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Radiation Era

(Radiation-Dominated Era)
(early universe era when radiation pressure dominated)

Radiation Era (aka Radiation-Dominated Era) refers to a period after the Big Bang when the Energy Density of radiation (Electromagnetic Radiation) was greater than the energy density of matter. As the universe expanded, at about 47,000 years after the Big Bang, this reversed, and subsequent time is termed the Matter Era or Matter-Dominated Era.

In simple terms, as the universe expanded, radiation energy density changed at a rate proportional to R-4 whereas that of matter changed at a rate proportional to R-3, allowing for the reversal (where R is the Scale Factor, an increasing number indicating the relative size of the universe as time passes).