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Inphysical cosmology, thehadron era[1]: 209 is an range of time in an obsolete model of the very early universe.[2]: xl It was said to have begun at a time of 10−44 seconds, or at 10−8 seconds,[3] and ended at 10−4 seconds. The temperature was high enough to allow the formation of hadron/anti-hadron pairs, which kept matter and anti-matter inthermal equilibrium. After the discovery ofquarks andgluons in the 1970s the model based on hadrons no longer made sense.[2]
However, hadron-antihadron pairs were only abundant for a brief time between about 5×10−5 seconds, the time of theQCD phase transition, and about 7×10−5 seconds, when the temperature of the universe dropped below thepion mass.[4] Before the QCD phase transition, during thequark epoch, the universe was hot enough thatquarks did not combine to form hadrons. At temperatures below the pion mass, most of the hadrons and anti-hadrons were eliminated inannihilation reactions, leaving the Universe dominated by photons, neutrinos and electron-positron pairs. This subsequent period is referred to as thelepton era.[3]
Combining particle physics models with the standardLambda-CDM model of cosmological expansion, suggests when expansion cooled the universe to a transition temperature around 150MeV hadron formation was favored. From this time untilneutrino decoupling,antimatter is an important constiutent of the universe.[5]
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