= Separately conserved visible and dark entropy
{title2=$\xi=s_{\rm SM}/s_\phi$}
After <thermal decoupling in cosmology>, two homogeneous sectors that evolve adiabatically without entropy injection satisfy $s_{\rm SM}a^3=\mathrm{constant}$ and $s_\phi a^3=\mathrm{constant}$, hence their <entropy density> ratio $\xi$ is fixed. At a common relativistic <temperature> before decoupling, $\xi=g_{*s}^{\rm SM}/g_{*s}^\phi$. Internal annihilations can change each sector's <temperature> and effective entropy degrees of freedom without changing its total comoving <entropy>.
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