Radiation mass density (${\rho }_{\mathrm{rad}}$)

For relativistic species at a common temperature, the mass-equivalent energy density is $\rho ={\pi }^2{g}_{\ast }(k_{B}T{)}^4/(30{\hslash }^3{c}^5)$. Using mass rather than energy density accounts for the powers of $c$ in the Friedmann equation.