Black hole remnants are not too fast to be dark matter

TL;DR

This paper argues that black hole remnants can still be viable dark matter candidates despite recoil velocities, as cosmic expansion ensures their velocities are compatible with cold dark matter constraints if formed early enough.

Contribution

It clarifies that recoil velocities of black hole remnants do not conflict with dark matter models if formed before nucleosynthesis, challenging previous claims.

Findings

Recoil velocities are compatible with dark matter if remnants form early

Cosmic expansion reduces recoil velocity impact over time

Remnants formed before nucleosynthesis are consistent with observations

Abstract

We comment on recent claims that recoil in the final stages of Hawking evaporation gives black hole remnants large velocities, rendering them inviable as a dark matter candidate. We point out that due to cosmic expansion, such large velocities at the final stages of evaporation are not in tension with the cold dark matter paradigm so long as they are attained at sufficiently early times. In particular, the predicted recoil velocities are robustly compatible with observations if the remnants form before the epoch of big bang nucleosynthesis, a requirement which is already imposed by the physics of nucleosynthesis itself.