The gravitational wave background (also GWB and stochastic background) is a random gravitational-wave signal potentially detectable by gravitational wave detection experiments. Since the background is supposed to be statistically random, it has yet been researched only in terms of such statistical descriptors as the

mean There are several kinds of mean in mathematics, especially in statistics. Each mean serves to summarize a given group of data, often to better understand the overall value ( magnitude and sign) of a given data set. For a data set, the '' ar ...

, the

variance In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbe ...

, etc.

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 sources include time-varying scalar (classical) fields in the early universe, "preheating" mechanisms after

inflation In economics, inflation is an increase in the general price level of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a reduct ...

involving energy transfer from inflaton particles to regular matter, 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 four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...

phase transition),

cosmic string Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry-breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking was not simp ...

s, etc. While these sources are more hypothetical, a detection of a background from them would be a major discovery of new physics. The detection of such an inflationary background 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 term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...

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 fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) a ...

Astrophysical sources

An astrophysical background is produced by the confused 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. Two large ...

and

Virgo Virgo may refer to: *Virgo (astrology), the sixth astrological sign of the zodiac * Virgo (constellation), a constellation *Virgo Cluster, a cluster of galaxies in the constellation Virgo *Virgo Stellar Stream, remains of a dwarf galaxy * Virgo Su ...

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 region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can def ...

or a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

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

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

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 depend 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.

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.

References

External links

Gravitational Wave Experiments and Early Universe Cosmology
{{Cosmology topics Effects of gravitation

background Background may refer to: Performing arts and stagecraft * Background actor * Background artist * Background light * Background music * Background story * Background vocals * ''Background'' (play), a 1950 play by Warren Chetham-Strode Record ...