The period from about 10−43 s (the Planck era) to 30 000 years after the Big Bang. During this time, the expansion of the Universe was dominated by the effects of radiation or high-speed particles (at high energies, all particles behave like radiation). The lepton era and the hadron era are both subdivisions of the radiation era. The radiation era was followed by the matter era, during which slow-moving particles dominated the expansion of the Universe.
Subjects: Astronomy and Astrophysics.