Realizing scale-invariant density perturbations in low-energy effective string theory

We discuss the realization of inflation and resulting cosmological perturbations in the low-energy effective string theory. In order to obtain nearly scale-invariant spectra of density perturbations and a suppressed tensor-to-scalar ratio, it is generally necessary that the dilaton field φ is effectively decoupled from gravity together with the existence of a slowly varying dilaton potential. We also study the effect of second-order corrections to the tree-level action which are the sum of a Gauss-Bonnet term coupled to φ and a kinetic term (∇φ)⁴. We find that it is possible to realize observationally supported spectra of scalar and tensor perturbations provided that the correction is dominated by the (∇φ)⁴ term even in the absence of the dilaton potential. When the Gauss-Bonnet term is dominant, tensor perturbations exhibit violent negative instabilities on small scales about a de Sitter background in spite of the fact that scale-invariant scalar perturbations can be achieved.