Editing Universe (section)

==== Hadrons ====
{{Main|Hadron}}
A hadron is a [[composite particle]] made of [[quark]]s [[bound state|held together]] by the [[strong force]]. Hadrons are categorized into two families: [[baryon]]s (such as [[proton]]s and [[neutron]]s) made of three quarks, and [[meson]]s (such as [[pion]]s) made of one quark and one [[antiparticle|antiquark]]. Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern universe.<ref name=Allday2002/>{{rp|118–123}}

From approximately 10<sup>−6</sup> seconds after the [[Big Bang]], during a period known as the [[hadron epoch]], the temperature of the universe had fallen sufficiently to allow quarks to bind together into hadrons, and the mass of the universe was dominated by [[hadron]]s. Initially, the temperature was high enough to allow the formation of hadron–anti-hadron pairs, which kept matter and antimatter in [[thermal equilibrium]]. However, as the temperature of the universe continued to fall, hadron–anti-hadron pairs were no longer produced. Most of the hadrons and anti-hadrons were then eliminated in particle–antiparticle [[annihilation]] reactions, leaving a small residual of hadrons by the time the universe was about one second old.<ref name=Allday2002>{{cite book|last1=Allday|first1=Jonathan|title=Quarks, Leptons and the Big Bang|date=2002|publisher=IOP Publishing|isbn=978-0-7503-0806-9|edition=2nd}}</ref>{{rp|244–266}}