Late universe dynamics with scale-independent linear couplings in the dark sector

TL;DR

This paper investigates scale-independent linear couplings in dark sector cosmological models, analyzing their dynamics and observational constraints, and finds that certain coupling forms are favored by supernova data.

Contribution

It introduces a general linear coupling model with scale-independent dynamics and assesses observational constraints, highlighting the preference for specific coupling forms in dark energy models.

Findings

Constant coupling part is unconstrained by SN Ia data.

Linear coupling proportional to dark energy density is preferred in strong coupling regimes.

Phantom models favor positive coupling, non-phantom models allow negative or zero coupling.

Abstract

We explore the dynamics of cosmological models with two coupled dark components with energy densities $\rho_A$ and $\rho_B$. We assume that the coupling is of the form $Q=Hq(\rho_A,\rho_B)$, so that the dynamics of the two components turns out to be scale independent, i.e. does not depend explicitly on the Hubble scalar $H$. With this assumption, we focus on the general linear coupling $q=q_o+q_A\rho_A+q_B\rho_B$, which may be seen as arising from any $q(\rho_A,\rho_B)$ at late time and leads in general to an effective cosmological constant. In the second part of the paper we consider observational constraints on the form of the coupling from SN Ia data, assuming that one of the components is cold dark matter. We find that the constant part of the coupling function is unconstrained by SN Ia data and, among typical linear coupling functions, the one proportional to the dark energy density $\rho_{A}$ is preferred in the strong coupling regime, $|q_{A}|>1$. While phantom models favor a positive coupling function, in non-phantom models, not only a negative coupling function is allowed, but the uncoupled sub-case falls at the border of the likelihood.