Flavour mixing in an expanding universe

Motivated by a microscopic model of string‐inspired foam, in which foamy structures are provided by brany point‐like defects (D‐particles) in space‐time, we discuss flavour mixing in curved space‐time for spin‐0 and spin‐1/2 particles. This can be view as the low energy limit of flavour non‐preserving interactions of stringy matter excitations with the defects, and non‐trivial space‐time background induced by quantum fluctuations of the D‐particles. We show, at late epochs of the Universe, that both the fermionic and the bosonic vacuum condensate behaves as a fluid with negative pressure and positive energy; however the equation of state has wfermion > ‐1/3 and therefore the contribution of fermion‐fluid flavour vacuum alone could not yield accelerating Universes. On the other hand, for the boson fluid the equation of state is, for late eras, close to wboson → ‐1, and hence overall the D‐foam universe appears accelerating at late eras.