Baryon loading parameter
= Baryon loading parameter
{title2=$R=3\bar\rho_b/(4\bar\rho_\gamma)$}
The inertia ratio of the <photon-baryon fluid> is $R=3\bar\rho_b/(4\bar\rho_\gamma)$. Photon momentum is weighted by the radiation enthalpy $4\bar\rho_\gamma/3$, so this ratio controls momentum exchange, the common sound speed and the offset of the acoustic oscillator. Background conservation gives $R\propto a$ and $\dot R=\mathcal H R$.