Gravitational Leptogenesis, Reheating, and Models of Neutrino Mass
  Gravitational leptogenesis refers to a class of baryogenesis models in which
the matter-antimatter asymmetry of the universe arises through the standard
model lepton-number gravitational anomaly. In these models chiral gravitational
waves source a lepton asymmetry in standard model neutrinos during the
inflationary epoch. We point out that gravitational leptogenesis can be
successful in either the Dirac or Majorana neutrino mass scenario. In the Dirac
mass scenario, gravitational leptogenesis predicts a relic abundance of sterile
neutrinos that remain out of equilibrium, and the lepton asymmetry carried by
the standard model sector is unchanged. In the Majorana mass scenario, the
neutrinos participate in lepton-number-violating interactions that threaten to
washout the lepton asymmetry during post-inflationary reheating. However, we
show that a complete (exponential) washout of the lepton asymmetry is prevented
if the lepton-number-violating interactions go out of equilibrium before all of
the standard model Yukawa interactions come into equilibrium. The baryon and
lepton asymmetries carried by right-chiral quarks and leptons are sequestered
from the lepton-number violation, and the washout processes only suppress the
predicted baryon asymmetry by a factor of $\varepsilon_{\rm w.o.} = \pm
O(0.1)$. The sign of $\varepsilon_{\rm w.o.}$ depends on the model parameters
in such a way that a future measurement of the primordial gravitational wave
chirality would constrain the scale of lepton-number violation (heavy Majorana
neutrino mass).
