Photon-baryon diffusion damping equation
= Photon-baryon diffusion damping equation
When gravity and expansion can be neglected and $R$ is constant, the <photon monopole> satisfies
$$
\ddot\Theta_0+
\frac{k^2}{3(1+R)\Gamma}
\left(\frac{R^2}{1+R}+\frac{16}{15}\right)\dot\Theta_0+
\frac{k^2}{3(1+R)}\Theta_0=0.
$$
The first damping contribution comes from <photon-baryon velocity slip>; the second comes from the <photon quadrupole>. This is a local form of <Silk damping>.