recombination

Recombination is the transition when matter density became equal to radiation density. Up until this point the universe had been "radiation dominated": there was more energy in the form of radiation than in the form of matter. However, radiation decreases in energy density faster than matter does (by an order of magnitude), so even though there was a billion to one ratio (or even greater) before in favour of radiation, the universe eventually became "matter dominated". At this point the Radiation Era is held to have ended.

Since radiation loses energy faster than matter, a matter dominated universe is inevitable. (source)

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