= Photon-to-baryon ratio
{title2=$s_\gamma=n_\gamma/n_B=1/\eta$}
The reciprocal of the <baryon-to-photon ratio>, not a synonym for it. In a thermal photon gas this number ratio is proportional to, but differs from, the thermodynamic <entropy> per baryon. The exact photon entropy per photon is $4u_\gamma/(3k_BTn_\gamma)\simeq3.60$, so the photon-sector entropy per baryon in units of $k_B$ is about $3.60s_\gamma$. The entropy sector must be specified when other relativistic species contribute.
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