On the merger rate of primordial black holes: effects of nearest neighbours distribution and clustering

TL;DR

This paper examines how the spatial distribution and initial clustering of primordial black holes affect the estimated merger rates, confirming existing constraints for narrow mass functions and suggesting tighter bounds for broad mass functions.

Contribution

It revisits the standard merger rate estimate by analyzing the impact of nearest neighbour distribution and clustering, providing refined constraints on primordial black hole dark matter fractions.

Findings

Constraints are robust for narrow mass functions.

Initial clustering may tighten bounds for broad mass functions.

Merger rate estimates depend on primordial power spectrum features.

Abstract

One of the seemingly strongest constraints on the fraction of dark matter in the form of primordial black holes (PBH) of ${\cal O}$(10)$\,M_\odot$ relies on the merger rate inferred from the binary BH merger events detected by LIGO/Virgo. The robustness of these bounds depends however on the accuracy with which the formation of PBH binaries in the early Universe can be described. We revisit the standard estimate of the merger rate, focusing on a couple of key ingredients: the spatial distribution of nearest neighbours and the initial clustering of PBHs associated to a given primordial power spectrum. Overall, we confirm the robustness of the results presented in the literature in the case of a narrow mass function (which constrain the PBH fraction of dark matter to be $f_{\rm PBH}\lesssim 0.001-0.01$). The initial clustering of PBHs might have an effect tightening the current constraint, but only for very broad mass functions, corresponding to wide bumps in the primordial power spectra extending at least over a couple of decades in $k$-space.