In physical cosmology, the radiation-dominated era was the first of the three phases of the known universe, the other two being the matter-dominated era and the dark-energy-dominated era. During this era, the dynamics of the universe were set by radiation, which refers generally to the constituents of the universe which moved relativistically, principally photons and neutrinos.

The radiation-dominated era occurred from the universe's creation until it was approximately 47,000 years old,[1] at a cosmological redshift of approximately 3600,[2] at which point the expansion of the universe caused the mass–energy density to surpass the radiation energy density.[3]

Even though the matter energy density surpassed the radiation density at this point, the universe remained optically thick to radiation until a cosmological redshift of approximately 1100, when the universe was about 378,000 years old. This second moment in time (close to the time of recombination) at which point the photons in that compose the cosmic microwave background radiation were last scattered, is often mistaken as marking the end of the radiation era.

For a radiation-dominated universe the evolution of the scale factor in the Friedmann–Lemaître–Robertson–Walker metric is obtained solving the Friedmann equations:

$a(t)\propto t^{1/2}. \,$[4]