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.
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.
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, 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; 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, Advanced LIGO began observations in 2015 and Advanced Virgo is expected to follow in 2016. 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 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 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. 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 or 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.
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 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.
As of 2015, gravitational waves have only been detected indirectly, and gravitational-wave astronomy lacks observational results. However, a number of gravitational-wave detectors are in operation with the aim of making gravitational-wave astronomy a reality. As a young area of research, it 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.
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, 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 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 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, 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. The high sensitivity of ground-based and space-based detection systems should be able to observe these elusive waves.
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