Gravitational Leptogenesis
  We introduce a dimension five CP violating coupling between the Ricci scalar and fermions. This operator splits the energy level between neutrinos and anti-neutrinos and can generate a lepton-asymmetry in the the radiation era if heavy Majorana neutrinos decouple at the GUT scale. This operator can also generate a lepton asymmetry during warm inflation if the light neutrinos have a Majorana mass m_\nu \simeq 0.25 eV which is observable in double beta decay experiments. 
Gaetano Lambiase, Subhendra Mohanty