Cosmological entropy perturbation
= Cosmological entropy perturbation
A cosmological entropy perturbation changes the relative composition of cosmic fluids while leaving the total curvature initially unchanged. For <cold dark matter> and photons, a common relative entropy variable is
$$
S_{c\gamma}=\delta_c-\frac34\delta_\gamma.
$$
= Isocurvature perturbation
{synonym}