The Stochastic Siren: Astrophysical Gravitational-Wave Background Measurements of the Hubble Constant

We report the first measurement of the Hubble constant H 0 using the stochastic gravitational-wave background arising from binary black hole mergers. This astrophysical background is sensitive to the expansion history of the Universe and thus can be used for cosmological parameter inference independently of not only electromagnetic methods, but also gravitational-wave standard siren approaches. We describe the background's cosmological dependence and show how it can be used as a ``stochastic siren'' to measure H 0. By analyzing existing resolved binary black hole mergers and the current non-detection of the background, we find that H 0 can be measured more accurately relative to using resolved mergers alone. We also note that the stochastic siren may serve a unique role in the Hubble tension in that the lower bound of the H 0 measurement would progressively increase with continued non-detection of the background.