|Binary systems made up of two massive objects orbiting each other are an important source for gravitational-wave astronomy. The system emits gravitational radiation as it orbits, these carry away energy and momentum, causing the orbit to shrink. Shown here is a binary white dwarf system, an important source for space-borne detectors like eLISA. The eventual merger of the white dwarfs may result in a supernova, represented by the explosion in the third panel.|
Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves (minute distortions of spacetime predicted by Einstein's theory of general relativity) to collect observational data about objects such as neutron stars and black holes, events such as supernovae, and processes including those of the early universe shortly after the Big Bang.
Gravitational waves have a solid theoretical basis, founded upon the theory of relativity. They were first predicted by Einstein in 1916; although a specific consequence of general relativity, they are a common feature of all theories of gravity that obey special relativity. Indirect observational evidence for their existence first came in 1974 from measurements of the Hulse–Taylor binary pulsar, whose orbit evolves exactly as would be expected for gravitational wave emission. Richard Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for this discovery. Subsequently, many other binary pulsars (including one double pulsar system) have been observed, all fitting gravitational-wave predictions.
On 11 February 2016 it was announced that LIGO had directly observed the first gravitational waves in September 2015. The second observation of gravitational waves was made on 26 December 2015 and announced on 15 June 2016.
Ordinary gravitational waves frequencies are very low and much harder to detect, while higher frequencies occur in more dramatic events and thus have become the first to be observed.
In 2015, the LIGO project was the first to directly observe gravitational waves using laser interferometers. The LIGO detectors observed gravitational waves from the merger of two stellar-mass black holes, matching predictions of general relativity. These observations demonstrated the existence of binary stellar-mass black hole systems, and were the first direct detection of gravitational waves and the first observation of a binary black hole merger. This finding has been characterized as revolutionary to science, because the verification of our ability to use gravitational-wave astronomy to progress in our search and exploration of dark matter and the big bang.
There are several current scientific collaborations for observing gravitational waves. There is a world-wide network of ground-based detectors, these are kilometre-scale laser interferometers including: the Laser Interferometer Gravitational-Wave Observatory (LIGO), a joint project between MIT, Caltech and the scientists of the LIGO Scientific Collaboration with detectors in Livingston, Louisiana and Hanford, Washington; Virgo, at the European Gravitational Observatory, Cascina, Italy; GEO600 in Sarstedt, Germany, and the Kamioka Gravitational Wave Detector (KAGRA), operated by the University of Tokyo in the Kamioka Observatory, Japan. LIGO and Virgo are currently being upgraded to their advanced configurations. Advanced LIGO began observations in 2015, detecting gravitational waves even though not having reached its design sensitivity yet; Advanced Virgo is expected to start observing in 2016. The more advanced KAGRA is scheduled for 2018. GEO600 is currently operational, but its sensitivity makes it unlikely to make an observation; its primary purpose is to trial technology.
An alternative means of observation is using pulsar timing arrays (PTAs). There are three consortia, the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA), which co-operate as the International Pulsar Timing Array. These use existing radio telescopes, but since they are sensitive to frequencies in the nanohertz range, many years of observation are needed to detect a signal and detector sensitivity improves gradually. Current bounds are approaching those expected for astrophysical sources.
Further in the future, there is the possibility of space-borne detectors. The European Space Agency has selected a gravitational-wave mission for its L3 mission, due to launch 2034, the current concept is the evolved Laser Interferometer Space Antenna (eLISA). Also in development is the Japanese Deci-hertz Interferometer Gravitational wave Observatory (DECIGO).
Astronomy has traditionally relied on electromagnetic radiation. Astronomy originated with visible-light astronomy and what could be seen with the naked eye. As technology advanced, it became possible to observe other parts of the electromagnetic spectrum, from radio to gamma rays. Each new frequency band gave a new perspective on the Universe and heralded new discoveries. Late in the 20th century, the detection of solar neutrinos founded the field of neutrino astronomy, giving an insight into previously invisible phenomena, such as the inner workings of the Sun. The observation of gravitational waves provides a further means of making astrophysical observations.
Gravitational waves provide complementary information to that provided by other means. By combining observations of a single event made using different means, it is possible to gain a more complete understanding of the source's properties. This is known as multi-messenger astronomy. Gravitational waves can also be used to observe systems that are invisible (or almost impossible to detect) to measure by any other means, for example, they provide a unique method of measuring the properties of black holes.
Gravitational waves can be emitted by many systems, but, to produce detectable signals, the source must consist of extremely massive objects moving at a significant fraction of the speed of light. The main source is a binary of two compact objects. Example systems include:
- Compact binaries made up of two closely orbiting stellar-mass objects, such as white dwarfs, neutron stars or black holes. Wider binaries, which have lower orbital frequencies, are a source for detectors like LISA. Closer binaries produce a signal for ground-based detectors like LIGO. Ground-based detectors could potentially detect binaries containing an intermediate mass black hole of several hundred solar masses.
- Supermassive black hole binaries, consisting of two black holes with masses of 105–109 solar masses. Supermassive black holes are found at the centre of galaxies. When galaxies merge, it is expected that their central supermassive black holes merge too. These are potentially the loudest gravitational-wave signals. The most massive binaries are a source for PTAs. Less massive binaries (about a million solar masses) are a source for space-borne detectors like LISA.
- Extreme-mass-ratio systems of a stellar-mass compact object orbiting a supermassive black hole. These are sources for detectors like LISA. Systems with highly eccentric orbits produce a burst of gravitational radiation as they pass through the point of closest approach; systems with near-circular orbits, which are expected towards the end of the inspiral, emit continuously within LISA's frequency band. Extreme-mass-ratio inspirals can be observed over many orbits. This makes them excellent probes of the background spacetime geometry, allowing for precision tests of general relativity.
In addition to binaries, there are other potential sources:
- Supernovae generate high-frequency bursts of gravitational waves that could be detected with LIGO or Virgo.
- Rotating neutron stars are a source of continuous high-frequency waves if they possess axial asymmetry.
- Early universe processes, such as inflation or a phase transition.
- Cosmic strings could also emit gravitational radiation if they do exist. Discovery of these gravitational waves would confirm the existence of cosmic strings.
Gravitational waves interact only weakly with matter. This is what makes them difficult to detect. It also means that they can travel freely through the Universe, and are not absorbed or scattered like electromagnetic radiation. It is therefore possible to see to the center of dense systems, like the cores of supernovae or the Galactic Centre. It is also possible to see further back in time than with electromagnetic radiation, as the early universe was opaque to light prior to recombination, but transparent to gravitational waves.
The ability of gravitational waves to move freely through matter also means that gravitational-wave detectors, unlike telescopes, are not pointed to observe a single field of view but observe the entire sky. Detectors are more sensitive in some directions than others, which is one reason why it is beneficial to have a network of detectors.
In cosmic inflation
Cosmic inflation, a hypothesized period when the universe rapidly expanded 10−36 s after the Big Bang, would have given rise to gravitational waves; they would have left a characteristic imprint in the polarization of the CMB radiation. It is possible to calculate the properties of the primordial gravitational waves from measurements of the patterns in the microwave radiation, and use this to learn about the early universe. Again, the gravitational waves are not directly detected, but their presence must be inferred from other astronomical techniques.
As a young area of research, gravitational-wave astronomy is still in development; however, there is consensus within the astrophysics community that this field will evolve to become an established component of 21st century multi-messenger astronomy.
Gravitational-wave observations complement observations in the electromagnetic spectrum. These waves also promise to yield information in ways not possible via detection and analysis of electromagnetic waves. Electromagnetic waves can be absorbed and re-radiated in ways that make extracting information about the source difficult. Gravitational waves, however, only interact weakly with matter, meaning that they are not scattered or absorbed. This should allow astronomers to view the center of a supernova, stellar nebulae, and even colliding galactic cores in new ways.
Ground-based detectors yield new information about the inspiral phase and mergers of binary stellar mass black holes, and binaries consisting of one such black hole and a neutron star (a candidate mechanism for some gamma ray bursts). They could also detect signals from core-collapse supernovae, and from periodic sources such as pulsars with small deformations. If there is truth to speculation about certain kinds of phase transitions or kink bursts from long cosmic strings in the very early universe (at cosmic times around 10−25 seconds), these could also be detectable. Space-based detectors like LISA should detect objects such as binaries consisting of two white dwarfs, and AM CVn stars (a white dwarf accreting matter from its binary partner, a low-mass helium star), and also observe the mergers of supermassive black holes and the inspiral of smaller objects (between one and a thousand solar masses) into such black holes. LISA should also be able to listen to the same kind of sources from the early universe as ground-based detectors, but at even lower frequencies and with greatly increased sensitivity.
Detecting emitted gravitational waves is a difficult endeavor. It involves ultra stable high quality lasers and detectors calibrated with a sensitivity of at least 2·10−22 Hz−1/2 as shown at the ground-based detector, GEO600. It has also been proposed that even from large astronomical events, such as supernova explosions, these waves are likely to degrade to vibrations as small as an atomic diameter.
- Peters, P.; Mathews, J. (1963). "Gravitational Radiation from Point Masses in a Keplerian Orbit". Physical Review. 131 (1): 435–440. Bibcode:1963PhRv..131..435P. doi:10.1103/PhysRev.131.435.
- Peters, P. (1964). "Gravitational Radiation and the Motion of Two Point Masses". Physical Review. 136 (4B): B1224–B1232. Bibcode:1964PhRv..136.1224P. doi:10.1103/PhysRev.136.B1224.
- Schutz, Bernard F. (1984). "Gravitational waves on the back of an envelope". American Journal of Physics. 52 (5): 412. Bibcode:1984AmJPh..52..412S. doi:10.1119/1.13627.
- Hulse, R. A.; Taylor, J. H. (1975). "Discovery of a pulsar in a binary system". The Astrophysical Journal. 195: L51. Bibcode:1975ApJ...195L..51H. doi:10.1086/181708.
- "The Nobel Prize in Physics 1993". Nobel Foundation. Retrieved 2014-05-03.
- Stairs, Ingrid H. (2003). "Testing General Relativity with Pulsar Timing". Living Reviews in Relativity. 6: 5. arXiv: . Bibcode:2003LRR.....6....5S. doi:10.12942/lrr-2003-5.
- LIGO Scientific Collaboration and Virgo Collaboration; Abbott, B. P.; Abbott, R.; Abbott, T. D.; Abernathy, M. R.; Acernese, F.; Ackley, K.; Adams, C.; Adams, T. (2016-06-15). "GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence". Physical Review Letters. 116 (24): 241103. doi:10.1103/PhysRevLett.116.241103. PMID 27367379.
- Moore, Christopher; Cole, Robert; Berry, Christopher (19 July 2013). "Gravitational Wave Detectors and Sources". Retrieved 17 April 2014.
- Overbye, Dennis (11 February 2016). "Physicists Detect Gravitational Waves, Proving Einstein Right". New York Times. Retrieved 11 February 2016.
- Krauss, Lawrence (11 February 2016). "Finding Beauty in the Darkness". New York Times. Retrieved 11 February 2016.
- Pretorius, Frans (2005). "Evolution of Binary Black-Hole Spacetimes". Physical Review Letters. 95 (12): 121101. arXiv: . Bibcode:2005PhRvL..95l1101P. doi:10.1103/PhysRevLett.95.121101. ISSN 0031-9007. PMID 16197061.
- Campanelli, M.; Lousto, C. O.; Marronetti, P.; Zlochower, Y. (2006). "Accurate Evolutions of Orbiting Black-Hole Binaries without Excision". Physical Review Letters. 96 (11): 111101. arXiv: . Bibcode:2006PhRvL..96k1101C. doi:10.1103/PhysRevLett.96.111101. ISSN 0031-9007. PMID 16605808.
- Baker, John G.; Centrella, Joan; Choi, Dae-Il; Koppitz, Michael; van Meter, James (2006). "Gravitational-Wave Extraction from an Inspiraling Configuration of Merging Black Holes". Physical Review Letters. 96 (11): 111102. arXiv: . Bibcode:2006PhRvL..96k1102B. doi:10.1103/PhysRevLett.96.111102. ISSN 0031-9007. PMID 16605809.
- Abbott, B. P.; Abbott, R.; Abbott, T. D.; Abernathy, M. R.; Acernese, F.; Ackley, K.; Adams, C.; Adams, T.; Addesso, P. (2016-02-11). "Observation of Gravitational Waves from a Binary Black Hole Merger". Physical Review Letters. 116 (6): 061102. arXiv: . Bibcode:2016PhRvL.116f1102A. doi:10.1103/PhysRevLett.116.061102. ISSN 0031-9007. PMID 26918975.
- Sesana, A. (22 May 2013). "Systematic investigation of the expected gravitational wave signal from supermassive black hole binaries in the pulsar timing band". Monthly Notices of the Royal Astronomical Society: Letters. 433 (1): L1–L5. arXiv: . Bibcode:2013MNRAS.433L...1S. doi:10.1093/mnrasl/slt034.
- "ESA's new vision to study the invisible universe". ESA. Retrieved 29 November 2013.
- Longair, Malcolm (2012). Cosmic century: a history of astrophysics and cosmology. Cambridge University Press. ISBN 1107669367.
- Bahcall, John N. (1989). Neutrino Astrophysics (Reprinted. ed.). Cambridge: Cambridge University Press. ISBN 052137975X.
- Bahcall, John (9 June 2000). "How the Sun Shines". Nobel Prize. Retrieved 10 May 2014.
- Nelemans, Gijs (7 May 2009). "The Galactic gravitational wave foreground". Classical and Quantum Gravity. 26 (9): 094030. arXiv: . Bibcode:2009CQGra..26i4030N. doi:10.1088/0264-9381/26/9/094030.
- Stroeer, A; Vecchio, A (7 October 2006). "The LISA verification binaries". Classical and Quantum Gravity. 23 (19): S809–S817. arXiv: . Bibcode:2006CQGra..23S.809S. doi:10.1088/0264-9381/23/19/S19.
- Abadie, J; Abbott, R.; Abernathy, M.; Accadia, T.; Acernese, F.; Adams, C.; Adhikari, R.; Ajith, P.; Allen, B.; Allen, G.; Amador Ceron, E.; Amin, R. S.; Anderson, S. B.; Anderson, W. G.; Antonucci, F.; Aoudia, S.; Arain, M. A.; Araya, M.; Aronsson, M.; Arun, K. G.; Aso, Y.; Aston, S.; Astone, P.; Atkinson, D. E.; Aufmuth, P.; Aulbert, C.; Babak, S.; Baker, P.; et al. (7 September 2010). "Predictions for the rates of compact binary coalescences observable by ground-based gravitational-wave detectors". Classical and Quantum Gravity. 27 (17): 173001. arXiv: . Bibcode:2010CQGra..27q3001A. doi:10.1088/0264-9381/27/17/173001.
- "Measuring Intermediate-Mass Black-Hole Binaries with Advanced Gravitational Wave Detectors". Gravitational Physics Group. University of Birmingham. Retrieved 28 November 2015.
- "Observing the invisible collisions of intermediate mass black holes". LIGO Scientific Collaboration. Retrieved 28 November 2015.
- Volonteri, Marta; Haardt, Francesco; Madau, Piero (10 January 2003). "The Assembly and Merging History of Supermassive Black Holes in Hierarchical Models of Galaxy Formation". The Astrophysical Journal. 582 (2): 559–573. arXiv: . Bibcode:2003ApJ...582..559V. doi:10.1086/344675.
- Sesana, A.; Vecchio, A.; Colacino, C. N. (11 October 2008). "The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays". Monthly Notices of the Royal Astronomical Society. 390 (1): 192–209. arXiv: . Bibcode:2008MNRAS.390..192S. doi:10.1111/j.1365-2966.2008.13682.x.
- Amaro-Seoane, Pau; Aoudia, Sofiane; Babak, Stanislav; Binétruy, Pierre; Berti, Emanuele; Bohé, Alejandro; Caprini, Chiara; Colpi, Monica; Cornish, Neil J; Danzmann, Karsten; Dufaux, Jean-François; Gair, Jonathan; Jennrich, Oliver; Jetzer, Philippe; Klein, Antoine; Lang, Ryan N; Lobo, Alberto; Littenberg, Tyson; McWilliams, Sean T; Nelemans, Gijs; Petiteau, Antoine; Porter, Edward K; Schutz, Bernard F; Sesana, Alberto; Stebbins, Robin; Sumner, Tim; Vallisneri, Michele; Vitale, Stefano; Volonteri, Marta; Ward, Henry; Babak, Stanislav; Binétruy, Pierre; Berti, Emanuele; Bohé, Alejandro; Caprini, Chiara; Colpi, Monica; Cornish, Neil J.; Danzmann, Karsten; Dufaux, Jean-François; Gair, Jonathan; Jennrich, Oliver; Jetzer, Philippe; Klein, Antoine; Lang, Ryan N.; Lobo, Alberto; Littenberg, Tyson; McWilliams, Sean T.; Nelemans, Gijs; Petiteau, Antoine; Porter, Edward K.; Schutz, Bernard F.; Sesana, Alberto; Stebbins, Robin; Sumner, Tim; Vallisneri, Michele; Vitale, Stefano; Volonteri, Marta; Ward, Henry (21 June 2012). "Low-frequency gravitational-wave science with eLISA/NGO". Classical and Quantum Gravity. 29 (12): 124016. arXiv: . Bibcode:2012CQGra..29l4016A. doi:10.1088/0264-9381/29/12/124016.
- Amaro-Seoane, P. (May 2012). "Stellar dynamics and extreme-mass ratio inspirals". arXiv: . Bibcode:2012arXiv1205.5240A.
- Berry, C. P. L.; Gair, J. R. (12 December 2012). "Observing the Galaxy's massive black hole with gravitational wave bursts". Monthly Notices of the Royal Astronomical Society. 429 (1): 589–612. arXiv: . Bibcode:2013MNRAS.429..589B. doi:10.1093/mnras/sts360.
- Amaro-Seoane, Pau; Gair, Jonathan R; Freitag, Marc; Miller, M Coleman; Mandel, Ilya; Cutler, Curt J; Babak, Stanislav (7 September 2007). "Intermediate and extreme mass-ratio inspirals—astrophysics, science applications and detection using LISA". Classical and Quantum Gravity. 24 (17): R113–R169. arXiv: . Bibcode:2007CQGra..24R.113A. doi:10.1088/0264-9381/24/17/R01.
- Gair, Jonathan; Vallisneri, Michele; Larson, Shane L.; Baker, John G. (2013). "Testing General Relativity with Low-Frequency, Space-Based Gravitational-Wave Detectors". Living Reviews in Relativity. 16: 7. arXiv: . Bibcode:2013LRR....16....7G. doi:10.12942/lrr-2013-7.
- Kotake, Kei; Sato, Katsuhiko; Takahashi, Keitaro (1 April 2006). "Explosion mechanism, neutrino burst and gravitational wave in core-collapse supernovae". Reports on Progress in Physics. 69 (4): 971–1143. arXiv: . Bibcode:2006RPPh...69..971K. doi:10.1088/0034-4885/69/4/R03.
- Abbott, B.; Adhikari, R.; Agresti, J.; Ajith, P.; Allen, B.; Amin, R.; Anderson, S.; Anderson, W.; Arain, M.; Araya, M.; Armandula, H.; Ashley, M.; Aston, S; Aufmuth, P.; Aulbert, C.; Babak, S.; Ballmer, S.; Bantilan, H.; Barish, B.; Barker, C.; Barker, D.; Barr, B.; Barriga, P.; Barton, M.; Bayer, K.; Belczynski, K.; Berukoff, S.; Betzwieser, J.; et al. (2007). "Searches for periodic gravitational waves from unknown isolated sources and Scorpius X-1: Results from the second LIGO science run". Physical Review D. 76 (8): 082001. arXiv: . Bibcode:2007PhRvD..76h2001A. doi:10.1103/PhysRevD.76.082001.
- "Searching for the youngest neutron stars in the galaxy". LIGO Scientific Collaboration. Retrieved 28 November 2015.
- Binétruy, Pierre; Bohé, Alejandro; Caprini, Chiara; Dufaux, Jean-François (13 June 2012). "Cosmological backgrounds of gravitational waves and eLISA/NGO: phase transitions, cosmic strings and other sources". Journal of Cosmology and Astroparticle Physics. 2012 (6): 027–027. arXiv: . Bibcode:2012JCAP...06..027B. doi:10.1088/1475-7516/2012/06/027.
- Damour, Thibault; Vilenkin, Alexander (2005). "Gravitational radiation from cosmic (super)strings: Bursts, stochastic background, and observational windows". Physical Review D. 71 (6): 063510. arXiv: . Bibcode:2005PhRvD..71f3510D. doi:10.1103/PhysRevD.71.063510.
- Schutz, Bernard F (21 June 2011). "Networks of gravitational wave detectors and three figures of merit". Classical and Quantum Gravity. 28 (12): 125023. arXiv: . Bibcode:2011CQGra..28l5023S. doi:10.1088/0264-9381/28/12/125023.
- Hu, Wayne; White, Martin (1997). "A CMB polarization primer". New Astronomy. 2 (4): 323–344. arXiv: . Bibcode:1997NewA....2..323H. doi:10.1016/S1384-1076(97)00022-5.
- Kamionkowski, Marc; Stebbins, Albert; Stebbins, Albert (1997). "Statistics of cosmic microwave background polarization". Physical Review D. 55 (12): 7368–7388. arXiv: . Bibcode:1997PhRvD..55.7368K. doi:10.1103/PhysRevD.55.7368.
- Price, Larry (September 2015). "Looking for the Afterglow: The LIGO Perspective" (PDF). LIGO Magazine (7): 10. Retrieved 28 November 2015.
- "PLANNING FOR A BRIGHT TOMORROW: PROSPECTS FOR GRAVITATIONAL-WAVE ASTRONOMY WITH ADVANCED LIGO AND ADVANCED VIRGO". LIGO Scientific Collaboration. Retrieved 31 December 2015.
- See Cutler & Thorne 2002, sec. 2.
- See Cutler & Thorne 2002, sec. 3.
- See Seifert F., et al. 2006, sec. 5.
- See Golm & Potsdam 2013, sec. 4.
- Cutler, Curt; Thorne, Kip S. (2002), "An overview of gravitational-wave sources", in Bishop, Nigel; Maharaj, Sunil D., Proceedings of 16th International Conference on General Relativity and Gravitation (GR16), World Scientific, p. 4090, arXiv: , Bibcode:2002gr.qc.....4090C, ISBN 981-238-171-6
- Thorne, Kip S. (1995), "Gravitational radiation", Particle and Nuclear Astrophysics and Cosmology in the Next Millenium: 160, arXiv: , Bibcode:1995pnac.conf..160T
- Gravitational Wave Astronomy, Max Planck Institute for Gravitational Physics, retrieved 24 January 2013
- Schutz, B. F. (1999), "Gravitational wave astronomy", Classical and Quantum Gravity, 16 (12A): A131–A156, arXiv: , Bibcode:1999CQGra..16A.131S, doi:10.1088/0264-9381/16/12A/307
- LIGO Magazine, LIGO Scientific Collaboration
- LIGO Scientific Collaboration
- AstroGravS: Astrophysical Gravitational-Wave Sources Archive
- 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).