 Photon-to-baryon ratio

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 $4u_{\gamma }/(3k_{B}T{n}_{\gamma })\simeq 3.60$, 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.