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