Hearing the Hubble tension with shouts and murmurs: measuring H₀ using the gravitational-wave background

Early- and late-universe measurements of the Hubble constant are in major disagreement, amounting to a conflict known as the Hubble tension. This tension has prompted the development of many novel approaches to measuring the Hubble constant, including the use of gravitational waves from individually-resolved compact object mergers, so-called "standard sirens." While the standard siren method has been well-explored, the gravitational-wave background arising from unresolved mergers has not yet been considered as a means to study the Hubble tension. In this talk, I will demonstrate how combining resolved binary black hole mergers with the astrophysical gravitational-wave background can be used to not only measure the Hubble constant, but also constrain the properties of the binary black hole population. I will present the results of applying our new method to existing and future gravitational-wave data. Our procedure serves as a means to probe the Hubble tension independently of conventional methods, while simultaneously providing insight on the population of binary black hole mergers across cosmic time.