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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.
The possible production of shadow matter during the period of cosmic inflation is considered. The superstring theory of Gross et al. (1985), which results in a gauge group E8 x E8, could, at low energies, result in the existence of two sectors: an observed sector associated with all familiar particles and interactions, and a hidden one whose particles couple only through gravitational interactions with ordinary matter. It is demonstrated here that if, in the early universe, an inflationary phase is associated with the breaking of one of the symmetries in the E8 x E8 theory, this strongly constrains the physics of both sectors if shadow matter is to be the missing mass in the universe.