GW190425, GW190521 and GW190814: Three candidate mergers of primordial
  black holes from the QCD epoch
  The two recent gravitational-wave events GW190425 and GW190814 from the third
observing run of LIGO/Virgo have both a companion which is unexpected if
originated from a neutron star or a stellar black hole, with masses
$[1.6-2.5]~M_\odot$ and $[2.5-2.7]~M_\odot$ and merging rates $
460^{+1050}_{-360} $ and $ 7^{+16}_{-6}$ events/yr/Gpc$^3$ respectively, at
90\% c.l.. Moreover, the recent event GW190521 has black hole components with
masses 67 and $91~M_\odot$, and therefore lies in the so-called
pair-instability mass gap, where there should not be direct formation of
stellar black holes. The possibility that all of these compact objects are
Primordial Black Holes (PBHs) is investigated. The known thermal history of the
Universe predicts that PBH formation is boosted at the time of the QCD
transition, inducing a peak in their distribution at this particular mass
scale, and a bump around $30-50~M_\odot$. We find that the merging rates
inferred from GW190425, GW190521 and GW190814 are consistent with PBH binaries
formed by capture in dense halos in the matter era or in the early universe. At
the same time, the rate of black hole mergers around $30~M_\odot$ and of
sub-solar PBH mergers do not exceed the LIGO/Virgo limits. Such PBHs could
explain a significant fraction, or even the totality of the Dark Matter, but
they must be sufficiently strongly clustered in order to be consistent with
current astrophysical limits.
