Probing the expansion history of the Universe with Bright and Spectral Sirens

Gravitational waves (GWs) generated from compact binary mergers can provide independent measurements of the Hubble constant, potentially resolving the Hubble tension. The luminosity distance of these events can be directly measured from the GW signal alone, and, together with a redshift estimate, provides an independent measurement of the Hubble constant. GW observations of binary neutron star mergers with an observed electromagnetic (EM) counterpart are expected to achieve a 2% precision in the Hubble constant measurement with 50 events. This is known as the "bright siren" approach. On the other hand, an estimate of a compact binary's redshift can also be obtained without an EM counterpart, for example, by comparing the observed (redshifted) mass distribution of the binaries with the mass distribution of the binary's population in their source frame, an approach known as the "spectral sirens". Thanks to this approach, binary black hole mergers at higher redshifts become ideal for inferring the Hubble parameter, H(z). In this talk, we present the predictions when combining the bright sirens and spectral sirens approaches with appropriate consideration of systematics using current and next-generation GW detectors to investigate their potential for exploring the expansion rate of the Universe across different redshifts.