The coincidence problem in the scenario of dark energy interacting with two fluids

TL;DR

This paper analyzes a cosmological model where dark energy interacts with dark matter and a third fluid, exploring stability conditions that shed light on the coincidence problem and the nature of the third fluid.

Contribution

It introduces a general framework for analyzing the stability of interacting dark energy models with two fluids, revealing conditions on the third fluid's pressure and possible identities.

Findings

Interaction proportional to the sum of DE and third fluid densities requires positive pressure for the third fluid.

Stability analysis constrains the equations of state of the three fluids involved.

Results suggest the third fluid could be radiation, unparticles, or warm dark matter.

Abstract

A cosmological model of dark energy interacting with dark matter and another general component of the universe is considered. The evolution equations for coincidence parameters r and s, which represent the ratios between the dark energy and the matter and the other cosmic fluid, respectively, are analyzed in terms of the stability of stationary solutions. The obtained general results allow to shed some light on the coincidence problem and in the equations of state of the three interacting fluids, due to the constraints imposes by the stability of the solutions. We found that for an interaction proportional to the sum of the DE density and the third fluid density, the hypothetical fluid must have positive pressure, which leads naturally to a cosmological scenario with radiation, unparticle or even some form of warm DM as the third interacting fluid.