Disformal couplings and the dark sector of the universe

TL;DR

This paper investigates how disformal and conformal couplings between dark matter and dark energy influence cosmic evolution, revealing that large conformal couplings are still viable and that disformal couplings often cause instabilities, complicating observational distinctions.

Contribution

It provides a detailed analysis of dark sector interactions with both conformal and disformal couplings, highlighting stability issues and observational challenges.

Findings

Large conformal couplings are not necessarily disallowed with disformal terms.

Negative disformal couplings often lead to instabilities in the scalar field.

Disentangling disformal from conformal couplings observationally is challenging.

Abstract

Interactions between dark matter and dark energy, allowing both conformal and and disformal couplings, are studied in detail. We discuss the background evolution, anisotropies in the cosmic microwave background and large scale structures. One of our main findings is that a large conformal coupling is not necessarily disallowed in the presence of a general disformal term. On the other hand, we find that negative disformal couplings very often lead to instabilities in the scalar field. Studying the background evolution and linear perturbations only, our results show that it is observationally challenging to disentangle disformal from purely conformal couplings.