  We study the consequences of CPT and lepton number violation in neutrino
sector. For CPT violation we take gravity with which neutrino and antineutrino
couple differently. Gravity mixes neutrino and antineutrino in an unequal ratio
to give two mass eigenstates. Lepton number violation interaction together with
CPT violation gives rise to neutrino-antineutrino oscillation. Subsequently, we
study the neutrino flavor mixing and oscillation under the influence of
gravity. It is found that gravity changes flavor oscillation significantly
which influences the relative abundance of different flavors in present
universe. We show that the neutrinoless double beta decay rate is modified due
to presence of gravity- the origin of CPT violation, as the mass of the flavor
state is modified.
