The String Coupling Accelerates the Expansion of the Universe

Generic cosmological models in non-critical string theory have a time-dependent dilaton background at a late epoch. The cosmological deceleration parameter q0 is given by the square of the string coupling, g2s, up to a negative sign. Hence the expansion of the Universe must accelerate eventually, and the observed value of q0 coresponds to g 2 s 0:6. In this scenario, the string coupling is asymptotically free at large times, but its present rate of change is imperceptibly small