= Photon-to-baryon ratio from density parameters
{title2=$s_\gamma\simeq0.33[m_p^4c^3/(\rho_c\hbar^3)]^{1/4}\Omega_\gamma^{3/4}/\Omega_B$}
With $n_\gamma=0.24Q^3$, $u_\gamma=0.66\hbar cQ^4$, $Q=k_BT/(\hbar c)$ and $n_B=\Omega_B\rho_c/m_p$, eliminate $Q$ using $u_\gamma=\Omega_\gamma\rho_cc^2$. The prefactor is $0.24/0.66^{3/4}=0.328$. Here $\rho_c$ is the <mass-equivalent cosmological critical density>; inserting an energy-density critical value without its $c^2$ conversion would give incorrect dimensions. A numerical estimate needs photon and baryon density parameters as additional inputs.
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