Closed baryonic Universe from photon cooling
The Universe may be critically closed by baryons and not conflict with standard big-bang nucleosynthesis if the comoving number density of photons is reduced after nucleosynthesis. This is accomplished at a temperature of order 10 keV by recoupling the photons to a cold hidden sector via a photon-mixing mechanism. Signatures from supernovae, neutrino backgrounds, and nucleosynthesis are discussed.
Bartlett, J. G.↗