Evidence for primordial black hole dark matter from LIGO/Virgo merger rates

Abstract

The LIGO/Virgo collaboration has by now observed or constrained the gravitational merger rates of different classes of compact objects. We consider the possibility that the bulk of these mergers are primordial black hole (PBH) mergers of PBHs formed during the QCD epoch making up the entirety of the dark matter. Having shown in a companion paper that mergers due to the initial binary population formed in the early Universe are likely negligible, we compute current merger rates in PBH clusters in which the typical PBH resides. We consider two scenarios: (i) the PBH mass function dictated by the QCD equation of state and (ii) the PBH mass function dictated by the existence of a peak in the inflationary perturbation spectrum. In the first scenario, which is essentially parameter free, we reproduce very well the merger rates for heavy BHs, the merger rate of mass-asymmetric BHs such as GW190814, a recently discovered merger of a 23$M_{\odot}$ black hole with a $2.6M_{\odot}$ object, and can naturally explain why LIGO/Virgo has not yet observed mergers of two light PBHs from the dominant $\sim 1\,M_{\odot}$ PBH population. In the second scenario, which has some parameter freedom, we match well the observed rate of heavy PBHs, but can currently not explain the rate for mass-asymmetric events. In either case it is tantalizing that in both scenarios PBH merger rates made with a minimum of assumptions match most LIGO/Virgo observed rates very well.