On phenomenological models of dark energy interacting with dark matter

TL;DR

This paper explores the implications of dark matter and dark energy interactions, showing they may affect gravitational dynamics and structure formation, and discusses the challenges in phenomenologically modeling these interactions.

Contribution

It demonstrates that dark sector coupling likely introduces additional terms in gravitational equations and highlights the difficulties in phenomenologically defining such interactions.

Findings

Dark sector coupling can induce viscous effects in cosmological equations.

Interactions may suppress small-scale structure formation.

Defining dark sector interactions phenomenologically is challenging.

Abstract

An interaction between dark matter and dark energy is usually introduced by a phenomenological modification of the matter conservation equations, while the Einstein equations are left unchanged. Starting from some general and fundamental considerations, in this work it is shown that a coupling in the dark sector is likely to introduce new terms also in the gravitational dynamics. Specifically in the cosmological background equations a bulk dissipative pressure, characterizing viscous effects and able to suppress structure formation at small scales, should appear from the dark coupling. At the level of the perturbations the analysis presented in this work reveals instead the difficulties in properly defining the dark sector interaction from a phenomenological perspective.