Comoving entropy
= Comoving entropy
{title2=$S_c=a^3s$}
<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.