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