= Photon continuity equation
{title2=$\dot\delta_\gamma+\tfrac43\nabla\cdot\mathbf v_\gamma-4\dot\phi=0$}
In <Newtonian gauge in cosmology>, the <Free-streaming photon Boltzmann equation> is $\dot\Theta+\mathbf e\cdot\nabla\Theta=\dot\phi-\mathbf e\cdot\nabla\psi$. Angular averaging and the <photon angular temperature moments> give the displayed <continuity equation>. Its dipole yields the <photon Euler equation>, with the sign of the <anisotropic stress> term determined by the stress convention.
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