Effective number of neutrino species
= Effective number of neutrino species
{title2=$N_{\rm eff}$}
{wiki}
The effective number of neutrino species parametrizes radiation beyond photons through
$$
\rho_r=\frac{\pi^2}{30}\left[2+\frac78\,2
\left(\frac4{11}\right)^{4/3}N_{\rm eff}\right]T_\gamma^4.
$$