Dark coupling

TL;DR

This paper develops a framework for dark matter-dark energy interactions, identifying conditions for stability, and finds that certain interacting models fit observational data without instabilities or phantom crossing.

Contribution

It introduces a generic coupling model between dark sectors, establishes stability conditions, and demonstrates compatibility with observational data for specific interaction scenarios.

Findings

Interaction models can be stable and fit observational data.

Large interaction strengths are compatible with current cosmological measurements.

Some previously proposed unstable scenarios are ruled out by stability analysis.

Abstract

The two dark sectors of the universe - dark matter and dark energy - may interact with each other. Background and linear density perturbation evolution equations are developed for a generic coupling. We then establish the general conditions necessary to obtain models free from early time non-adiabatic instabilities. As an application, we consider a viable universe in which the interaction strength is proportional to the dark energy density. The scenario does not exhibit "phantom crossing" and is free from instabilities, including early ones. A sizeable interaction strength is compatible with combined WMAP, HST, SN, LSS and H(z) data. Neutrino mass and/or cosmic curvature are allowed to be larger than in non-interacting models. Our analysis sheds light as well on unstable scenarios previously proposed.