An Etymological Dictionary of Astronomy and Astrophysics
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The era following the hadronic era, when the Universe consisted mainly of leptons and photons. It began when the temperature dropped below 10¹² degrees kelvin some 10^-4 seconds after the Big Bang, and it lasted until the temperature fell below 10¹⁰ degrees kelvin, at an era of about 1 second.

→ lepton; → era.