Grand unified theories and the lepton number of the Universe

Present observations are consistent with a number of neutrinos in the Universe much greater than the number of photons. We explore the possibility of the production and/or survival of a large lepton number within the framework of grand unified models. We find that standard unified theories with gauge-invariant initial asymmetries may give lepton numbers much greater than the photon number (even with zero initial baryon and lepton numbers), and lead to the possibility that the present Universe is hot in baryons (nBn_γ) but cold in leptons (n_νn_γ). Detection of a large lepton number may thus provide a probe of initial conditions.