Primordial Black Holes Confront LIGO/Virgo data: Current situation
  The LIGO and Virgo Interferometers have so far provided 11 gravitational-wave
(GW) observations of black-hole binaries. Similar detections are bound to
become very frequent in the near future. With the current and upcoming wealth
of data, it is possible to confront specific formation models with
observations. We investigate here whether current data are compatible with the
hypothesis that LIGO/Virgo black holes are of primordial origin. We compute in
detail the mass and spin distributions of primordial black holes (PBHs), their
merger rates, the stochastic background of unresolved coalescences, and
confront them with current data from the first two observational runs, also
including the recently discovered GW190412. We compute the best-fit values for
the parameters of the PBH mass distribution at formation that are compatible
with current GW data. In all cases, the maximum fraction of PBHs in dark matter
is constrained by these observations to be $f_{\text{PBH}}\approx {\rm
few}\times 10^{-3}$. We discuss the predictions of the PBH scenario that can be
directly tested as new data become available. In the most likely formation
scenarios where PBHs are born with negligible spin, the fact that at least one
of the components of GW190412 is moderately spinning is incompatible with a
primordial origin for this event, unless accretion or hierarchical mergers are
significant. In the absence of accretion, current non-GW constraints already
exclude that LIGO/Virgo events are all of primordial origin, whereas in the
presence of accretion the GW bounds on the PBH abundance are the most stringent
ones in the relevant mass range. A strong phase of accretion during the cosmic
history would favour mass ratios close to unity, and a redshift-dependent
correlation between high masses, high spins and nearly-equal mass binaries,
with the secondary component spinning faster than the primary.
