S-duality Invariant Perturbations in String Cosmology

TL;DR

This paper explores how S-duality symmetry influences the generation of curvature and isocurvature perturbations in string cosmology models, including pre-big bang scenarios, revealing conditions for scale-invariant spectra.

Contribution

It demonstrates the use of SL(2,R) symmetry to derive spectral indices of perturbations in axion-dilaton-moduli models, including a string-motivated case with dual axion fields.

Findings

Axion fluctuations can produce scale-invariant spectra.

SL(2,R) symmetry constrains perturbation spectra.

String-motivated models with multiple axions are analyzed.

Abstract

We investigate the generation of curvature and isocurvature (dilaton, moduli and axion) perturbations in a general class of axion-dilaton-moduli models,including the pre-big bang scenario. Allowing for an arbitrary coupling constant between the dilaton field and the axion field, we exploit the SL(2,R) symmetry of the theory to obtain the spectral indices of the field perturbations in a pre-big bang type scenario. Axion field fluctuations about a homogeneous background field can yield a scale-invariant (Harrison-Zel'dovich) spectrum. As an example we present a string-motivated case with SL(2,R)xSL(2,R) symmetry, where a second axion field arises from the compactification of the ten-dimensional theory to four dimensions.