Tale of stable interacting dark energy, observational signatures, and the $H_0$ tension (1805.08252v3)

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 $\sigma_8$ 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 $\Lambda$CDM model, finding that the latter is always preferred with the strength of the evidence ranging from positive to very strong.

• The paper proposes two novel stable interacting dark energy models that avoid early-time instabilities, particularly when dark energy crosses the phantom divide.
• Observational data analysis shows a preference for the dark energy equation of state being in the phantom region ($w_x < -1$), implying energy transfer from dark energy to dark matter.
• These new models partially alleviate the $H_0$ tension but do not resolve other cosmological tensions like $\sigma_8$ as effectively as the standard $\Lambda$CDM model, which remains statistically favored.

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

The research paper addresses an advanced cosmological topic by exploring a new class of stable interacting dark energy (IDE) models, which feature interactions between dark matter (DM) and dark energy (DE). The paper proposes two novel models designed to overcome the issue of linear perturbation instabilities commonly found in conventional IDE models at early times. This work is built on the framework where the interaction between dark components is adjusted by an equation state-dependent parameter for dark energy, allowing stability across the entire parameter space.

The significance of this research lies in formulating and analyzing models that remain stable when the dark energy equation of state (EoS), $w_x$, crosses the possibly problematic phantom divide at $w_x = -1$. Such behavior usually leads to instabilities in conventional interaction models. This paper provides a resolution by introducing EoS-dependent interaction terms, resulting in a stable perturbative evolution for both phantom and non-phantom domains of $w_x$.

Key Findings and Methodology

• Model Proposal: Two IDE models were proposed with interaction terms designed to ensure stability in perturbations. The innovative aspect lies in how these interaction terms depend on the EoS parameter for dark energy, effectively circumventing instability issues.
• Constraints from Observational Data: Utilizing a range of observational datasets, including CMB data from the Planck satellite and multiple astrophysical measurements like Baryon Acoustic Oscillations and supernovae luminosity distances, robust constraints on these IDE models were derived. The datasets collectively span different cosmic eras, providing comprehensive coverage of universe evolution.
• Phantom Dark Energy Preference: The analysis reveals a mild, statistically significant preference for the DE EoS to be in the phantom region ($w_x < -1$) up to a 95% confidence level. This suggests an energy transfer direction from DE to DM, characterized by a coupling function $Q < 0$.
• Cosmological Tensions: Regarding the tensions in standard cosmology, the models partially alleviate the $H_0$ tension—a discrepancy between the locally measured and cosmic microwave background-inferred Hubble constant. However, they do not sufficiently address other tensions, such as those related to $\sigma_8$, a measure of matter clustering.
• Bayesian Model Comparison: Through Bayesian inference, it was demonstrated that while these new IDE models present a viable alternative, the standard $\Lambda$CDM model remains preferred, with the evidence varying from positive to very strong depending on the datasets.

Implications and Future Directions

The paper offers significant theoretical advancements in understanding the interaction between dark components. While the findings provide new insights into the nature and dynamics of dark energy, especially in interaction with dark matter, the preference towards $\Lambda$CDM from a Bayesian perspective suggests further refinement and consideration of additional parameters or alternative models.

Future developments may involve extending the IDE models to incorporate other cosmic phenomena, such as massive neutrinos, and testing against additional or more precise datasets. Furthermore, the paper's approach to handling dark sector interactions can inspire future work to explore underlying fundamental physics theories, potentially involving novel scalar fields or modifications to gravity, which might further elucidate the nature of dark energy and its role in the universe's expansion history.