Dark degeneracy I: Dynamical or interacting dark energy?

TL;DR

This paper explores the dark degeneracy in cosmology, showing how different dark energy models can produce similar observational effects, and analyzes how this degeneracy affects parameter constraints using cosmological data.

Contribution

It explicitly characterizes the conditions for dark degeneracy in the fluid description and performs a parameter estimation analysis to understand its impact on cosmological constraints.

Findings

Degeneracy exists between non-interacting dynamical and interacting vacuum dark energy models.

Dark degeneracy weakens constraints on m0 and , affecting tension assessments.

Phenomenology of different dark energy models can be remarkably similar despite underlying differences.

Abstract

We revisit the dark degeneracy that arises from the Einstein equations relating geometry to the total cosmic substratum but not resolving its individual components separately. We establish the explicit conditions for the dark degeneracy in the fluid description of the dark sector. At the background level, this degeneracy can be formally understood in terms of a unified dark sector Equation of State (EoS) that depends both on the dynamical nature of the dark energy (DE) as well as on its interaction with the pressureless dark matter. For linear perturbations, the degeneracy arises for specified DE pressure perturbations (or sound speed, equivalently) and DE anisotropic stress. Specializing to the degeneracy between non-interacting dynamical DE and interacting vacuum DE models, we perform a parameter estimation analysis for a range of dynamical DE parametrizations, where for illustration we explicitly break the degeneracy at the linear level by adopting a luminal sound speed for both scenarios. We conduct this analysis using cosmological background data alone and in combination with Planck measurements of the cosmic microwave background radiation. We find that although the overall phenomenology between the dynamical DE and interacting approaches is similar, there are some intriguing differences. In particular, there is an ambiguity in the strength of constraints on $\Omega_{m0}$ and $\sigma_8$, which are considerably weakened for interacting vacuum DE, indicating that the dark degeneracy can change the significance of tensions in cosmological parameters inferred from different data sets.