Stringy bounces and gradient instabilities

TL;DR

This paper explores nonlocal dilaton potentials in bouncing cosmologies, identifies gradient instabilities as non-generic, and proposes criteria to avoid them, supporting a consistent spectrum of curvature perturbations.

Contribution

It introduces a gauge- and frame-invariant framework for analyzing fluctuations in nonlocal dilaton-driven bouncing models and provides criteria to prevent gradient instabilities.

Findings

Gradient instabilities are non-generic in these models.

Explicit examples show stability with a quasi-flat curvature spectrum.

Proposed criteria effectively avoid pathological instabilities.

Abstract

Bouncing solutions are obtained from a generally covariant action characterized by a potential which is a nonlocal functional of the dilaton field at two separated space-time points. Gradient instabilities are shown to arise in this context but they are argued to be nongeneric. After performing a gauge-invariant and frame-invariant derivation of the evolution equations of the fluctuations, a heuristic criterion for the avoidance of pathological instabilities is proposed and corroborated by a number of explicit examples that turn out to be compatible with a quasi-flat spectrum of curvature inhomogeneities for typical wavelengths larger than the Hubble radius.