ON THE GRAVITY THEORIES AND COSMOLOGY FROM STRINGS

We have studied the conformal properties and cosmological solutions of the four-dimensional gravitational action for different supersymmetric and non-supersymmetric string scenarios, assuming flat directions in the configuration space of moduli and dilaton fields. We have explicitly shown the equivalence between different conformal frames and chosen a preferred frame in which the relevant masses are constant. The theory of gravity for the radiation dominated era is General Relativity. During the matter dominated era all analyzed examples are in conflict with the bounds on the time variation of gauge couplings and/or with the post-newtonian bounds of General Relativity.