Gravitational-wave astronomy
Gravitational-wave astronomy is a subfield of astronomy concerned with the detection and study of gravitational waves emitted by astrophysical sources.
Gravitational waves are minute distortions or ripples in spacetime caused by the acceleration of massive objects. They are produced by cataclysmic events such as the merger of binary black holes, the coalescence of binary neutron stars, supernova explosions and processes including those of the early universe shortly after the Big Bang. Studying them offers a new way to observe the universe, providing valuable insights into the behavior of matter under extreme conditions. Similar to electromagnetic radiation which involves transport of energy via propagation of electromagnetic field fluctuations, gravitational radiation involves fluctuations of the relatively weaker gravitational field. The existence of gravitational waves was first suggested by Oliver Heaviside in 1893 and then later conjectured by Henri Poincaré in 1905 as the gravitational equivalent of electromagnetic waves before they were predicted by Albert Einstein in 1916 as a corollary to his theory of general relativity.
File:LIGO measurement of gravitational waves.png|thumb|upright=1.5|Data about the first observation of gravitational waves from the two LIGO detectors and Virgo interferometer
In 1978, Russell Alan Hulse and Joseph Hooton Taylor Jr. provided the first experimental evidence for the existence of gravitational waves by observing two neutron stars orbiting each other and won the 1993 Nobel Prize in physics for their work. In 2015, nearly a century after Einstein's forecast, the first direct observation of gravitational waves as a signal from the merger of two black holes confirmed the existence of these elusive phenomena and opened a new era in astronomy. Subsequent detections have included binary black hole mergers, neutron star collisions, and other violent cosmic events. Gravitational waves are now detected using laser interferometry, which measures tiny changes in the length of two perpendicular arms caused by passing waves. Observatories like LIGO, Virgo and KAGRA use this technology to capture the faint signals from distant cosmic events. LIGO co-founders Barry C. Barish, Kip S. Thorne, and Rainer Weiss were awarded the 2017 Nobel Prize in Physics for their ground-breaking contributions in gravitational wave astronomy.
Potential and challenges
When distant astronomical objects are observed using electromagnetic waves, different phenomena like scattering, absorption, reflection, refraction, etc. cause information loss. There are various regions in space only partially penetrable by photons, such as the insides of nebulae, the dense dust clouds at the galactic core, the regions near black holes, etc. Gravitational astronomy has the potential to be used in parallel with electromagnetic astronomy to study the universe at a better resolution. In an approach known as multi-messenger astronomy, gravitational wave data is combined with data from other wavelengths to get a more complete picture of astrophysical phenomena. Gravitational wave astronomy helps understand the early universe, test theories of gravity, and reveal the distribution of dark matter and dark energy. In particular, it can help find the Hubble constant, which describes the rate of accelerated expansion of the universe. All of these open doors to a physics beyond the Standard Model.Challenges that remain in the field include noise interference, the lack of ultra-sensitive instruments, and the detection of low-frequency waves. Ground-based detectors face problems with seismic vibrations produced by environmental disturbances and the limitation of the arm length of detectors due to the curvature of the Earth's surface. In the future, the field of gravitational wave astronomy will try develop upgraded detectors and next-generation observatories, along with possible space-based detectors such as LISA. LISA will be able to listen to distant sources like compact supermassive black holes in the galactic core and primordial black holes, as well as low-frequency sensitive signals sources such as binary white dwarf merger and sources from the early universe.
Gravitational waves
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1893 and then later by Henri Poincaré in 1905 as waves similar to electromagnetic waves but the gravitational equivalent.Gravitational waves were later predicted in 1916 by Albert Einstein on the basis of his general theory of relativity as ripples in spacetime. Later he refused to accept gravitational waves. Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation. Newton's law of universal gravitation, part of classical mechanics, does not provide for their existence, since that law is predicated on the assumption that physical interactions propagate instantaneously – showing one of the ways the methods of Newtonian physics are unable to explain phenomena associated with relativity.
The first indirect evidence for the existence of gravitational waves came in 1974 from the observed orbital decay of the Hulse–Taylor binary pulsar, which matched the decay predicted by general relativity as energy is lost to gravitational radiation. In 1993, Russell A. Hulse and Joseph Hooton Taylor Jr. received the Nobel Prize in Physics for this discovery.
Direct observation of gravitational waves was not made until 2015, when a signal generated by the merger of two black holes was received by the LIGO gravitational wave detectors in Livingston, Louisiana, and in Hanford, Washington. The 2017 Nobel Prize in Physics was subsequently awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the direct detection of gravitational waves.
In gravitational-wave astronomy, observations of gravitational waves are used to infer data about the sources of gravitational waves. Sources that can be studied this way include binary star systems composed of white dwarfs, neutron stars, and black holes; events such as supernovae; and the formation of the early universe shortly after the Big Bang.
Instruments for different frequencies
Collaboration between detectors aids in collecting unique and valuable information, owing to different specifications and sensitivity of each.Image:Gravitational-wave detector sensitivities and astrophysical gravitational-wave sources.png|thumb|upright=2| Noise curves for a selection of gravitational-wave detectors as a function of frequency. At very low frequencies are pulsar timing arrays, at low frequencies are space-borne detectors, and at high frequencies are ground-based detectors. The characteristic strain of potential astrophysical sources are also shown. To be detectable the characteristic strain of a signal must be above the noise curve.
High frequency
There are several ground-based laser interferometers which span several miles/kilometers, including: the two Laser Interferometer Gravitational-Wave Observatory detectors in Washington and Louisiana, USA; Virgo, at the European Gravitational Observatory in Italy; GEO600 in Germany, and the Kamioka Gravitational Wave Detector in Japan. While LIGO, Virgo, and KAGRA have made joint observations to date, GEO600 is currently utilized for trial and test runs due to lower sensitivity of its instruments and has not participated in joint runs with the others recently.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 of the verification of our ability to use gravitational-wave astronomy to progress in our search and exploration of dark matter and the big bang.
Detection of an event by three or more detectors allows the celestial location to be estimated from the relative delays.
Low frequency
An alternative means of observation is using pulsar timing arrays. There are three consortia, the European Pulsar Timing Array, the North American Nanohertz Observatory for Gravitational Waves, and the Parkes Pulsar Timing Array, 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.In June 2023, four PTA collaborations, the three mentioned above and the Chinese Pulsar Timing Array, delivered independent but similar evidence for a stochastic background of nanohertz gravitational waves. Each provided an independent first measurement of the theoretical Hellings-Downs curve, i.e., the quadrupolar correlation between two pulsars as a function of their angular separation in the sky, which is a telltale sign of the gravitational wave origin of the observed background. The sources of this background remain to be identified, although binaries of supermassive black holes are the most likely candidates.