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The Universe may be critically closed by baryons and not conflict with standard big-band 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.
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Bartlett et al. (1991) studied this question.
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