Abstract
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.
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