On the Merger Rate of Primordial Black Holes in Cosmic Voids

TL;DR

This paper estimates the merger rate of primordial black holes in cosmic voids and suggests that a small but significant fraction of gravitational wave events could originate from these secluded regions, with minimal redshift evolution.

Contribution

It provides the first detailed calculation of PBH merger rates specifically within cosmic voids and their potential contribution to gravitational wave detections.

Findings

Approximately 2-3 events per year from cosmic voids.

Merger rate evolution shows minimal sensitivity to redshift.

The PBH mass and fraction relation aligns with observed data.

Abstract

Cosmic voids are known as underdense substructures of the cosmic web that cover a large volume of the Universe. It is known that cosmic voids contain a small number of dark matter halos, so the existence of primordial black holes (PBHs) in these secluded regions of the Universe is not unlikely. In this work, we calculate the merger rate of PBHs in dark matter halos structured in cosmic voids and determine their contribution to gravitational wave events resulting from black hole mergers recorded by the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO)-Advanced Virgo (aVirgo) detectors. Relying on the PBH scenario, the results of our analysis indicate that about $2 \sim 3$ annual events of binary black hole mergers out of all those recorded by the aLIGO-aVirgo detectors should belong to cosmic voids. We also calculate the redshift evolution of the merger rate of PBHs in cosmic voids. The results show that the evolution of the merger rate of PBHs has minimum sensitivity to the redshift changes, which seems reasonable while considering the evolution of cosmic voids. Finally, we specify the behavior of the merger rate of PBHs as a function of their mass and fraction in cosmic voids and we estimate $\mathcal{R} (M_{PBH}, f_{PBH})$ relation, which is well compatible with our findings.