Observing gravitational waves from the first generation of black holes

TL;DR

This paper explores the potential of third-generation gravitational wave detectors to observe mergers of first-generation black hole seeds at high redshift, providing insights into early cosmic structure formation.

Contribution

It demonstrates that proposed third-generation detectors could observe seed black-hole mergers and measure their distances, offering new ways to study early universe structure formation.

Findings

Detectors could observe a few to tens of seed black-hole mergers in three years.

A network of detectors may measure source distances with ~40% accuracy.

Observations could distinguish high-redshift black hole formation models.

Abstract

The properties of the first generation of black-hole seeds trace and distinguish different models of formation of cosmic structure in the high-redshift universe. The observational challenge lies in identifying black holes in the mass range ~100-1000 solar masses at redshift z~10. The typical frequencies of gravitational waves produced by the coalescence of the first generation of light seed black-hole binaries fall in the gap between the spectral ranges of low-frequency space-borne detectors (e.g., LISA) and high-frequency ground-based detectors (e.g., LIGO, Virgo and GEO 600). As such, these sources are targets for proposed third-generation ground-based instruments, such as the Einstein Telescope which is currently in design study. Using galaxy merger trees and four different models of black hole accretion - which are meant to illustrate the potential of this new type of source rather than to yield precise event-rate predictions - we find that such detectors could observe a few to a few tens of seed black-hole merger events in three years and provide, possibly unique, information on the evolution of structure in the corresponding era. We show further that a network of detectors may be able to measure the luminosity distance to sources to a precision of ~40%, allowing us to be confident of the high-redshift nature of the sources.