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]
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 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]
Astrophysical sources
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 105–109solar 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]
Detection
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]
^Sesana, A. (22 May 2013). "Systematic investigation of the expected gravitational wave signal from supermassive black hole binaries in the pulsar timing band". Monthly Notices of the Royal Astronomical Society: Letters. 433 (1): L1–L5.
arXiv:1211.5375.
Bibcode:
2013MNRAS.433L...1S.
doi:
10.1093/mnrasl/slt034.
S2CID11176297.
^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.
arXiv:astro-ph/0207276.
Bibcode:
2003ApJ...582..559V.
doi:
10.1086/344675.
S2CID2384554.
^Sesana, A.; Vecchio, A.; Colacino, C. N. (11 October 2008). "The stochastic gravitational-wave background from massive black hole binary systems: implications for observations with Pulsar Timing Arrays". Monthly Notices of the Royal Astronomical Society. 390 (1): 192–209.
arXiv:0804.4476.
Bibcode:
2008MNRAS.390..192S.
doi:
10.1111/j.1365-2966.2008.13682.x.
S2CID18929126.
^"Probing the Universe's Secrets: Key Evidence for NanoHertz Gravitational Waves". scitechdaily.com. Chinese Academy of Sciences. 2 July 2023. Retrieved 21 July 2023. Chinese scientists has recently found key evidence for the existence of nanohertz gravitational waves, marking a new era in nanoHertz gravitational research.