The interval, commencing about 10−5 s after the Big Bang, in which the various kinds of lepton were the main contributors to the density of the Universe. Pairs of leptons and antileptons were created in large numbers in the early Universe but, as the Universe cooled, most lepton species were annihilated. The lepton era immediately followed the hadron era, and is a subdivision of the radiation era. The end of the lepton era is usually taken to have occurred when most electron–positron pairs were annihilated, at a temperature of 5 × 109 K, about 1 second after the Big Bang.
Subjects: Astronomy and Astrophysics.