Universe

==== Photons ====
{{Main|Photon epoch}}
{{See also|Photino}}
A photon is the [[quantum]] of [[light]] and all other forms of [[electromagnetic radiation]]. It is the [[force carrier|carrier]] for the [[electromagnetic force]]. The effects of this [[force]] are easily observable at the [[microscopic scale|microscopic]] and at the [[macroscopic scale|macroscopic]] level because the photon has zero [[rest mass]]; this allows long distance [[fundamental interaction|interactions]].<ref name="OpenStax-college-physics"/>{{rp|1470}}

The photon epoch started after most leptons and anti-leptons were [[annihilation|annihilated]] at the end of the lepton epoch, about 10 seconds after the Big Bang. Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch. For the remainder of the photon epoch the universe contained a hot dense [[plasma (physics)|plasma]] of nuclei, electrons and photons. About 380,000 years after the Big Bang, the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms. As a result, photons no longer interacted frequently with matter and the universe became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in temperature and density detectable in the CMB were the early "seeds" from which all subsequent [[structure formation]] took place.<ref name=Allday2002 />{{rp|244–266}}
{{Big Bang timeline|state=collapsed}}