Metric Perturbations in Dilaton-Driven Inflation

TL;DR

This paper calculates the spectrum of scalar and tensor metric perturbations generated during dilaton-driven inflation in string cosmology, revealing nearly Planckian spectra linked to fundamental parameters of the model.

Contribution

It provides a detailed perturbative analysis of metric perturbations in dilaton-driven inflation, addressing gauge issues and deriving nearly Planckian spectra.

Findings

Scalar and tensor spectra are nearly Planckian.

The spectra's temperature relates to fundamental string cosmology parameters.

A gauge-invariant approach ensures reliable perturbation calculations.

Abstract

We compute the spectrum of scalar and tensor metric perturbations generated, as amplified vacuum fluctuations, during an epoch of dilaton-driven inflation of the type occurring naturally in string cosmology. In the tensor case the computation is straightforward while, in the scalar case, it is made delicate by the appearance of a growing mode in the familiar longitudinal gauge. In spite of this, a reliable perturbative calculation of perturbations far outside the horizon can be performed by resorting either to appropriate gauge invariant variables, or to a new coordinate system in which the growing mode can be "gauged down". The simple outcome of this complicated analysis is that both scalar and tensor perturbations exhibit nearly Planckian spectra, whose common "temperature" is related to some very basic parameters of the string-cosmology background.