Interacting dark energy models in Cosmology and large-scale structure observational tests

TL;DR

This paper investigates models of interacting dark energy and dark matter in cosmology, deriving analytical growth rate expressions, testing them against observational data, and assessing galaxy cluster virial states to explore the dark sector interaction.

Contribution

It introduces phenomenological interaction models between dark energy and dark matter, derives analytical growth rate formulas, and tests these models with observational data and galaxy cluster analyses.

Findings

Analytic growth rate expressions are derived for interaction models.

Non-zero interaction cannot be fitted by the growth index approximation in one model.

Galaxy cluster virial ratios provide constraints on dark sector interactions.

Abstract

Modern Cosmology offers us a great understanding of the universe with striking precision, made possible by the modern technologies of the newest generations of telescopes. The standard cosmological model, however, is not absent of theoretical problems and open questions. One possibility that has been put forward is the existence of a coupling between dark sectors. The idea of an interaction between the dark components could help physicists understand why we live in an epoch of the universe where dark matter and dark energy are comparable in terms of energy density, which can be regarded as a coincidence given that their time evolutions are completely different. We introduce the interaction phenomenologically and proceed to test models of interaction with observations of redshift-space distortions. In a flat universe composed only of those two fluids, we consider separately two forms of interaction, through terms proportional to the densities of both dark energy and dark matter. An analytic expression for the growth rate approximated as $f = \Omega_{\mathrm{DM}}^{\gamma}$, where $\Omega_{\mathrm{DM}}$ is the percentage contribution from the dark matter to the content of the universe and $\gamma$ is the growth index, is derived in terms of the interaction strength and of other parameters of the model in the first case, while for the second model we show that a non-zero interaction cannot be accommodated by the index growth approximation. The successful expressions obtained are then used to compare the predictions with growth of structure observational data in a MCMC code. We also employ observations of galaxy clusters to assess their virial state via the modified Layzer-Irvine equation in order to detect signs of an interaction, obtaining measurements of observed virial ratios, interaction strength, rest virial ratio and departure from equilibrium for a set of clusters. [abridged]