Hearing gravity from the cosmos: GWTC-2 probes general relativity at cosmological scales

Gravitational-wave (GW) catalogs are rapidly increasing in number, allowing for robuststatistical analyses of the population of compact binaries. Nonetheless, GW inference of cosmology has typically relied on additional electromagnetic counterparts or galaxy catalogs. I present a new probe of cosmological modifications of general relativity with GW data only. I focus on deviations of the GW luminosity distance constrained with the astrophysical population of binary black holes (BBHs). The three key observables are 1) the number of events as a function of luminosity distance, 2) the stochastic GW background of unresolved binaries and 3) the location of any feature in the source mass distribution, such as the pair instability supernova (PISN) gap. Modifications of the GW luminosity distance are a priori degenerate with the unknown evolution of the merger rate and source masses. However, a large damping of the GW amplitude predicts a reduction of the events and lowering of the edges of the PISN gap with redshift that is against standard astrophysical expectations. Applying a hierarchical Bayesian analysis to the current LIGO-Virgo catalog (GWTC-2), the strongest constraints to date are placed on deviations from the GW luminosity distance, finding c(M) = -3.2(-2.0)(+3.4) at 68% C.L., which is similar to 10 times better than multi-messenger GW170817 bounds. These modifications also affect the determination of the BBH masses, which is crucial to accommodate the high-mass binary GW190521 away from the PISN gap. In this analysis it is found that the maximum mass of 99% of the population shifts to lower masses with increased uncertainty, m(99%) = 46.2(-9.1)(+11.4) M-circle dot at 68% C.L. Testing gravity at large scales with the population of BBHs will become increasingly relevant with future catalogs, providing an independent and self-contained test of the standard cosmological model. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

This study presents a novel approach to probe cosmological modifications of general relativity using GW data only, specifically focusing on deviations of the GW luminosity distance constrained with the astrophysical population of binary black holes. The hierarchical Bayesian analysis on the LIGO-Virgo catalog provides the strongest constraints to date on deviations from the GW luminosity distance, leading to important implications for the standard cosmological model and the determination of BBH masses. The findings suggest that testing gravity at large scales with the population of BBHs will become increasingly relevant with future catalogs, offering an independent and self-contained test of the standard cosmological model.