Subhorizon radiation acoustic oscillations
= Subhorizon radiation acoustic oscillations
{title2=$\Delta_r\sim C\cos(k\tau/\sqrt3+\vartheta)$}
= Radiation acoustic oscillation
{synonym}
Pressure-supported radiation perturbations well inside the horizon oscillate rather than exhibit secular gravitational growth. In the ideal relativistic fluid, $c_s^2=1/3$ and the leading conformal frequency is $k/\sqrt3$. Their rapidly changing source has a small late-time response in the slowly varying cold-matter density mode.