  We explore leptogenesis induced by the propagation of neutrinos in
gravitational backgrounds that may occur in string theory. The first background
is due to linear dilatons and the associated Kalb-Ramond field (axion) in four
non-compact space-time dimensions of the string, and can be described within
the framework of local effective lagrangians. The axion is linear in the time
coordinate of the Einstein frame and gives rise to a constant torsion which
couples to the fermion spin through a gravitational covariant derivative. This
leads to different energy-momentum dispersion relations for fermions and
anti-fermions and hence leptogenesis. The next two backgrounds go beyond the
local effective lagrangian framework. One is a stochastic (Lorentz Violating)
Finsler metric which again leads to different dispersion relations between
fermions and antifermions. The third background of primary interest is the one
due to populations of stochastically fluctuating point-like space-time defects
that can be encountered in string/brane theory (D0 branes). Only neutral matter
interacts non-trivially with these defects, as a consequence of charge
conservation. Hence, such a background singles out neutrinos among the Standard
Model excitations as the ones interacting predominantly with the defects. The
back-reaction of the defects on the space-time due to their interaction with
neutral matter results in stochastic Finsler-like metrics (similar to our
second background). On average, the stochastic fluctuations of the D0 brane
defects preserve Lorentz symmetry, but their variance is non-zero.
Interestingly, the particle/antiparticle asymmetry comes out naturally to
favour matter over antimatter in this third background, once the effects of the
kinematics of the scattering of the D branes with matter is incorporated.
