Radio sirens: inferring $H_0$ with binary black holes and neutral hydrogen in the era of the Einstein Telescope and the SKA Observatory

TL;DR

This paper proposes a novel method combining gravitational wave data and neutral hydrogen intensity mapping to measure the Universe's expansion rate, achieving about 8% precision on H_0 with next-generation observatories.

Contribution

It introduces 'radio sirens', a new approach that uses hydrogen maps as redshift priors for GW events to improve cosmological measurements.

Findings

Constrains H_0 to ~8% precision using ~3,000 GW events.

Achieves ~90% improvement over methods without hydrogen map information.

Demonstrates the potential of next-generation GW and intensity mapping surveys for cosmology.

Abstract

A new synergy between gravitational waves (GWs) and the study of the large-scale structure of the Universe is now emerging. Along this line of research, we combine simulated observations of stellar-origin black hole mergers and neutral hydrogen 21 cm line intensity mapping to probe the expansion rate of the Universe through the distance-redshift relation. GW signals from binary black holes provide direct distance information, while neutral hydrogen intensity maps offer a tomographic view of the large-scale structure of the Universe. Using the 3-dimensional density fields of hydrogen as a redshift prior for GW events, we explore a novel dark-sirens-like approach, here termed radio sirens, to measure the late-time expansion history of the Universe. We study the performance of the next-generation GW observatories, such as the Einstein Telescope, to ensure enough statistics and access to high-redshift data. On the other hand, future spectroscopic intensity mapping surveys with the SKA-Mid telescope are expected to trace the underlying dark matter distribution at large scales up to redshift $z\sim 3$. This combined methodology allows us to constrain the Hubble constant to $\sim 8\%$ precision, using around 3,000 GW events with signal-to-noise ratios greater than 150. This corresponds to an improvement of around $90\%$ compared to not considering the information from the neutral hydrogen maps.