Cosmological adiabatic sound speed
= Cosmological adiabatic sound speed
{title2=$c_a^2=\bar P'/\bar\rho'$}
The squared adiabatic sound speed is the ratio of background pressure and energy-density time derivatives. For a <barotropic equation of state> it is $dP/d\rho$. The <non-adiabatic pressure perturbation> is the difference $\delta P-c_a^2\delta\rho$; in a general field or multicomponent model this background quantity need not equal the rest-frame propagation speed.