The Primordial Gravitational Wave Background in String Cosmology

TL;DR

This paper calculates the spectrum of primordial gravitational waves in string cosmology, considering the effects of the dilaton field and different modeling approaches, providing parameter-free predictions compatible with observations.

Contribution

It presents a detailed, parameter-free prediction of the gravitational wave spectrum in string cosmology, including the dilaton's effects and multiple modeling approaches.

Findings

Spectrum features depend on the dilaton field.

Explicit predictions without free parameters.

Compatibility with observational bounds.

Abstract

We find the spectrum P(w)dw of the gravitational wave background produced in the early universe in string theory. We work in the framework of String Driven Cosmology, whose scale factors are computed with the low-energy effective string equations as well as selfconsistent solutions of General Relativity with a gas of strings as source. The scale factor evolution is described by an early string driven inflationary stage with an instantaneous transition to a radiation dominated stage and successive matter dominated stage. This is an expanding string cosmology always running on positive proper cosmic time. A careful treatment of the scale factor evolution and involved transitions is made. A full prediction on the power spectrum of gravitational waves without any free-parameters is given. We study and show explicitly the effect of the dilaton field, characteristic to this kind of cosmologies. We compute the spectrum for the same evolution description with three differents approachs. Some features of gravitational wave spectra, as peaks and asymptotic behaviours, are found direct consequences of the dilaton involved and not only of the scale factor evolution. A comparative analysis of different treatments, solutions and compatibility with observational bounds or detection perspectives is made.