Detecting a stochastic gravitational-wave background with LIGO-Virgo-Kagra

In under 10 years since the first direct observation of a gravitational wave (GW), GW Astronomy has made giant leaps, going from detecting loud binary black hole (BBH) coalescences, to binary neutron star (BNS) and black hole -- neutron star (BHNS) binary events, all the way to large-volume population analyses. Yet the vast majority of events are too faint to be directly detected with coherent, so-called matched filter searches, and what’s more, there are a slew of other potential sources of GWs that have yet to be identified. The collection of all GWs that may not be directly resolved by our detectors build up incoherently, giving rise to a cumulative signal we refer to as the gravitational-wave background (GWB). Given its incoherent and indeterminate nature, the GWB is best described by stochastic variables, and searched for via stochastic analyses. Different sources of GWs will contribute to the GWB differenly, hence stochastic analyses may be tailored to search for specific trademarks such as frequency dependence or directional dependence to perform component separation and give optimal results. As most GWB sources targeted by stochastic analyses are persistent, some of them may also be detected with continuous wave search methods, which are consistently more sensitive to the signal, but require more accurate source modelling. It is important to note that, until now, no source of GWs has been confidently identified using a stochastic search.

In this talk, I will start by describing the sources and signals contributing to the GWB, putting them in perspective within the context of astrophysics and cosmology. I will then introduce the stochastic GW analysis methods we routinely employ within the Laser Interferometer Gravitational-wave Observatory (LIGO), Virgo, and Kagra (LVK) collaboration to search for GWBs, and present recent search results. These include upper limits on a number of different target signals from both stochastic and continuous wave searches. I will finally present forecasts for when and how we expect to detect a GWB from BBHs and BNSs with the LVK detectors.