 October, identified GW150914 as a real event, with a significance of over 5.1 sigma or a confidence level of 99.99994%.[14] The signal was seen at Livingston 7 milliseconds before it was seen at Hanford, which is consistent with the light travel time between the two sites (gravitational waves propagate at the speed of light).[1][12]

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 detect the signal.[2] GEO600 (near Hanover, Germany) was not sensitive enough to detect the signal.[1] Consequently, neither of these detectors was able to confirm the signal measured by LIGO.[2]

Black hole merger

The event happened at a distance of 410+160
−180 megaparsecs[1][15] (determined by the amplitude of the signal),[2] or 1.3±0.6 billion light years, corresponding to a cosmological redshift of 0.09+0.03
−0.04 (90% confidence 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 36+5
−4 times and 29±4 times the mass of the Sun, resulting in a post-merger black hole of 62±4 solar masses. The missing 3.0±0.5 solar masses of energy was radiated away in the form of gravitational waves, in accordance with mass–energy equivalence.

The peak power of the radiated gravitational waves — about 3.6×1049 watts — was more than the combined power of all light radiated by all the stars in the observable universe.[2][12][16][17] "The total power output in the gravitational waves during the brief collision was 50 times greater than all of the power put out by all of the stars in the universe put together," Kip Thorne said.

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 before they merged. This close orbital radius implies that the objects had to be black holes, as no other known objects with these masses could orbit each other this closely before merging. A black hole-neutron star pair would have merged at a lower frequency. 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.[1][12][15]

The decay of the waveform after it peaked was consistent with the damped oscillations of a black hole relaxing to a final merged configuration.[1] Although the inspiral motion can be described well from the signal analytics, 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.[12][18]

Location of the event

Gravitational wave instruments are all-sky monitors with no ability to spatially resolve signals. A network of instruments is needed to reconstruct the location of the event on the sky. With only the two LIGO instruments in observational mode, GW150914’s source location could only be reconstructed to a banana-shaped area. This was done via analysis of the 6.9+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). This area of the sky was targeted by follow-up observations covering radio, optical, near infra-red, X-ray, and gamma-ray wavelengths along with searches for coincident neutrinos.[15]

Announcement

The announcement of the detection was made on 11 February 2016[2] at a news conference in Washington, D.C. by David Reitze, the executive director of LIGO,[3] with a panel comprising Gabriela González, Rainer Weiss and Kip Thorne.[2]

The initial announcement paper was published during the news conference in Physical Review Letters[1], with further papers either published shortly afterwards[19] or immediately available in preprint form (see the LIGO Open Science Center and preprints on ArXiv).

Implications

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.[20][1]

Astrophysics

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.[19]

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. Natal kicks (the velocity a black hole gains at its formation in a core-collapse supernova event) cannot always be high, 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.[19]

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.[19][1]

Gravitons

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 masslessness of the photon; 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 2.16×10−58 kg (corresponding to 1.2×10−22 eV/c2 or a Compton wavelength of greater than 1013 km).[20][1]

Future detections

Given the brightness of this detection, it is expected that this may be the first of several detections during the first year of operation by the Advanced LIGO detectors. Over its next observing campaign, it is expected to detect five more black hole mergers like GW150914, and to detect 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.[5] 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.[1]

Space-borne observatories

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.[19]

References

  1. ^ a b c d e f g h i j k l m n Abbott, Benjamin P.; et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Phys. Rev. Lett. 116: 061102. doi:10.1103/PhysRevLett.116.061102.
  2. ^ a b c d e f g h i j k Castelvecchi, Davide; Witze, Alexandra (11 February 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. Retrieved 11 February 2016.
  3. ^ a b "Einstein's gravitational waves 'seen' from black holes". BBC News. 11 February 2016.
  4. ^ Twilley, Nicola (February 2016). "Gravitational Waves Exist: The Inside Story of How Scientists Finally Found Them". The New Yorker.
  5. ^ a b c Naeye, Robert (11 February 2016). "Gravitational Wave Detection Heralds New Era of Science". Sky and Telescope. Retrieved 11 February 2016.
  6. ^ Einstein, A (June 1916). "Näherungsweise Integration der Feldgleichungen der Gravitation". Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften Berlin. part 1: 688–696.
  7. ^ Einstein, A (1918). "Über Gravitationswellen". Sitzungsberichte der Königlich Preussischen Akademie der Wissenschaften Berlin. part 1: 154–167.
  8. ^ Finley, Dave. "Einstein's gravity theory passes toughest test yet: Bizarre binary star system pushes study of relativity to new limits". Phys.Org.
  9. ^ "Gravitational wave detection a step closer with Advanced LIGO". SPIE Newsroom. Retrieved 4 January 2016.
  10. ^ "LIGO Hanford's H1 Achieves Two-Hour Full Lock". February 2015.
  11. ^ "Gravitational-wave rumours in overdrive". Nature. 12 January 2016. Retrieved 11 February 2016.
  12. ^ a b c d e f "Observation Of Gravitational Waves From A Binary Black Hole Merger" (PDF). LIGO. 11 February 2016. Retrieved 11 February 2016.
  13. ^ Cho, Adrian (11 February 2016). "Here's the first person to spot those gravitational waves". Science (magazine). doi:10.1126/science.aaf4039.
  14. ^ LIGO’s First-Ever Detection of Gravitational Waves Opens a New Window on the Universe
  15. ^ a b c "Properties of the binary black hole merger GW150914" (PDF). 11 February 2016. {{cite journal}}: Cite journal requires |journal= (help)
  16. ^ Harwood, W. (11 February 2016). "Einstein was right: Scientists detect gravitational waves in breakthrough". CBS News. Retrieved 12 February 2016.
  17. ^ Drake, Nadia (11 February 2016). "Found! Gravitational Waves, or a Wrinkle in Spacetime". National Geographic News. Retrieved 12 February 2016.
  18. ^ "LIGO detects first ever gravitational waves – from two merging black holes". Physics World. 11 February 2016. Retrieved 12 February 2016.
  19. ^ a b c d e Abbott, Benjamin P.; et al. (LIGO Scientific Collaboration and Virgo Collaboration) (20 February 2016). "Astrophysical implications of the binary black-hole merger GW150914". The Astrophysical Journal. Retrieved 11 February 2016.
  20. ^ a b Abbott, Benjamin P.; et al. (LIGO Scientific Collaboration and Virgo Collaboration) (11 February 2016). "Tests of general relativity with GW150914". LIGO. Retrieved 12 February 2016.
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