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. These observations show the effect of gravitational-wave emission, but do not allow us to measure the gravitational-waves themselves.
In 2014, the BICEP2 collaboration announced they have found evidence for gravitational waves in the cosmic microwave background (CMB). These gravitational waves could be a relic of cosmic inflation, a period when the universe rapidly expanded 10−36 s after the Big Bang; they leave 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.
There are currently several scientific collaborations hoping to make the first direct detection of 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 and Caltech which has detectors in in Livingston, Louisiana and Hanford, Washington; Virgo, at the European Gravitational Observatory, Cascina, Italy; GEO 600 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 and are due to begin operation in 2015, the more advanced KAGRA is scheduled for 2018. GEO 600 is currently operational, but its sensitivity makes it unlikely to make an observation; its primary purpose is to trial technology. It is hoped that ground-based detectors shall make their first detection by the end of the decade.
An alternative means of detection 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 using new parts of the electromagnetic spectrum, from gamma rays to radio. 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 detection of gravitational waves will provide 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. Examples 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.
- 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 lack holes merge too. THhese 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 holes. 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. The imprint in the CMB of gravitational waves generated by inflation has been detected by BICEP2.
- Cosmic strings could also emit gravitational radiation if they do exist. Discovery of these gravitational waves would confirm the existence of cosmic strings.
Since no gravitational waves have yet been detected, it is possible there are further sources yet to be considered.
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 centre 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.
So far, gravitational waves have only been detected indirectly, and gravitational-wave astronomy remains more of a possibility than an actuality. However, a number of gravitational-wave detectors are in operation with the aim of making gravitational-wave astronomy a reality. This young area of research is still in the developmental stages, however there is consensus within the astrophysics community that this field will evolve to become an established component of 21st century multi-messenger astronomy, and that gravitational-wave astronomers, working with ground and spaced-based detectors, will stand shoulder-to-shoulder with gamma-ray, x-ray, optical, infrared and radio astronomers in exploring the cosmos in the years to come.
Detecting gravitational waves promises to 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 behave, in some analogous to sound waves, in that they behave like composites of harmonic waves. These harmonic composite waveforms carry the signature of their origin only and radiate in a way that doesn't allow them to become distorted due to interactions with matter between the source and the detectors. This should allow astronomers to view the center of a supernova, stellar nebula's, and even colliding galactic cores in new ways.
Terrestrial detectors are expected to 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 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 are rather difficult endeavor. It involves ultra stable high quality lasers and detectors calibrated with a sensitivity of at least 2·10^(-22)/√Hz as shown at the ground-based detector, GEO-600. 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. In theory, the high sensitivity of terrestrial and space-based detection systems should be able to observe these elusive waves.
- 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:astro-ph/0307536. Bibcode:2003LRR.....6....5S. doi:10.12942/lrr-2003-5.
- The BICEP2 Collaboration (2014). "BICEP2 2014 I: Detection of B-mode Polarization at Degree Angular Scales".
- Hu, Wayne; White, Martin (1997). "A CMB polarization primer". New Astronomy 2 (4): 323–344. arXiv:astro-ph/9706147. 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:astro-ph/9611125. Bibcode:1997PhRvD..55.7368K. doi:10.1103/PhysRevD.55.7368.
- 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:1211.5375. 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 Pres. 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.
- Moore, Christopher; Cole, Robert; Berry, Christopher (19 July 2013). "Gravitational Wave Detectors and Sources". Retrieved 17 April 2014.
- Nelemans, Gijs (7 May 2009). "The Galactic gravitational wave foreground". Classical and Quantum Gravity 26 (9): 094030. arXiv:0901.1778. 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:astro-ph/0605227. 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:1003.2480. Bibcode:2010CQGra..27q3001A. doi:10.1088/0264-9381/27/17/173001.
- 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:astro-ph/0207276. 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:0804.4476. 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:1202.0839. Bibcode:2012CQGra..29l4016A. doi:10.1088/0264-9381/29/12/124016.
- 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:1210.2778. 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:astro-ph/0703495. 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:1212.5575. 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:astro-ph/0509456. 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:gr-qc/0605028. Bibcode:2007PhRvD..76h2001A. doi:10.1103/PhysRevD.76.082001.
- 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:1201.0983. 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:hep-th/0410222. 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:1102.5421. Bibcode:2011CQGra..28l5023S. doi:10.1088/0264-9381/28/12/125023.
- Cf. Thorne 1995.
- See Golm & Potsdam 2013, sec. 4
- 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:gr-qc/0204090, 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:gr-qc/9506086, Bibcode:1995pnac.conf..160T
- Golm; Potsdam, Gravitational Wave Astronomy, Max Planck Institute for Gravitational Physics, retrieved 24 January 2013
- Seifert F., Kwee P., Heurs M., Willke B, Danzmann K (2006), GEO600 Sensetivity
- AstroGravS: Astrophysical Gravitational-Wave Sources Archive
- Physics Group
- LIGO affiliate research center