An early epoch in the history of the → Universe when the
radiation → density parameter was Ω_{r}≈ 1,
while other density parameters had negligible contributions. A
radiation-dominated Universe is characterized by
R/R_{0} ∝ t^{1/2}, where
R is the → cosmic scale factor and t is time.
According to the → Big Bang model, the radiation-dominated phase
was followed by the → matter-dominated phase.