Daughters mimic sterile neutrinos (almost!) perfectly

Abstract

Since only recently, cosmological observations are sensitive to hot dark matter (HDM) admixtures with sub-eV mass, $m_\text{hdm}^\text{eff}$ < eV, that are not fully-thermalised, $N_\text{eff}$ < 1. We argue that their almost automatic interpretation as a sterile neutrino species is neither from theoretical nor practical parsimony principles preferred over HDM formed by decay products (daughters) of an out-of-equilibrium particle decay. While daughters mimic sterile neutrinos in $N_\text{eff}$ and $m_\text{hdm}^\text{eff}$, there are opportunities to assess this possibility in likelihood analyses. Connecting cosmological parameters and moments of momentum distribution functions, we show that --also in the case of mass-degenerate daughters with indistinguishable main physical effects-- the mimicry breaks down when the next moment, the skewness, is considered. Predicted differences of order one in the root-mean-squares of absolute momenta are too small for current sensitivities.