In
physical cosmology, the
hadron epoch was the period in the evolution of the early universe during which the mass of the universe was dominated by
hadrons. It started approximately 10
-6 seconds after the
Big Bang, when the temperature of the universe had fallen sufficiently to allow the
quarks from the preceding
quark epoch to bind together into hadrons. Initially the temperature was high enough to allow the formation of hadron/anti-hadron pairs, which kept matter and anti-matter 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
annihilation reactions, leaving a small residue of hadrons. The elimination of anti-hadrons was completed by one second after the Big Bang, when the following
lepton epoch began.