An Etymological Dictionary of Astronomy and Astrophysics
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A critical change in the history of the Universe, which occurred after the radiation era, when the density of energy contained within matter exceeded the density of energy contained within radiation. This transition started about 5000 years after the Big Bang, when the temperature had fallen to 3 x 10⁴ K. Later, 380 000 years after the Big Bang, when the temperature was 3000 K, matter and radiation were no longer coupled together and the Universe became transparent.

→ matter; → era.