Nonsingular Dilaton Cosmology in the String Frame

One of the paramount problems manifest in modern cosmological theories is that of the initial singularity. Gasperini and Veneziano have proposed the pre-big-bang (PBB) model [2] based on the assumption that the Universe evolved out of a ten-dimensional Minkowski spacetime which is an exact vacuum of string theory, in hopes of resolving the singularity problem. Recently, Brandenberger, Easther and Maia [3] managed to construct nonsingular cosmological solutions based on initial conditions resembling those of the pre-big-bang scenario. They began with dilaton gravity and the low energy effective action of string theory in the Einstein frame, adding to it specially constructed higher derivative terms. As the string frame is the fundamental frame with respect to string theory , it is important to reconsider the analysis of [3], performing the calculations directly in the string frame. The Einstein frame metric g̃μν is related to the string frame metric gμν via a conformal transformation [5]