Comment on "keV Neutrino Dark Matter in a Fast Expanding Universe" by Biswas et al

TL;DR

This paper critiques a previous claim that a non-standard fast-expanding universe suppresses dark matter relic density, clarifying that the earlier conclusion was based on an incorrect assumption about entropy contributions.

Contribution

The authors correct a misconception in prior work by showing the assumption about entropy contributions from the extra component is invalid, impacting relic density calculations.

Findings

The previous claim about relic density suppression is incorrect.

The extra component does not contribute to entropic degrees of freedom.

Relic density calculations must exclude the extra component's entropy contribution.

Abstract

Biswas et al. found that the thermal relic density of a dark matter particle freezing out while the universe's energy density is dominated by a non-standard extra component $\phi$, whose energy density redshifts faster than radiation, can be greatly suppressed. Here we show that this result, which contradicts extensive previous literature, is incorrect: the mistake lies with the assumption that the (decoupled) extra component $\phi$ contributes to the entropic degrees of freedom relevant for dark matter freeze out. If this were the case, a completely different approach would be needed to calculate the dark matter relic abundance, with dramatically different results.