Editing Big Bang (section)

=== Cooling ===
{{Main|Big Bang nucleosynthesis|Cosmic microwave background}}
[[File:2MASS LSS chart-NEW Nasa.jpg|thumb|right|upright=2.5|Panoramic view of the entire [[Infrared#Regions within the infrared|near-infrared]] sky reveals the distribution of galaxies beyond the [[Milky Way]]. Galaxies are color-coded by [[redshift]].|alt=A map of the universe, with specks and strands of light of different colors.]]
The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. [[Explicit symmetry breaking|Symmetry-breaking]] phase transitions put the [[fundamental force]]s of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10<sup>−12</sup> seconds.<ref name=":1" /><ref name="kolb_c7">{{harvnb|Kolb|Turner|1988|loc=chpt. 7}}</ref>

After about 10<sup>−11</sup> seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in [[particle accelerator]]s. At about 10<sup>−6</sup> seconds, [[quark]]s and [[gluon]]s combined to form [[baryon]]s such as [[proton]]s and [[neutron]]s. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass [[annihilation]] immediately followed, leaving just one in 10<sup>8</sup> of the original matter particles and none of their [[antiparticle]]s.<ref>{{cite web | last=Weenink | first=Jan | date=February 26, 2009 | title=Baryogenesis | url=https://webspace.science.uu.nl/~proko101/JanGWeenink_bg3.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://webspace.science.uu.nl/~proko101/JanGWeenink_bg3.pdf |archive-date=2022-10-09 |url-status=live | publisher=Tomislav Prokopec }}</ref> A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by [[photon]]s (with a minor contribution from [[neutrino]]s).

A few minutes into the expansion, when the temperature was about a billion [[kelvin]] and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's [[deuterium]] and [[helium]] [[atomic nucleus|nuclei]] in a process called [[Big Bang nucleosynthesis]] (BBN).<ref name="kolb_c4"/> Most protons remained uncombined as hydrogen nuclei.<ref name="peacock_c9"/>

As the universe cooled, the [[rest mass|rest energy]] density of matter came to gravitationally dominate over that of the photon and neutrino radiation at a time of about 50,000 years. At a time of about 380,000 years, the universe cooled enough that electrons and nuclei combined into neutral [[atom]]s (mostly [[hydrogen]]) in an event called [[Recombination (cosmology)|recombination]]. This process made the previously opaque universe transparent, and the photons that last scattered during this epoch comprise the cosmic microwave background.<ref name="peacock_c9">{{harvnb|Peacock|1999|loc=chpt. 9}}</ref>