Comment on “keV neutrino dark matter in a fast expanding universe” by Biswas et al.

Abstract

Biswas et al. [1] 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 ϕ, 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 ϕ 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.