# A Preferred Mass Range for Primordial Black Hole Formation and Black Holes as Dark Matter Revisited

## TL;DR

This paper revisits the idea that primordial black holes within a specific mass range could account for dark matter, emphasizing the role of non-thermal histories and spectral index in their formation and abundance.

## Contribution

It demonstrates that a preferred mass range for primordial black holes as dark matter is robust under certain spectral conditions and discusses formation likelihood and relic density constraints.

## Key findings

- Black holes in a specific mass range could constitute all dark matter.
- Non-thermal histories favor formation of black holes in this mass range.
- Observational constraints limit the relic density of primordial black holes.

## Abstract

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.