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

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 process of PBH formation has so far not been considered in numerical simulations. In this work we simulate 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 PBH < 1. The distribution of gravitational-wave events estimated from the volume sensitivity could explain mergers around 30-50 M ⊙, 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 ⊙, as suggested by the GWTC-3 catalog, possibly pointing to a mixed population of astrophysical and primordial black holes.