Adiabatic instability in coupled dark energy-dark matter models

TL;DR

This paper analyzes the stability of coupled dark energy-dark matter models, revealing that strong coupling leads to an adiabatic instability characterized by exponential growth of small-scale modes, impacting model viability.

Contribution

It demonstrates that strong coupling in dark energy-dark matter models causes an inherent instability in the adiabatic regime, clarifying conditions for avoiding this instability.

Findings

Strong coupling induces a Jeans-like instability.

Instability causes exponential growth of small-scale perturbations.

Weak or non-adiabatic regimes can evade the instability.

Abstract

We consider theories in which there exists a nontrivial coupling between the dark matter sector and the sector responsible for the acceleration of the universe. Such theories can possess an adiabatic regime in which the quintessence field always sits at the minimum of its effective potential, which is set by the local dark matter density. We show that if the coupling strength is much larger than gravitational, then the adiabatic regime is always subject to an instability. The instability, which can also be thought of as a type of Jeans instability, is characterized by a negative sound speed squared of an effective coupled dark matter/dark energy fluid, and results in the exponential growth of small scale modes. We discuss the role of the instability in specific coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and clarify for these theories the regimes in which the instability can be evaded due to non-adiabaticity or weak coupling.