Formation of primordial black hole binaries and their merger rates

TL;DR

This paper reviews the formation mechanisms and merger rates of primordial black hole binaries, emphasizing early Universe formation and the effects of interactions on merger rate suppression, especially relevant if primordial black holes constitute a significant dark matter component.

Contribution

It provides a detailed analysis of how primordial black hole interactions influence binary formation and merger rates, highlighting the conditions under which these effects are significant.

Findings

Early Universe binaries dominate present merger rates

Interactions can suppress merger rates if PBH abundance exceeds 1%

Formation of a harder binary population contributes to mergers when PBH constitute over 10% of dark matter

Abstract

We review the theory behind the formation of primordial black hole binaries and their merger rates. We consider the binary formation in the early and late Universe, emphasising the former as it gives the dominant contribution of the present primordial black hole merger rate. The binaries formed in the early Universe are highly eccentric so their interactions with other primordial black holes can significantly increase their coalescence times and thereby suppress the merger rate. We discuss in detail how the suppression of the merger rate arising from such interactions can be estimated and how such interactions lead to the formation of another, much harder, binary population that contributes to the present merger rate if more than 10% of dark matter consists of primordial black holes with a relatively narrow mass distribution. When the primordial abundance is below 1%, encounters between primordial black holes are unlikely and their effect on the merger rate becomes negligible.