LIGO/Virgo black holes and dark matter: The effect of spatial clustering

TL;DR

This paper explores how spatial clustering of primordial black holes affects their merger rates and the implications for dark matter composition, showing that clustering can relax existing constraints and be distinguished through gravitational wave observations.

Contribution

It demonstrates that clustering of primordial black holes can significantly alter merger rate constraints and proposes a method to distinguish clustered distributions via gravitational wave background observations.

Findings

Clustering can relax the primordial black hole dark matter fraction constraints.

Merger rate evolution with redshift can break degeneracies caused by clustering.

LIGO's sensitivity can differentiate clustered distributions through stochastic gravitational wave background.

Abstract

We discuss the effect of clustering for the determination of the merger rate of binary black holes in the LIGO mass range. While for a Poissonian initial distribution, and assuming isolated binaries, the allowed fraction of Primordial Black Holes (PBHs) to dark matter (DM) is a few percent, we show that this bound can be relaxed if PBHs are clustered. More precisely we show that for large clustering the merger rate can drop with increasing fraction of PBHs, introducing a degeneracy in the parameters of the theory consistent with a given present merger rate, and allowing all the DM to be in the form of stellar mass PBHs. This degeneracy can however be broken by looking at the evolution of the merger rate with redshift. For a simple clustering model that we consider, we show that the LIGO projected sensitivity can disentangle, through the observation of a stochastic background of gravitational waves, different clustered distributions having the same present merger rate.