Such processes could have hypothetically created leptons in the early universe. In these processes baryon number is also non-conserved, and thus baryons should have been created along with leptons. Such non-conservation of baryon number is indeed assumed to have happened in the early universe, and is known as baryogenesis. However, in some theoretical models, it is suggested that leptogenesis also occurred prior to baryogenesis; thus the term leptogenesis is often used to imply the non-conservation of leptons without corresponding non-conservation of baryons. In the Standard Model, the difference between the lepton number and the baryon number is precisely conserved, so that leptogenesis without baryogenesis is impossible. Thus such leptogenesis implies extensions to the Standard Model.