lepton epoch

{{Short description|Period in the evolution of the early universe}}

{{Physical cosmology}}

In cosmological models of the Big Bang, the lepton epoch was the period in the evolution of the early universe in which the leptons dominated the mass of the Universe. It started roughly 1 second after the Big Bang, after the majority of hadrons and anti-hadrons annihilated each other at the end of the hadron epoch.{{Cite journal|last1=Fromerth|first1=M.J.|last2=Kuznetsova|first2=I.|last3=Labun|first3=L.|last4=Letessier|first4=J.|last5=Rafelski|first5=J.|year=2012|title=From Quark-Gluon Universe to Neutrino Decoupling: 200 < T < 2MeV|url=http://www.actaphys.uj.edu.pl/vol43/abs/v43p2261|journal=Acta Physica Polonica B|language=en|volume=43|issue=12|pages=2261|doi=10.5506/APhysPolB.43.2261|issn=0587-4254|arxiv=1211.4297|s2cid=118448487}} During the lepton epoch, the temperature of the Universe was still high enough to create neutrino and electron-positron pairs. Approximately 10 seconds after the Big Bang, the temperature of the universe had fallen to the point where electron-positron pairs were gradually annihilated.{{cite web|title=Thermal history of the Universe and early growth of density fluctuations|url=http://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf|work=Guinevere Kauffmann|publisher=Max Planck Institute for Astrophysics|access-date=2016-01-06}} A small residue of electrons needed to charge-neutralize the Universe{{Clarify|date=March 2023|reason=The article needs to explain why an excess of electrons renders the Universe charge-neutral. It would seem to do the opposite.}} remained along with free streaming neutrinos: an important aspect of this epoch is the neutrino decoupling.{{Cite journal|last1=Birrell|first1=Jeremiah|last2=Yang|first2=Cheng Tao|last3=Rafelski|first3=Johann|year=2015|title=Relic neutrino freeze-out: Dependence on natural constants|journal=Nuclear Physics B|language=en|volume=890|pages=481–517|doi=10.1016/j.nuclphysb.2014.11.020|arxiv=1406.1759|bibcode=2015NuPhB.890..481B|doi-access=free}} The Big Bang nucleosynthesis epoch follows, overlapping with the photon epoch.{{cite web|title=First few minutes|work=Eric Chaisson|publisher=Harvard Smithsonian Center for Astrophysics|url=https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/fr_1/fr_1_part3.html|access-date=2016-01-06}}{{cite web|title=Timeline of the Big Bang|work=The physics of the Universe|url=http://www.physicsoftheuniverse.com/topics_bigbang_timeline.html|access-date=2016-01-06}}