Tale of stable interacting dark energy, observational signatures, and the $H_0$ tension

TL;DR

This work constructs two stable interacting dark energy models where the DM–DE coupling scales with the DE equation of state to avoid early-time perturbation instabilities across the full $w_x$ range. By analyzing Planck, BAO, JLA, and cosmic chronometer data, the authors constrain the coupling $\xi$ and find a mild preference for phantom $w_x$ ($w_x<-1$), with energy transfer tending from DE to DM ($Q<0$). The models modestly alleviate the $H_0$ tension but do not resolve the $\sigma_8$ discrepancy, and Bayesian evidence continues to favor $\Lambda$CDM over the IDE scenarios across data combinations. Overall, the approach provides a well-motivated framework to test stable DM–DE interactions and assess their impact on key cosmological tensions, while highlighting the continued primacy of $\Lambda$CDM in model selection given current data.

Abstract

We investigate the observational consequences of a novel class of stable interacting dark energy (IDE) models, featuring interactions between dark matter (DM) and dark energy (DE). In the first part of our work, we start by considering two IDE models which are known to present early-time linear perturbation instabilities. Applying a transformation depending on the dark energy equation of state (EoS) to the DM-DE coupling, we then obtain two novel stable IDE models. Subsequently, we derive robust and accurate constraints on the parameters of these models, assuming a constant EoS $w_x$ for the DE fluid, in light of some of the most recent publicly available cosmological data. These include Cosmic Microwave Background (CMB) temperature and polarization anisotropy measurements from the \textit{Planck} satellite, a selection of Baryon Acoustic Oscillation measurements, Supernovae Type-Ia luminosity distance measurements from the JLA sample, and measurements of the Hubble parameter up to redshift $2$ from cosmic chronometers. Our analysis displays a mild preference for the DE fluid residing in the phantom region ($w_x<-1$), with significance up to 95\% confidence level, while we obtain new upper limits on the coupling parameter between the dark components. The preference for a phantom DE suggests a coupling function $Q<0$, thus a scenario where energy flows from the DE to the DM. We also examine the possibility of addressing the $H_0$ and $σ_8$ tensions, finding that only the former can be partially alleviated. Finally, we perform a Bayesian model comparison analysis to quantify the possible preference for the two IDE models against the standard concordance $Λ$CDM model, finding that the latter is always preferred with the strength of the evidence ranging from positive to very strong.

Figures (6)

• Figure 1: 1D marginalized posterior distributions for a selection of cosmological parameters of the IDErc1 model, plotted in arbitrary units and normalized to the maximum of the posterior. The posteriors plotted are obtained from the CMB-only dataset (gray curve), the CMB+BAO dataset combination (red curve), and the CMB+BAO+JLA+CC dataset combination (green curve).
• Figure 2: As in Fig. \ref{['1D_1']} but for the IDErc2 model.
• Figure 3: Triangular plot showing the 1D marginalized posterior distributions (along the diagonal), and 2D joint posterior distributions for some selected parameters of the IDErc1 and IDErc2 models, obtained from the CMB+BAO dataset combination.
• Figure 4: As in Fig. \ref{['2D_BAO_1']} but for the CMB+BAO+JLA+CC dataset combination.
• Figure 5: Theoretical prediction for CBM TT power spectrum in presence of the DM-DE coupling for different values of $\xi$. Left panel (right panel) shows the effects for the scenarios IDErc1 (IDErc2), respectively.