The Photon-to-Neutrino Temperature Ratio When Entropy Is Released After Neutrino Decoupling

In standard cosmology the photon-to-neutrino temperature ratio is fixed at (11/4)^(1/3) because electron-positron annihilation heats the photons after neutrinos have decoupled. If post-bounce reheating or a late entropy release overlaps the decoupling era, part of that heat reaches the neutrinos and the ratio falls. This study computes the ratio as a function of the shared fraction and identifies how much sharing the observed radiation density tolerates. The study is computed from stated assumptions and is not a measurement. If a fraction 0 of the annihilation-era entropy release is shared with the neutrino sector, the photon-to-neutrino temperature ratio falls from its standard value to 1.401. If a fraction 0.1 of the annihilation-era entropy release is shared with the neutrino sector, the photon-to-neutrino temperature ratio falls from its standard value to 1.355.