Leptogenesis by curvature coupling of heavy neutrinos

We introduce a CP-violating coupling between the Ricci curvature and the heavy right-handed neutrinos. This splits the Majorana masses of the left- and the right-handed heavy neutrinos. When the heavy neutrinos decay, their decay rates are different into left and right chirality light neutrinos. A time-varying nonzero Ricci curvature can give rise to a net lepton asymmetry. The source of a nonzero curvature in a radiation era is the quantum fluctuations of primordial fields.