Photon-baryon velocity slip
= Photon-baryon velocity slip
{title2=$s=v_\gamma-v_b$}
The velocity slip is the difference between the bulk velocities of photons and baryons. <Thomson scattering> suppresses it at a rate $\Gamma(1+R^{-1})$, where $R=3\rho_b/(4\rho_\gamma)$ and $\Gamma=-\dot\tau>0$. Its first correction in the <tight-coupling approximation> produces heat-conduction damping of acoustic waves.