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.
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.
Collaboration between detectors aids in collecting unique and valuable information, owing to different specifications and sensitivity of each.
There are several ground-based
laser interferometers which span several miles/kilometers, including: the two
Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in WA and LA, USA;
Virgo, at the
European Gravitational Observatory in Italy;
GEO600 in Germany, and the
Kamioka Gravitational Wave Detector (KAGRA) 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.
High frequency
In 2015, the
LIGO project was the first to
directly observe gravitational waves using laser interferometers.[3][4] The LIGO detectors observed gravitational waves from the merger of two
stellar-mass black holes, matching predictions of
general relativity.[5][6][7] 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.[8] 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.
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.[12] 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.[13][14][15][16] The sources of this background remain to be identified, although binaries of
supermassive black holes are the most likely candidates.[17]
Astronomy has traditionally relied on
electromagnetic radiation. Originating with the visible band, 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.[19] During the 20th century, indirect and later direct
measurements of high-energy, massive particles provided an additional window into the cosmos. Late in the 20th century, the detection of
solar neutrinos founded the field of
neutrino astronomy, giving an insight into previously inaccessible phenomena, such as the inner workings of the
Sun.[20][21] The observation of
gravitational waves provides a further means of making astrophysical observations.
Russell Hulse and
Joseph Taylor were awarded the 1993
Nobel Prize in Physics for showing that the orbital decay of a pair of neutron stars, one of them a pulsar, fits general relativity's predictions of gravitational radiation.[22] Subsequently, many other binary pulsars (including one
double pulsar system) have been observed, all fitting gravitational-wave predictions.[23] In 2017, the Nobel Prize in Physics was awarded to
Rainer Weiss,
Kip Thorne and
Barry Barish for their role in the first detection of gravitational waves.[24][25][26]
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) 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.[27][28] Closer binaries produce a signal for ground-based detectors like
LIGO.[29] Ground-based detectors could potentially detect binaries containing an
intermediate mass black hole of several hundred solar masses.[30][31]
Supermassive black hole binaries, consisting of two black holes with masses of 105–109solar 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.[32] These are potentially the loudest gravitational-wave signals. The most massive binaries are a source for
PTAs.[33] Less massive binaries (about a million solar masses) are a source for space-borne detectors like
LISA.[34]
Extreme-mass-ratio systems of a stellar-mass compact object orbiting a supermassive black hole.[35] These are sources for detectors like
LISA.[34] Systems with highly
eccentric orbits produce a burst of gravitational radiation as they pass through the point of closest approach;[36] systems with near-circular orbits, which are expected towards the end of the inspiral, emit continuously within LISA's frequency band.[37] 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.[38]
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.[39]
Rotating neutron stars are a source of continuous high-frequency waves if they possess axial asymmetry.[40][41]
Cosmic strings could also emit gravitational radiation if they do exist.[43] 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 Center. 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.[44]
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.[45] Directionalization is also poor, due to the small number of detectors.
In cosmic inflation
Cosmic inflation, a hypothesized period when the universe rapidly expanded during the first 10−36 seconds after the
Big Bang, would have given rise to gravitational waves; that would have left a characteristic imprint in the
polarization of the CMB radiation.[46][47]
It is possible to calculate the properties of the primordial gravitational waves from measurements of the patterns in the
microwave radiation, and use those calculations to learn about the early universe. [how?]
Development
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.[48]
Gravitational-wave observations complement observations in the
electromagnetic spectrum.[49][48] 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 have yielded new information about the inspiral phase and mergers of binary systems of two
stellar mass black holes, and merger of two
neutron stars. 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.[50] 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.[51]
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.[52] 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.[53]
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^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.
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