Baryon-to-photon ratio ( $η$ )

The baryon-to-photon ratio is the number density of baryons divided by the number density of photons. In the present universe it is of order

1 0^{- 9}

.

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- Baryon-to-photon dilution by entropy injection Baryon-to-photon ratio

Baryon-to-photon dilution by entropy injection ( $η^{+} / η^{-} = (1 + ρ_{χ} / ρ_{γ})^{- 3/4}$ )

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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