  Propagation of fermion in curved space-time generates gravitational
interaction due to the coupling between spin of the fermion and space-time
curvature. This gravitational interaction, which is an axial-vector appears as
CPT violating term in the Lagrangian. It is seen that this space-time
interaction can generate neutrino asymmetry in Universe. If the back-ground
metric is spherically asymmetric, say, of a rotating black hole, this
interaction is non-zero, thus the net difference to the number density of the
neutrino and anti-neutrino is nonzero.
