The Clustering Evolution of Primordial Black Holes
  Primordial black holes might comprise a significant fraction of the dark
matter in the Universe and be responsible for the gravitational wave signals
from black hole mergers observed by the LIGO/Virgo collaboration. The spatial
clustering of primordial black holes might affect their merger rates and have a
significant impact on the constraints on their masses and abundances. We
provide some analytical treatment of the primordial black hole spatial
clustering evolution, compare our results with some of the existing N-body
numerical simulations and discuss the implications for the black hole merger
rates. If primordial black holes contribute to a small fraction of the dark
matter, primordial black hole clustering is not relevant. On the other hand,
for a large contribution to the dark matter, we argue that the clustering may
increase the late time Universe merger rate to a level compatible with the
LIGO/Virgo detection rate. As for the early Universe merger rate of black hole
binaries formed at primordial epochs, clustering alleviates the LIGO/Virgo
constraints, but does not evade them.
