Nonsingular Bianchi type I cosmological solutions from the 1-loop superstring effective action

Abstract

Nonsingular Bianchi type I solutions are found from the effective action with a superstring-motivated Gauss-Bonnet term. These anisotropic nonsingular solutions evolve from the asymptotic Minkowski region, subsequently superinflate, and then smoothly continue either to Kasner-type (expanding in two directions and shrinking in one direction) or to Friedmann-type (expanding in all directions) solutions. We also find a new kind of singularity which arises from the fact that the anisotropic expansion rates are a multiple-valued function of time. The initial singularity in the isotropic limit of this model belongs to this new kind of singularity. In our analysis the anisotropic solutions are likely to be singular when the superinflation is steep. As for the cosmic no-hair conjecture, our results suggest that the kinetic-driven superinflation of our model does not isotropize the space-time.