(cosmology, now chiefly historical) In the Big Bang theory, the hot and dense plasma that made up the matter in the cosmos following the initial baryogenesis at an early stage of its expansion and cooling, from which the first atoms formed and photons decoupled. The emission of photons in this phase is regarded as the source of the cosmic microwave background.
Related dictionary entries include baryogenesis, lambda, superbounce, leptogenesis, inflation, cosmoparticle.
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