Comoving entropy ($S_c=a^{3}s$)

Comoving entropy is the entropy density multiplied by the physical volume factor $a^3$ for a fixed comoving region in an expanding homogeneous universe. In adiabatic expansion it is conserved. Separate decoupled radiation sectors conserve their own comoving entropy, so disappearance of massive species can heat the still-equilibrated sector relative to a decoupled species.