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First observation of gravitational waves

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

On 11 February 2016, the LIGO, Virgo and GEO600 collaborations announced the first detection of gravitational waves.[1][2][3][4] The signal was named GW150914.[1][5] The detected waveform, observed on 14 September 2015,[6] 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 observations demonstrated the existence of binary stellar-mass black hole systems and the first observation of a binary black hole merger.

Event detection

Gravitational wave signal GW150914 was detected by the LIGO detectors in Hanford, Washington State, and Livingston, Louisiana, USA, at 09:50:45 UTC on 14 September 2014. The signal lasted over 0.2 seconds, and increased in frequency and amplitude in about 8 cycles from 35 to 150 Hz, where the amplitude reached a maximum. 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. 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.[1][7]

At the time of the event, the Virgo gravitational wave detector was offline and undergoing an upgrade, and GEO600 was not sensitive enough to detect the signal.[1]

Black hole merger

Analysis of the signal 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 event happened at a distance of 410+160
−180 megaparsecs,[1][8] or 1.3±0.6 billion light years. The peak radiated gravitational wave power was more than the combined light power radiated by all the stars in the observable universe.[7][4]

Across the 0.2 second duration of the detectable signal, the relative velocity of the black holes increased from 30% to 60% of the speed of light. The frequency of the signal, at 150Hz, means that the object were orbiting each other at a distance of a few hundred kilometers before they merged. This close orbital radius implies that the objects had to be black holes, as no other known object could get this close to each other before merging. A black hole-neutron star pair would have merged at a lower frequency, while a pair of neutron stars would not have sufficient mass to account for the merger.[1][7]

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] The post-merger object is thought to be a rotating black hole.[7]

LIGO detectors

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 separated by 3,002 kilometers (1,865 mi). 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] It was during this period that the event was detected since formal science did not begin until 18 September 2015.

Announcement

The announcement of the detection was made on 11 February 2016.[3] There were many rumors about a possible detection prior to announcement, starting soon after the event by a tweet by Lawrence Krauss on 25 September 2015.[11]

Implications for future detections

It is expected that this will merely be the first of several detections during the first year of operation by the Advanced LIGO detectors. Efforts are underway to significantly enhance the global gravitational-wave detector network. These include further commissioning of the Advanced LIGO detectors to reach design sensitivity, which will allow detection of binaries like GW150914 with three times higher signal to noise ratio, covering events three times the distance into space, and increasing the likelihood of event detections by a factor of 33 = 27. 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]

References

  1. ^ a b c d e f g h Abbott, B.P.; et al. (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Phys. Rev. Lett. 116: 061102. doi:10.1103/PhysRevLett.116.061102. {{cite journal}}: Explicit use of et al. in: |author= (help)
  2. ^ Overbye, Dennis (11 February 2016). "Physicists Detect Gravitational Waves, Proving Einstein Right". New York Times. Retrieved 11 February 2016.
  3. ^ a b Clark, Stuart (11 February 2016). "Gravitational waves: scientists announce 'we did it!' – live". the Guardian. Retrieved 11 February 2016.
  4. ^ a b Castelvecchi, Davide; Witze, Witze (11 February 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. Retrieved 11 February 2016.
  5. ^ Naeye, Robert (11 February 2016). "Gravitational Wave Detection Heralds New Era of Science". Sky and Telescope. Retrieved 11 February 2016.
  6. ^ "Gravitational waves from black holes detected". BBC News. 11 February 2016.
  7. ^ a b c d "Observation Of Gravitational Waves From A Binary Black Hole Merger" (PDF). LIGO. 11 February 2016. Retrieved 11 February 2016.
  8. ^ "Properties of the binary black hole merger GW150914" (PDF). 11 February 2016. {{cite journal}}: Cite journal requires |journal= (help)
  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.
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