ACCELERATION OF THE UNIVERSE IN TYPE-0 NONCRITICAL STRINGS

Abstract

Presently there is preliminary observational evidence that the cosmological constant might be nonzero, and hence that our universe is eternally accelerating (de Sitter). This poses fundamental problems for string theory, since a scattering matrix is not well defined in such universes. In a previous paper we have presented a model, based on (nonequilibrium) noncritical strings, which is characterized by eventual "graceful" exit from a de Sitter phase. The model is based on a type-0 string theory, involving D3 brane worlds, whose initial quantum fluctuations induce the noncriticality. We argue in this paper that this model is compatible with the current observations. A crucial role for the correct "phenomenology" of the model is played by the relative magnitude of the flux of the five-form of the type 0 string to the size of five of the extra dimensions, transverse to the direction of the flux-field. We do not claim, at this stage at least, that this model is a realistic physical model for the universe, but we find it interesting that the model cannot be ruled out immediately, at least on phenomenological grounds.