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

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