Observable gravitational waves in pre-big bang cosmology: an update

TL;DR

This paper explores the potential for pre-big bang string cosmology models to produce observable gravitational waves and scalar perturbations compatible with current data, emphasizing future tests via CMB and GW observations.

Contribution

It demonstrates that pre-big bang models can generate detectable gravitational wave backgrounds while matching scalar perturbation data, within a broad parameter space.

Findings

Pre-big bang models can produce observable gravitational waves.

Compatibility with Planck scalar perturbation data is achievable.

Future observations can test and constrain these models.

Abstract

In the light of the recent results concerning CMB observations and GW detection we address the question of whether it is possible, in a self-consistent inflationary framework, to simultaneously generate a spectrum of scalar metric perturbations in agreement with Planck data and a stochastic background of primordial gravitational radiation compatible with the design sensitivity of aLIGO/Virgo and/or eLISA. We suggest that this is possible in a string cosmology context, for a wide region of the parameter space of the so-called pre-big bang models. We also discuss the associated values of the tensor-to-scalar ratio relevant to the CMB polarization experiments. We conclude that future, cross-correlated results from CMB observations and GW detectors will be able to confirm or disprove pre-big bang models and -- in any case -- will impose new significant constraints on the basic string theory/cosmology parameters.