Simulations of PBH formation at the QCD epoch and comparison with the GWTC-3 catalog

TL;DR

This study simulates primordial black hole formation during the QCD epoch, accounting for the changing equation of state, and compares the resulting merger rates with gravitational wave observations, suggesting PBHs could explain some GW events.

Contribution

First simulation of PBH formation incorporating the QCD epoch's varying equation of state, revealing significant impacts on PBH mass distribution and merger rates.

Findings

PBH mass distributions are significantly affected by QCD epoch physics.

Merger rates of PBHs match GWTC-3 observations for certain dark matter fractions.

Possible explanation for GW events around 30-50 solar masses and asymmetric mergers.

Abstract

The probability of primordial black hole (PBH) formation is known to be boosted during the Quantum Chromodynamics (QCD) crossover due to a slight reduction of the equation of state. This induces a high peak and other features in the PBH mass distribution. But the impact of this variation during the PBH formation has been so far neglected. In this work we simulate for the first time the formation of PBHs by taking into account the varying equation of state at the QCD epoch, compute the over-density threshold using different curvature profiles and find that the resulting PBH mass distributions are significantly impacted. The expected merger rate distributions of early and late PBH binaries is comparable to the ones inferred from the GWTC-3 catalog for dark matter fractions in PBHs within $0.1 < f_{\rm PBH} <1 $. The distribution of gravitational-wave events estimated from the volume sensitivity could explain mergers around $30-50 M_\odot$, with asymmetric masses like GW190814, or in the pair-instability mass gap like GW190521. However, none of the considered cases leads to a multi-modal distribution with a secondary peak around $8-15 M_\odot$, as suggested by the GWTC-3 catalog, possibly pointing to a mixed population of astrophysical and primordial black holes.