Effective dark energy equation of state in interacting dark energy models

TL;DR

This paper derives an exact and approximate expression for the effective dark energy equation of state in models where dark matter and dark energy interact, highlighting potential misestimations in cosmological parameters if interactions are ignored.

Contribution

It introduces a new analytical framework for understanding the evolution of dark energy's equation of state in interacting models, including examples of mimicking effects.

Findings

Derived an exact expression for the effective dark energy equation of state.

Provided a simple analytical approximation for its evolution.

Showed non-phantom models can mimic phantom dark energy background evolution.

Abstract

In models where dark matter and dark energy interact non-minimally, the total amount of matter in a fixed comoving volume may vary from the time of recombination to the present time due to energy transfer between the two components. This implies that, in interacting dark energy models, the fractional matter density estimated using the cosmic microwave background assuming no interaction between dark matter and dark energy will in general be shifted with respect to its true value. This may result in an incorrect determination of the equation of state of dark energy if the interaction between dark matter and dark energy is not properly accounted for, even if the evolution of the Hubble parameter as a function of redshift is known with arbitrary precision. In this paper we find an exact expression, as well as a simple analytical approximation, for the evolution of the effective equation of state of dark energy, assuming that the energy transfer rate between dark matter and dark energy is described by a simple two-parameter model. We also provide analytical examples where non-phantom interacting dark energy models mimic the background evolution and primary cosmic microwave background anisotropies of phantom dark energy models.