Quantum Inhomogeneities in String Cosmology

TL;DR

This paper analyzes quantum fluctuations in string cosmology scenarios, deriving bounds on background parameters and conditions for generating scale-invariant spectra and magnetic seed fields, while discussing challenges in transitioning to the radiation era.

Contribution

It provides a detailed computation of quantum fluctuation spectra in specific string cosmology models and explores conditions for realistic cosmological features.

Findings

Bounds on moduli from fluctuation dominance

Conditions for scale-invariant axionic spectra

Potential issues with radiation era transition

Abstract

Within two specific string cosmology scenarios --differing in the way the pre- and post-big bang phases are joined-- we compute the size and spectral slope of various types of cosmologically amplified quantum fluctuations that arise in generic compactifications of heterotic string theory. By further imposing that these perturbations become the dominant source of energy at the onset of the radiation era, we obtain physical bounds on the background's moduli, and discuss the conditions under which both a (quasi-) scale-invariant spectrum of axionic perturbations and sufficiently large seeds for the galactic magnetic fields are generated. We also point out a potential problem with achieving the exit to the radiation era when the string coupling is near its present value.