= Baryon-to-photon dilution by entropy injection
{title2=$\eta^+/\eta^-=(1+\rho_\chi/\rho_\gamma)^{-3/4}$}
Thermal <photon number density> is proportional to $T^3$. A sudden photon heating factor $(1+r)^{1/4}$ therefore raises photon number by $(1+r)^{3/4}$ and dilutes the <baryon-to-photon ratio> by its inverse when baryon number is conserved. Comparing <Big Bang nucleosynthesis> and <Cosmic microwave background> baryon-density determinations can test entropy injection between the two epochs, subject to changes in the expansion rate and nuclear abundances.
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