The graviton background at early times in string cosmology

TL;DR

This paper explores the unique properties of the stochastic graviton background predicted by string cosmology, highlighting its large density at small energies and potential to reveal pre-big bang information.

Contribution

It identifies distinctive features of the graviton spectrum in string cosmology, emphasizing its large density and resilience to thermal distortion at early times.

Findings

Graviton density is parametrically large at small energies.

Spectrum shape remains undistorted by thermal effects in the Planck era.

Spectrum can carry information about the pre-big bang phase.

Abstract

We discuss some peculiar properties of the stochastic graviton background predicted by string cosmology. At Planckian times, for the values of the parameters of the model which are more interesting for the detection in gravitational wave experiments, the number density of gravitons is parametrically large compared to Planck density, and is peaked at small energies. The large parameter is related to the duration of the string phase and is a characteristic of string cosmology. The typical interaction time is parametrically larger than the Planck or string time. Therefore the shape of the graviton spectrum is not distorted by thermal effects in the Planck era and can carry informations on the pre-big bang phase suggested by string cosmology.