On compatibility of string effective action with an accelerating universe

Abstract

In this paper, we fully investigate the cosmological effects of the moduli-dependent one-loop corrections to the gravitational couplings of the string effective action to explain the cosmic acceleration problem in the early (and/or late) universe. These corrections comprise a Gauss–Bonnet (GB) invariant multiplied by universal non-trivial functions of the common modulus σ and the dilaton ϕ. The model exhibits several features of cosmological interest, including the transition between deceleration and acceleration phases. By considering some phenomenologically motivated ansatze for one of the scalars and/or the scale factor (of the universe), we also construct a number of interesting inflationary potentials. In all examples under consideration, we find that the model leads only to a standard inflation (w ⩾ −1) when the numerical coefficient δ associated with modulus-GB coupling is positive, while the model can also lead to a non-standard inflation (w < −1), if δ is negative. In the absence of (or trivial) coupling between the GB term and the scalars, there is no crossing between the w < −1 and w > −1 phases, while this is possible with non-trivial GB couplings, even for the constant dilaton phase of the standard picture. Within our model, after a sufficient amount of e-folds of expansion, the rolling of both fields ϕ and σ can be small. In turn, any possible violation of equivalence principle or deviations from the standard general relativity may be small enough to easily satisfy all astrophysical and cosmological constraints.