Comoving entropy conservation
= Comoving entropy conservation
{title2=$g_{*s}T^3a^3=\mathrm{constant}$}
Adiabatic expansion conserves <entropy> in a comoving volume: $s a^3$ is constant. With $s=(2\pi^2/45)g_{*s}T^3$, it gives $(a_i/a_0)^3=(g_{*s,0}/g_{*s,i})(T_0/T_i)^3$. Changes in <entropy> <degrees of freedom> must therefore be retained when transporting an early relic density to today, rather than setting $aT$ constant across all particle thresholds.
// Target: cosmology.bigb