Stochastic Gravitational Wave Background from PBH-ABH Mergers

TL;DR

This paper investigates the stochastic gravitational wave background generated by mergers of primordial black holes with astrophysical black holes, highlighting detection challenges and the inappropriateness of treating local signals as isotropic backgrounds.

Contribution

It introduces the study of gravitational wave backgrounds from mixed primordial and astrophysical black hole mergers, a previously unexplored source.

Findings

Detection of such backgrounds is very challenging.

Treating Milky Way signals as isotropic background is inappropriate.

The binary formation rate in the Milky Way is very low.

Abstract

The measurement of gravitational waves produced by binary black-hole mergers at the Advanced LIGO has encouraged extensive studies on the stochastic gravitational wave background. Recent studies have focused on gravitational wave sources made of the same species, such as mergers from binary primordial black holes or those from binary astrophysical black holes. In this paper, we study a new possibility -- the stochastic gravitational wave background produced by mergers of one primordial black hole and one astrophysical black hole. Such systems are necessarily present if primordial black holes exist. We study the isotropic gravitational wave background produced through the history of the Universe. We find it is very challenging to detect such a signal. We also demonstrate that it is improper to treat the gravitational waves produced by such binaries in the Milky Way as a directional stochastic background, due to a very low binary formation rate.