The gravitational wave background (also GWB and stochastic background) is a random background of

gravitational wave Gravitational waves are oscillations of the gravitational field that Wave propagation, travel through space at the speed of light; they are generated by the relative motion of gravity, gravitating masses. They were proposed by Oliver Heaviside i ...

s permeating the

Universe The universe is all of space and time and their contents. It comprises all of existence, any fundamental interaction, physical process and physical constant, and therefore all forms of matter and energy, and the structures they form, from s ...

, which is detectable by gravitational-wave experiments, like

pulsar timing array A pulsar timing array (PTA) is a set of galactic pulsars that is monitored and analyzed 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 p ...

s. 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.

Sources of a stochastic background

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 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 In economics, inflation is an increase in the average price of goods and services in terms of money. This increase is measured using a price index, typically a consumer price index (CPI). When the general price level rises, each unit of curre ...

involving energy transfer from inflaton particles to regular matter, cosmological phase transitions in the early universe (such as the

electroweak In particle physics, the electroweak interaction or electroweak force is the unified description of two of the fundamental interactions of nature: electromagnetism (electromagnetic interaction) and the weak interaction. Although these two forc ...

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 Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe, the cosmos. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', with the meaning of "a speaking of the wo ...

and on

high-energy physics Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the stu ...

Astrophysical sources

An astrophysical background is produced by the combined noise of many weak, independent, and unresolved astrophysical sources. 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 The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Prior to LIG ...

and

Virgo Virgo may refer to: Arts and entertainment * Virgo (film), a 1970 Egyptian film * Virgo (character), several Marvel Comics characters * Virgo Asmita, a character in the manga ''Saint Seiya: The Lost Canvas'' * ''Virgo'' (album), by Virgo Four, ...

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 A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

or a

neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...

—in the rapid collapse during the final moments of an explosive

supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

event, which can lead to such formations, gravitational waves may theoretically be liberated. Also, in rapidly rotating neutron stars there is a whole class of instabilities driven by the emission of gravitational waves. The nature of source also depends on the sensitive frequency band of the signal. The current generation of ground based experiments like

and

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. An alternative means of observation is using

s (PTAs). Three consortia—the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the

Parkes Pulsar Timing Array Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...

(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.

Supermassive black hole A supermassive black hole (SMBH or sometimes SBH) 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 (). Black holes are a class of astronomical ...

s with masses of 10⁵–10⁹

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es 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. 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.

Detection

On 11 February 2016, the

and

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. 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 A pulsar (''pulsating star, on the model of quasar'') 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 pointin ...

. Observations from EPTA,

Parkes Observatory Parkes Observatory is a radio astronomy observatory, located north of the town of Parkes, New South Wales, Australia. It hosts Murriyang, the 64 m CSIRO Parkes Radio Telescope also known as "The Dish", along with two smaller radio telescopes. T ...

and Chinese Pulsar Timing Array (CPTA) were also published on the same day, providing cross validation of the evidence for the GWB using different telescopes and analysis methods. These observations provided the first measurement of the theoretical Hellings-Downs curve, i.e., the quadrupolar and higher multipolar 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 gravitational-wave background cannot be identified without further observations and analyses, although binaries of

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) 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 (). Black holes are a class of astronomical ...

s are leading candidates.

References

External links