We consider a model of an expanding Universe in string theory that yields
`environmental' CPT violation for fermions, in the sense of different
dispersion relations for fermions and antifermions. These are induced by a
cosmological background with constant torsion provided by the Kalb-Ramond
antisymmetric tensor field (axion) of the string gravitational multiplet. This
effect induces different densities of neutrinos and antineutrinos while in
chemical equilibrium, offering new scenarios for leptogenesis and baryogenesis
even in the absence of CP violation.Comment: 8 pages, some clarifying remarks and references added, correction of
typos in the text below eq. (26), no effects on conclusion
