= Number density of a relativistic neutrino relative to photons
{title2=$n_\nu/n_\gamma=\tfrac{3g_\nu}{8}(T_\nu/T_\gamma)^3$}
= Relativistic-neutrino number-density ratio
{synonym}
With negligible <chemical potential>, the <Fermi-Dirac distribution> has $3/4$ of the boson number density per degree of freedom at equal temperature. A species with $g_\nu$ populated states therefore has $n_\nu/n_\gamma=(3g_\nu/8)(T_\nu/T_\gamma)^3$, since <photons> have two states. This remains the number-density ratio after the <neutrino> becomes massive if its comoving particle number is conserved.
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