Slow-Roll Suppression of Adiabatic Instabilities in Coupled Scalar Field-Dark Matter Models

TL;DR

This paper investigates how slow-roll dynamics in coupled scalar field-dark matter models suppress adiabatic instabilities, highlighting conditions under which instabilities can still develop, especially in non-adiabatic regimes.

Contribution

It demonstrates that slow-roll behavior stabilizes perturbations in coupled scalar field-dark matter models, providing insights into the conditions that lead to or prevent instabilities.

Findings

Slow-roll dynamics suppress adiabatic instabilities.

Instabilities can occur for large coupling constants.

Non-adiabatic solutions with oscillations can amplify instabilities.

Abstract

We study the evolution of linear density perturbations in the context of interacting scalar field-dark matter cosmologies, where the presence of the coupling acts as a stabilization mechanism for the runaway behavior of the scalar self-interaction potential as in the case of the Chameleon model. We show that in the "adiabatic" background regime of the system the rise of unstable growing modes of the perturbations is suppressed by the slow-roll dynamics of the field. Furthermore the coupled system behaves as an inhomogeneous adiabatic fluid. In contrast instabilities may develop for large values of the coupling constant, or along non-adiabatic solutions, characterized by a period of high-frequency dumped oscillations of the scalar field. In the latter case the dynamical instabilities of the field fluctuations, which are typical of oscillatory scalar field regimes, are amplified and transmitted by the coupling to dark matter perturbations.