Dark energy and cosmological solutions in second-order string gravity

We study the cosmological evolution based on a D-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (a dilaton or compactification modulus). A barotropic perfect fluid coupled to the scalar field is also allowed. Phase space analysis and the stability of asymptotic solutions are performed for a number of models which include (i) a fixed scalar field, (ii) a linear dilaton in the string frame, and (iii) a logarithmic modulus in the Einstein frame. We confront analytical solutions with observational constraints for the deceleration parameter and show that Gauss-Bonnet gravity alone (i.e., with no matter fields) may not explain the current acceleration of the universe. We also study the future evolution of the universe using the Gauss-Bonnet parametrization and find that big rip singularities can be avoided even in the presence of a phantom fluid because of the balance between the fluid and curvature corrections. A non-minimal coupling between the fluid and the modulus field also opens up the interesting possibility of avoiding a big rip regardless of the details of the fluid equation of state.