Universe

==== Leptons ====
{{Main|Lepton}}
A lepton is an [[elementary particle|elementary]], [[half-integer spin]] particle that does not undergo strong interactions but is subject to the [[Pauli exclusion principle]]; no two leptons of the same species can be in exactly the same state at the same time.<ref>{{cite encyclopedia
 |title=Lepton (physics)
 |url=http://www.britannica.com/EBchecked/topic/336940/lepton
 |encyclopedia=[[Encyclopædia Britannica]]
 |access-date=September 29, 2010
|archive-date=May 11, 2015
|archive-url=https://web.archive.org/web/20150511203531/http://www.britannica.com/EBchecked/topic/336940/lepton
 |url-status=live
 }}</ref> Two main classes of leptons exist: [[electric charge|charged]] leptons (also known as the ''electron-like'' leptons), and neutral leptons (better known as [[neutrino]]s). Electrons are stable and the most common charged lepton in the universe, whereas [[muon]]s and [[tau (particle)|taus]] are unstable particles that quickly decay after being produced in [[high energy physics|high energy]] collisions, such as those involving [[cosmic ray]]s or carried out in [[particle accelerator]]s.<ref>{{cite book
 | last=Harari | first=H.
 | year=1977
 | chapter=Beyond charm
 | title=Weak and Electromagnetic Interactions at High Energy, Les Houches, France, Jul 5 – Aug 14, 1976
 | editor1-last=Balian | editor1-first=R.
 | editor2-last=Llewellyn-Smith | editor2-first=C.H.
 | series=Les Houches Summer School Proceedings
 | volume=29 | page=613
 | publisher=[[North-Holland Publishing Company|North-Holland]]
}}</ref><ref>{{cite conference
 |author=Harari H.
 |title=Three generations of quarks and leptons
 |url=https://www.slac.stanford.edu/cgi-bin/getdoc/slac-pub-1974.pdf
 |book-title=Proceedings of the XII Rencontre de Moriond
 |editor1=E. van Goeler
 |editor2=Weinstein R.
 |page=170
 |year=1977
 |id=SLAC-PUB-1974
 |conference=
 |access-date=May 29, 2020
|archive-date=May 13, 2020
|archive-url=https://web.archive.org/web/20200513180308/https://www.slac.stanford.edu/cgi-bin/getdoc/slac-pub-1974.pdf
 |url-status=live
 }}</ref> Charged leptons can combine with other particles to form various [[composite particle]]s such as [[atom]]s and [[positronium]]. The [[electron]] governs nearly all of [[chemistry]], as it is found in [[atom]]s and is directly tied to all [[chemical property|chemical properties]]. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter.<ref>{{cite press release
 |publisher=[[Massachusetts Institute of Technology|MIT News Office]]
 |date=April 18, 2007
|title=Experiment confirms famous physics model
 |url=http://web.mit.edu/newsoffice/2007/neutrino.html
 |access-date=June 2, 2015
|archive-date=July 5, 2013
|archive-url=https://web.archive.org/web/20130705100832/http://web.mit.edu/newsoffice/2007/neutrino.html
 |url-status=live
 }}</ref>

The [[lepton epoch]] was the period in the evolution of the early universe in which the [[lepton]]s 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]]. During the lepton epoch the temperature of the universe was still high enough to create lepton–anti-lepton pairs, so leptons and anti-leptons were in thermal equilibrium. Approximately 10 seconds after the Big Bang, the temperature of the universe had fallen to the point where lepton–anti-lepton pairs were no longer created.<ref>{{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=January 6, 2016|archive-date=August 21, 2016|archive-url=https://web.archive.org/web/20160821041542/http://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf|url-status=live}}</ref> Most leptons and anti-leptons were then eliminated in [[annihilation]] reactions, leaving a small residue of leptons. The mass of the universe was then dominated by [[photon]]s as it entered the following [[photon epoch]].<ref>{{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=January 6, 2016|archive-date=December 4, 2013|archive-url=https://web.archive.org/web/20131204050252/https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/fr_1/fr_1_part3.html|url-status=live}}</ref><ref>{{cite web|title=Timeline of the Big Bang|work=The physics of the Universe|url=https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html|access-date=January 6, 2016|archive-date=March 30, 2020|archive-url=https://web.archive.org/web/20200330140345/https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html|url-status=live}}</ref>
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