Gravitational wave background

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. The signal may be intrinsically random, due to stochastic processes in the early Universe and the relic gravitational waves produced directly by cosmic inflation, or it may be produced by an incoherent superposition of a large number of weak independent unresolved gravitational-wave sources, such as supermassive black-hole binaries.

Source: Wikipedia — Gravitational wave background (CC BY-SA 4.0)