A simple example of "non-minimal" Pre-Big Bang scenario

TL;DR

This paper presents a non-minimal pre-big bang cosmological model that can generate PTA signals by incorporating high-energy string sources and their effects on the evolution of the early universe.

Contribution

It introduces a fluid-dynamical model of string sources with effective viscosity, demonstrating how it breaks S-duality symmetry in tensor-axion spectra within a non-minimal pre-big bang scenario.

Findings

The model can produce PTA signals consistent with observations.

High-energy string sources influence the evolution of the string phase.

Viscosity effects break S-duality symmetry in perturbation spectra.

Abstract

We give an example of non-minimal pre-big bang scenario able to produce the PTA signal considering a modified evolution of the high-curvature string phase, including the contribution of high-energy string sources. We use a fluid-dinamical model of sources and show that their effective viscosity breaks the $S$-duality symmetry of the tensor-axion perturbation spectra, as in general expected for the non-minimal scenario.