A new approach to constrain the Hubble expansion rate at high redshifts by gravitational waves

TL;DR

This paper proposes a novel method using gravitational wave observations of binary black hole mergers to infer the universe's expansion rate at high redshifts by analyzing the relationship between mass ratios and redshift, supported by simulation data.

Contribution

It introduces a new approach to constrain high-redshift Hubble expansion using mass ratio-redshift correlations in BBH mergers, supported by hydrodynamical simulations.

Findings

Expected correlation between mass ratio and redshift of BBHs.

Potential to measure H(z) at z ≈ 2 with LISA.

Discovery of a trend linking seed mass to BBH mass ratio.

Abstract

Detection of massive binary black hole (BBH) mergers at high redshifts is a target for LISA space mission. While the individual masses of a BBH merger are redshifted, the mass ratio of BBH mergers is independent of their redshift. Therefore, if there is an independent correlation between the mass ratio and redshift, such a relationship can be used to i) infer the redshift of the merging binaries, and together with the luminosity distance measurement ($D_L$), constrain the expansion rate of the universe at high redshifts $H(z)$, and ii) constrain models of supermassive black hole seed formation in the universe assuming a fixed cosmology. We discuss why there is an expected relation between the mass ratio of the massive BBHs with their redshift and show the clues for this relation by analyzing cosmological hydrodynamical simulations of BBH mergers. This approach opens up the possibility of directly measuring the expansion rate at redshift $z \approx 2$ with LISA for the first time. Moreover, we discover a trend between seed mass and mass ratio of massive BBHs which by itself is a major result that could be exploited to constrain the formation scenarios of supermassive BH seeds.