Photon epoch

The early-universe era when photons dominated cosmic energy density. It ended as matter became the dominant component.

Radiation-dominated era: An early-universe period when relativistic particles, chiefly photons and neutrinos, dominated cosmic energy density. The photon epoch is the photon-dominated portion of this broader radiation-driven era.
Photon: The quantum of electromagnetic radiation, carrying energy and momentum without rest mass. Photons supplied the dominant energy density named by this epoch.
Big Bang nucleosynthesis: The formation of light atomic nuclei during the universe's first minutes. It occurred during the earlier radiation-dominated history leading into the photon epoch.
Acoustic oscillations: Pressure-driven waves in the early photon-baryon plasma that imprinted a preferred cosmic clustering scale. Photon pressure helped drive these waves before photons decoupled from matter.
Matter-radiation equality: The cosmological transition when matter and radiation had equal energy densities. This boundary marks the end of photon domination and the onset of matter domination.
Cosmic neutrino background: The relic population of neutrinos left from the hot early universe. Relativistic neutrinos contributed substantially to radiation density alongside photons.
Planck epoch: The earliest proposed era of the universe, when quantum-gravitational effects would have been essential. It represents a far earlier stage than the photon-dominated universe.
Silk damping: The erasure of small-scale density fluctuations through photon diffusion in the early universe. Photon transport smoothed perturbations while radiation and baryons were tightly coupled.
Cosmic microwave background: The relic electromagnetic radiation released when the early universe became transparent. Its photons preserve a record of conditions near the end of photon domination.
Baryonic matter: Ordinary matter made primarily of protons and neutrons, together with their atomic constituents. Its energy density grew in relative importance as photon energy diluted.

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Lepton epochRelated: It followed the transfer of energy from annihilating leptons to photons.