A preferred mass range for primordial black hole formation and black holes as dark matter revisited

Bird et al. [1] and Sasaki et al. [2] 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.