Gravitational wave (GW) memory is an outstanding prediction of general relativity. The term "memory" signifies the enduring imprint left on space. Among the various sources of GW memory, the type generated by the gravitational radiation of coalescences of binary black holes holds the most promise for eventual observation. Nonetheless, up until the present moment, no such detection has been made.
In this report, we demonstrate that numerous instances of these memory events can collectively give rise to a stochastic gravitational wave memory background. This background is marginally detectable after the deployment of space-based detectors, such as Taiji, TianQin, and LISA.
Concurrently, other radiations carrying energy fluxes can also produce GW memory. Here we investigate the gravitational wave memory produced by various backgrounds, including the cosmic microwave background (CMB), cosmic neutrino background (CνB), and gravitational wave background. We find that the behavior of these phenomena exhibits a connection to the quadrupole structure of the universe's expansion and is marginally detectable without even needing to wait for future, more sophisticated detectors.
The calculations are directly obtained from standard cosmology and general relativity, without introducing any new theory.