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The gravitational wave background (also GWB and stochastic background) is a random background of gravitational waves permeating the Universe, which is detectable by gravitational-wave experiments, like pulsar timing arrays. [1] The signal may be intrinsically random, like from stochastic processes in the early Universe, or may be produced by an incoherent superposition of a large number of weak independent unresolved gravitational-wave sources, like supermassive black-hole binaries. Detecting the gravitational wave background can provide information that is inaccessible by any other means about astrophysical source population, like hypothetical ancient supermassive black-hole binaries, and early Universe processes, like hypothetical primordial inflation and cosmic strings. [2]
Several potential sources for the background are hypothesized across various frequency bands of interest, with each source producing a background with different statistical properties. The sources of the stochastic background can be broadly divided into two categories: cosmological sources, and astrophysical sources.
Cosmological backgrounds may arise from several early universe sources. Some examples of these primordial sources include time-varying inflationary scalar fields in the early universe, "preheating" mechanisms after inflation involving energy transfer from inflaton particles to regular matter, cosmological phase transitions in the early universe (such as the electroweak phase transition), cosmic strings, etc. While these sources are more hypothetical, a detection of a primordial gravitational wave background from them would be a major discovery of new physics and would have a profound impact on early-universe cosmology and on high-energy physics. [3] [4]
An astrophysical background is produced by the combined noise of many weak, independent, and unresolved astrophysical sources. [2] For instance the astrophysical background from stellar mass binary black-hole mergers is expected to be a key source of the stochastic background for the current generation of ground based gravitational-wave detectors. LIGO and Virgo detectors have already detected individual gravitational-wave events from such black-hole mergers. However, there would be a large population of such mergers which would not be individually resolvable which would produce a hum of random looking noise in the detectors. Other astrophysical sources which are not individually resolvable can also form a background. For instance, a sufficiently massive star at the final stage of its evolution will collapse to form either a black hole or a neutron star—in the rapid collapse during the final moments of an explosive supernova event, which can lead to such formations, gravitational waves may theoretically be liberated. [5] [6] Also, in rapidly rotating neutron stars there is a whole class of instabilities driven by the emission of gravitational waves.[ citation needed ]
The nature of source also depend on the sensitive frequency band of the signal. The current generation of ground based experiments like LIGO and Virgo are sensitive to gravitational-waves in the audio frequency band between approximately 10 Hz to 1000 Hz. In this band the most likely source of the stochastic background will be an astrophysical background from binary neutron-star and stellar mass binary black-hole mergers. [7]
An alternative means of observation is using pulsar timing arrays (PTAs). Three consortia—the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA)—coordinate as the International Pulsar Timing Array. They use radio telescopes to monitor the galactic array of millisecond pulsars, which form a galactic-scale detector sensitive to gravitational waves with low frequencies in the nanohertz to 100 nanohertz range. With existing telescopes, many years of observation are needed to detect a signal and detector sensitivity improves gradually. Sensitivity bounds are approaching those expected for astrophysical sources. [8]
Supermassive black holes with masses of 105–109 solar masses are found at the centers of galaxies. It is not known which came first, supermassive black holes or galaxies, or how they evolved. When galaxies merge, it is expected that their central supermassive black holes merge too. [9] These supermassive binaries produce potentially the loudest low-frequency gravitational-wave signals; the most massive of them are potential sources of a nanohertz gravitational wave background, which is in principle detectable by PTAs. [10]
On 11 February 2016, the LIGO and Virgo collaborations announced the first direct detection and observation of gravitational waves, which took place in September 2015. In this case, two black holes had collided to produce detectable gravitational waves. This is the first step to the potential detection of a GWB. [13] [14]
On 28 June 2023, the North American Nanohertz Observatory for Gravitational Waves collaboration announced evidence for a GWB using observational data from an array of millisecond pulsars. [15] [16] Observations from EPTA, [17] Parkes Observatory [18] and Chinese Pulsar Timing Array (CPTA) [19] [20] were also published on the same day, providing cross validation of the evidence for GWB using different telescopes and analysis methods. [21] These observations provided the 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. [22] The sources of this gravitational-wave background can not be identified without further observations and analyses, although binaries of supermassive black holes are leading candidates. [1]
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalises special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of second order partial differential equations.
The following is a timeline of gravitational physics and general relativity.
A supermassive black hole is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.
The Laser Interferometer Space Antenna (LISA) is a planned space probe to detect and accurately measure gravitational waves—tiny ripples in the fabric of spacetime—from astronomical sources. LISA will be the first dedicated space-based gravitational-wave observatory. It aims to measure gravitational waves directly by using laser interferometry. The LISA concept has a constellation of three spacecraft arranged in an equilateral triangle with sides 2.5 million kilometres long, flying along an Earth-like heliocentric orbit. The distance between the satellites is precisely monitored to detect a passing gravitational wave.
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray particles.
A pulsar is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Earth, and is responsible for the pulsed appearance of emission. Neutron stars are very dense and have short, regular rotational periods. This produces a very precise interval between pulses that ranges from milliseconds to seconds for an individual pulsar. Pulsars are one of the candidates for the source of ultra-high-energy cosmic rays.
A millisecond pulsar (MSP) is a pulsar with a rotational period less than about 10 milliseconds. Millisecond pulsars have been detected in radio, X-ray, and gamma ray portions of the electromagnetic spectrum. The leading theory for the origin of millisecond pulsars is that they are old, rapidly rotating neutron stars that have been spun up or "recycled" through accretion of matter from a companion star in a close binary system. For this reason, millisecond pulsars are sometimes called recycled pulsars.
Tests of general relativity serve to establish observational evidence for the theory of general relativity. The first three tests, proposed by Albert Einstein in 1915, concerned the "anomalous" precession of the perihelion of Mercury, the bending of light in gravitational fields, and the gravitational redshift. The precession of Mercury was already known; experiments showing light bending in accordance with the predictions of general relativity were performed in 1919, with increasingly precise measurements made in subsequent tests; and scientists claimed to have measured the gravitational redshift in 1925, although measurements sensitive enough to actually confirm the theory were not made until 1954. A more accurate program starting in 1959 tested general relativity in the weak gravitational field limit, severely limiting possible deviations from the theory.
The Virgo interferometer is a large Michelson interferometer designed to detect the gravitational waves predicted by general relativity. It is located in Santo Stefano a Macerata, near the city of Pisa, Italy. The instrument's two arms are three kilometres long, housing its mirrors and instrumentation inside an ultra-high vacuum.
Gravitational waves are waves of the intensity of gravity that are generated by the accelerated masses of binary stars and other motions of gravitating masses, and 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 the gravitational equivalent of electromagnetic waves. Gravitational waves are sometimes called gravity waves, but gravity waves typically refer to displacement waves in fluids. In 1916 Albert Einstein demonstrated that gravitational waves result from his general theory of relativity as ripples in spacetime.
A gravitational-wave detector is any device designed to measure tiny distortions of spacetime called gravitational waves. Since the 1960s, various kinds of gravitational-wave detectors have been built and constantly improved. The present-day generation of laser interferometers has reached the necessary sensitivity to detect gravitational waves from astronomical sources, thus forming the primary tool of gravitational-wave astronomy.
Gravitational-wave astronomy is an emerging field of science, concerning the observations of gravitational waves to collect relatively unique data and make inferences 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.
In cosmology, primordial black holes (PBHs) are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating primordial black holes without the supernova compression typically needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of stellar black holes.
A pulsar timing array (PTA) is a set of galactic pulsars that is monitored and analysed to search for correlated signatures in the pulse arrival times on Earth. As such, they are galactic-sized detectors. Although there are many applications for pulsar timing arrays, the best known is the use of an array of millisecond pulsars to detect and analyse long-wavelength gravitational wave background. Such a detection would entail a detailed measurement of a gravitational wave (GW) signature, like the GW-induced quadrupolar correlation between arrival times of pulses emitted by different millisecond pulsar pairings that depends only on the pairings' angular separations in the sky. Larger arrays may be better for GW detection because the quadrupolar spatial correlations induced by GWs can be better sampled by many more pulsar pairings. With such a GW detection, millisecond pulsar timing arrays would open a new low-frequency window in gravitational-wave astronomy to peer into potential ancient astrophysical sources and early Universe processes, inaccessible by any other means.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a consortium of astronomers who share a common goal of detecting gravitational waves via regular observations of an ensemble of millisecond pulsars using the Green Bank Telescope, Arecibo Observatory, the Very Large Array, and the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Future observing plans include up to 25% total time of the Deep Synoptic Array 2000 (DSA2000). This project is being carried out in collaboration with international partners in the Parkes Pulsar Timing Array in Australia, the European Pulsar Timing Array, and the Indian Pulsar Timing Array as part of the International Pulsar Timing Array.
The International Pulsar Timing Array (IPTA) is a multi-institutional, multi-telescope collaboration comprising the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar Timing Array (PPTA) in Australia, and the Indian Pulsar Timing Array Project (InPTA). The goal of the IPTA is to detect ultra-low-frequency gravitational waves, such as from mergers of supermassive black holes, using an array of approximately 30 pulsars. This goal is shared by each of the participating institutions, but they have all recognized that their goal will be achieved more quickly by combining their respective efforts and resources.
A binary black hole (BBH), or black hole binary, is a system consisting of two black holes in close orbit around each other. Like black holes themselves, binary black holes are often divided into stellar binary black holes, formed either as remnants of high-mass binary star systems or by dynamic processes and mutual capture; and binary supermassive black holes, believed to be a result of galactic mergers.
The first direct observation of gravitational waves was made on 14 September 2015 and was announced by the LIGO and Virgo collaborations on 11 February 2016. Previously, gravitational waves had been inferred only indirectly, via their effect on the timing of pulsars in binary star systems. The waveform, detected by both LIGO observatories, matched the predictions of general relativity for a gravitational wave emanating from the inward spiral and merger of a pair of black holes of around 36 and 29 solar masses and the subsequent "ringdown" of the single resulting black hole. The signal was named GW150914. It was also the first observation of a binary black hole merger, demonstrating both the existence of binary stellar-mass black hole systems and the fact that such mergers could occur within the current age of the universe.
Chiara Mingarelli is an Italian-Canadian astrophysicist who researches gravitational waves. She is an assistant professor of physics at Yale University since 2023, and previously an assistant professor at the University of Connecticut (2020–2023). She is also a science writer and communicator.
The Hellings-Downs curve is an analytical tool that helps to find patterns in pulsar timing data in an effort to detect long wavelength gravitational waves. More precisely, the Hellings-Downs curve refers to the wave-like shape predicted to appear in a plot of timing residual correlations versus the angle of separation between pairs of pulsars. This theoretical correlation function assumes a gravitational wave background that is isotropic and Einsteinian.
Chinese scientists has recently found key evidence for the existence of nanohertz gravitational waves, marking a new era in nanoHertz gravitational research.
Four independent collaborations have spotted a background of gravitational waves that passes through our Galaxy, opening a new window on the astrophysical and cosmological processes that could produce such waves.