Sound Speed and Viscosity of Semi-Relativistic Relic Neutrinos
  Generalized fluid equations, using sound speed $c_{\rm eff}^2$ and viscosity
$c_{\rm vis}^2$ as effective parameters, provide a convenient phenomenological
formalism for testing the relic neutrino "null hypothesis," i.e. that that
neutrinos are relativistic and free-streaming prior to recombination. In this
work, we relax the relativistic assumption and ask "to what extent can the
generalized fluid equations accommodate finite neutrino mass?" We consider both
the mass of active neutrinos, which are largely still relativistic at
recombination $m^2 / T^2 \sim 0.2$, and the effect of a semi-relativistic
sterile component. While there is no one-to-one mapping between mass/mixing
parameters and $c_{\rm eff}^2$ and $c_{\rm vis}^2$, we demonstrate that the
existence of a neutrino mass could induce a bias to measurements of $c_{\rm
eff}^2$ and $c_{\rm vis}^2$ at the level of $0.01 m^2 / T^2 \sim 10^{-3}$.
