Cosmic Conundra Explained by Thermal History and Primordial Black Holes
  A universal mechanism may be responsible for several unresolved cosmic
conundra. The sudden drop in the pressure of relativistic matter at
$W^{\pm}/Z^{0}$ decoupling, the quark--hadron transition and $e^{+}e^{-}$
annihilation enhances the probability of primordial black hole (PBH) formation
in the early Universe. Assuming the amplitude of the primordial curvature
fluctuations is approximately scale-invariant, this implies a multi-modal PBH
mass spectrum with peaks at $10^{-6}$, 1, 30, and $10^{6}\,M_{\odot}$. This
suggests a unified PBH scenario which naturally explains the dark matter and
recent microlensing observations, the LIGO/Virgo black hole mergers, the
correlations in the cosmic infrared and X-ray backgrounds, and the origin of
the supermassive black holes in galactic nuclei at high redshift. A distinctive
prediction of our model is that LIGO/Virgo should observe black hole mergers in
the mass gaps between 2 and $5\,M_{\odot}$ (where no stellar remnants are
expected) and above $65\,M_{\odot}$ (where pair-instability supernovae occur)
and low-mass-ratios in between. Therefore the recent detection of events
GW190425, GW190814 and GW190521 with these features is striking confirmation of
our prediction and may indicate a primordial origin for the black holes. In
this case, the exponential sensitivity of the PBH abundance to the equation of
state would offer a unique probe of the QCD phase transition. The detection of
PBHs would also offer a novel way to probe the existence of new particles or
phase transitions with energy between $1\,{\rm MeV}$ and $10^{10}\,$GeV.
