Thegravitational wave background (alsoGWB andstochastic background) is a random background ofgravitational waves permeating theUniverse, which is detectable by gravitational-wave experiments, likepulsar timing arrays.[1] The signal may be intrinsically random, like from stochastic processes in the early Universe and therelic gravitational waves produced directly from inflation, 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 hypotheticalprimordial inflation andcosmic 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. There have been ongoing effort to detectrelic gravitational waves originating directly from inflation.[3] Some examples of these primordial sources include time-varying inflationary scalar fields in the early universe, "preheating" mechanisms afterinflation involving energy transfer from inflaton particles to regular matter,cosmological phase transitions in the early universe (such as theelectroweak 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-universecosmology and onhigh-energy physics.[4][5]
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 andVirgo 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 ablack hole or aneutron star—in the rapid collapse during the final moments of an explosivesupernova event, which can lead to such formations, gravitational waves may theoretically be liberated.[6][7] 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 depends on the sensitive frequency band of the signal. The current generation of ground based experiments likeLIGO andVirgo 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.[8]
An alternative means of observation is usingpulsar timing arrays (PTAs). Three consortia—theEuropean Pulsar Timing Array (EPTA), theNorth American Nanohertz Observatory for Gravitational Waves (NANOGrav), and theParkes Pulsar Timing Array (PPTA)—coordinate as theInternational 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.[9]
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.[10] 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 byPTAs.[11]
Plot of correlation between pulsars observed by NANOGrav (2023) vs angular separation between pulsars, compared with a theoretical Hellings–Downs model (dashed purple) and if there were no gravitational wave background (solid green)[12][13]
On 11 February 2016, theLIGO andVirgo 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.[14][15]
On 28 June 2023, theNorth American Nanohertz Observatory for Gravitational Waves collaboration announced evidence for a GWB using observational data from an array of millisecondpulsars.[16][17] Observations fromEPTA,[18]Parkes Observatory[19] andChinese Pulsar Timing Array (CPTA)[20][21] were also published on the same day, providing cross validation of the evidence for the GWB using different telescopes and analysis methods.[22] These observations provided the first measurement of the theoreticalHellings-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.[23][24]
The sources of this gravitational-wave background cannot be identified without further observations and analyses, although binaries ofsupermassive black holes are leading candidates.[1]
