Evolution of cosmological perturbations in nonsingular string cosmologies

In a class of nonsingular cosmologies derived from higher-order corrections to the low-energy bosonic string action, we derive evolution equations for the most general cosmological scalar, vector, and tensor perturbations. In the large scale limit, the evolutions of both scalar and tensor perturbations are characterized by conserved quantities, the usual curvature perturbation in the uniform-field gauge, and the tensor-type perturbed metric. The vector perturbation is not affected, being described by the conservation of the angular momentum of the fluid component in the absence of any additional dissipative process. For the scalar- and tensor-type perturbations, we show how, given a background evolution during kinetic driven inflation of the dilaton field, we can obtain the final power spectra generated from the vacuum quantum fluctuations of the metric and the dilaton field during the inflation.