= Photon energy density
{title2=$\rho_\gamma=\int d^3p\,Ef$}
The temporal-temporal component of the photon <stress-energy tensor> in a local <orthonormal tetrad>. For an isotropic <one-particle distribution function> expressed using comoving energy $\epsilon=aE$, it is $\bar\rho_\gamma=4\pi a^{-4}\int d\epsilon\,\epsilon^3\bar f(\epsilon)$. A thermal <photon temperature perturbation> gives a first-order directional energy perturbation four times its fractional temperature change. Boundary conditions on the distribution justify this factor through <integration by parts>.
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