Interaction in the dark sector

TL;DR

This paper proposes phenomenological models for the interaction between dark matter and dark energy, based on their energy density ratios, and demonstrates how to distinguish these interactions from noninteracting models using background evolution data.

Contribution

It introduces criteria for formulating interaction terms between dark matter and dark energy that are independent of gravity theories, based on their density evolution.

Findings

Proposed new phenomenological interaction models compatible with cosmic evolution.

Identified degeneracies with noninteracting models that can be broken at the background level.

Analyzed existing interaction terms from literature for consistency with observations.

Abstract

It may well happen that the two main components of the dark sector of the Universe, dark matter and dark energy, do not evolve separately but interact nongravitationally with one another. However, given our current lack of knowledge on the microscopic nature of these two components there is no clear theoretical path to determine their interaction. Yet, over the years, phenomenological interaction terms have been proposed on mathematical simplicity and heuristic arguments. In this paper, based on the likely evolution of the ratio between the energy densities of these dark components, we lay down reasonable criteria to obtain phenomenological, useful, expressions of the said term independent of any gravity theory. We illustrate this with different proposals which seem compatible with the known evolution of the Universe at the background level. Likewise, we show that two possible degeneracies with noninteracting models are only apparent as they can be readily broken at the background level. Further, we analyze some interaction terms that appear in the literature.