Lower bound on the primordial black hole merger rate
  We derive a lower bound on the merger rate of primordial black hole (PBH)
binaries by estimating the maximal fraction of binaries that were perturbed
between formation in the early Universe and merger, and computing a
conservative merger rate of perturbed binaries. This implies robust constraints
on the PBH abundance in the range $1-100 M_\odot$. We further show that
LIGO/Virgo design sensitivity has the potential to reach the PBH mass range of
$10^{-2}-10^3 M_\odot$. The constraint from the merger rate of perturbed
binaries is stronger if PBHs are initially spatially clustered.
