A Preferred Mass Range for Primordial Black Hole Formation and Black
  Holes as Dark Matter Revisited
  Bird, et. al. and Sasaki, et. al. have recently proposed the intriguing
possibility that the black holes detected by LIGO could be all or part of the
cosmological dark matter. This offers an alternative to WIMPs and axions, where
dark matter could be comprised solely of Standard Model particles. The mass
range lies within an observationally viable window and the predicted merger
rate can be tested by future LIGO observations. In this paper, we argue that
non-thermal histories favor production of black holes near this mass range --
with heavier ones unlikely to form in the early universe and lighter black
holes being diluted through late-time entropy production. We discuss how this
prediction depends on the primordial power spectrum, the likelihood of black
hole formation, and the underlying model parameters. We find the prediction for
the preferred mass range to be rather robust assuming a blue spectral index
less than two. We consider the resulting relic density in black holes, and
using recent observational constraints, establish whether they could account
for all of the dark matter today.
