The Adiabatic Instability on Cosmology's Dark Side

TL;DR

This paper analyzes a small scale instability in coupled dark energy models, where strong matter-dark energy coupling leads to negative sound speed squared, offering a new way to constrain such theories.

Contribution

It generalizes previous instability analyses to a broader class of coupled dark energy models and provides a redshift-dependent method to test their viability.

Findings

Identifies a negative sound speed squared instability in strongly coupled models.

Provides a general framework applicable to many coupled dark energy theories.

Offers a new observational tool to rule out certain dark energy models.

Abstract

We consider theories with a nontrivial coupling between the matter and dark energy sectors. We describe a small scale instability that can occur in such models when the coupling is strong compared to gravity, generalizing and correcting earlier treatments. The instability is characterized by a negative sound speed squared of an effective coupled dark matter/dark energy fluid. Our results are general, and applicable to a wide class of coupled models and provide a powerful, redshift-dependent tool, complementary to other constraints, with which to rule many of them out. A detailed analysis and applications to a range of models are presented in a longer companion paper.