Lepton epoch

An early-universe era after the hadron epoch when leptons dominated the energy density. It ended as most leptons and antileptons annihilated, heating the photon bath.

Electron–positron annihilation: The reaction in which an electron and positron convert mainly into photons. This annihilation released much of the epoch’s lepton rest energy into radiation.
Lepton: A fundamental fermion that does not experience the strong interaction. Leptons supplied the particles whose abundance gave this epoch its name.
Big Bang: The expansion of the universe from an earlier hot, dense state, beginning about 13.8 billion years ago. The lepton epoch was a brief stage in the universe’s early expansion.
Baryon asymmetry: The observed excess of matter over antimatter in the universe. It helps explain why annihilation left residual matter rather than eliminating all particles.
Neutrino decoupling: The early-universe transition after which neutrinos interacted too rarely to remain in thermal equilibrium with other particles. Neutrinos stopped sharing the photon bath’s later heating from electron–positron annihilation.
Electron: A negatively charged elementary lepton found in atoms and many particle reactions. Electrons and positrons remained abundant as the universe cooled through this era.
Planck epoch: The earliest proposed era of the universe, when quantum-gravity effects are expected to dominate. It lies far earlier than the particle-physics eras that include lepton dominance.
Leptogenesis: A class of theories proposing that a lepton–antilepton imbalance arose in the early universe. A primordial lepton asymmetry could affect particle abundances during this era.
Big Bang nucleosynthesis: The production of the universe’s light atomic nuclei during its first minutes. Nucleosynthesis followed this epoch, after annihilation and cooling altered the radiation bath.
Neutrino: A neutral, weakly interacting elementary particle that exists in three known flavors. Relic neutrinos carried a substantial share of the epoch’s relativistic energy.

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Hadron eraRelated: It followed the hadron era as annihilations left a universe with few hadrons.