Gauss-Bonnet Chern-Simons gravitational wave leptogenesis
  The gravitational Chern-Simons term coupled to an evolving axion is known to
generate lepton number through the gravitational anomaly. We examine this
leptogenesis scenario in the presence of the Gauss-Bonnet term over and above
the gravitational Chern-Simons term. We find that the lepton production can be
exponentially enhanced. The Gauss-Bonnet term creates CP-violating instability
of gravitational waves that may appear transiently after inflation, and during
the period of instability elliptically polarized gravitational waves are
exponentially amplified at sub-horizon scales. This instability does not affect
the spectrum of the cosmic microwave background as it occurs at much shorter
length scales. In a typical scenario based on natural inflation, the observed
baryon asymmetry of the Universe corresponds to the UV cutoff scale at
$10^{14-16}$ GeV.
