Photon-baryon acoustic oscillator
= Photon-baryon acoustic oscillator
{title2=$\omega^2=k^2/[3(1+R)]$}
With $v_b=v_\gamma$ at leading order and $c_s^2=1/[3(1+R)]$, the <photon continuity equation> and total Euler equation give $\ddot\delta_\gamma+\dot R\dot\delta_\gamma/(1+R)+k^2\delta_\gamma/[3(1+R)]=4\ddot\phi+4\dot R\dot\phi/(1+R)-4k^2\psi/3$. The <baryon loading parameter> changes inertia and shifts the gravitational equilibrium. The leading equations omit diffusion damping.