[Paper Review] On the dynamics of a dark sector coupling

Weiqiang Yang, Supriya Pan+2 more|arXiv (Cornell University)|Sep 29, 2022

Cosmology and Gravitation Theories141 references4 citations

TL;DR

This paper investigates dynamical dark sector coupling in interacting dark energy models, where both the coupling parameter and dark energy equation of state evolve over time. Using CMB, BAO, and Pantheon supernova data, it finds 95% CL evidence for early dark energy and dynamical coupling, with specific models alleviating both the $H_0$ and $S_8$ tensions.

ABSTRACT

Interacting dark energy models may play a crucial role in explaining several important observational issues in modern cosmology and also may provide a solution to current cosmological tensions. Since the phenomenology of the dark sector could be extremely rich, one should not restrict the interacting models to have a coupling parameter which is constant in cosmic time, rather allow for its dynamical behavior, as it is common practice in the literature when dealing with other dark energy properties, as the dark energy equation of state. We present here a compendium of the current cosmological constraints on a large variety of interacting models, investigating scenarios where the coupling parameter of the interaction function and the dark energy equation of state can be either constant or dynamical. For the most general schemes, in which both the coupling parameter of the interaction function and the dark energy equation of state are dynamical, we find $95\%$~CL evidence for a dark energy component at early times and slightly milder evidence for a dynamical dark coupling for the most complete observational data set exploited here, which includes CMB, BAO and Supernova Ia measurements. Interestingly, there are some cases where a dark energy component different from the cosmological constant case at early times together with a coupling different from zero today, can alleviate both the $H_0$ and $S_8$ tension for the full dataset combination considered here. Due to the energy exchange among the dark sectors, the current values of the matter energy density and of the clustering parameter $σ_8$ are shifted from their $Λ$CDM-like values. This fact makes future surveys, especially those focused on weak lensing measurements, unique tools to test the nature and the couplings of the dark energy sector.

Motivation & Objective

• To explore the cosmological implications of time-varying coupling between dark matter and dark energy, moving beyond constant coupling assumptions.
• To investigate whether dynamical coupling and evolving dark energy equation of state can resolve current cosmological tensions, particularly $H_0$ and $S_8$.
• To assess the Bayesian evidence for interacting models with both dynamical coupling and dynamical dark energy equation of state using comprehensive observational data.
• To examine how energy exchange between dark sectors alters $\Omega_{m0}$ and $\sigma_8$, affecting constraints from weak lensing and large-scale structure.
• To identify viable scenarios where non-constant coupling and early dark energy components jointly reduce tensions with $H_0$ and $S_8$ measurements.

Proposed method

• The study employs a general interacting dark energy model with a coupling function $\xi(a) = \xi_0 + \xi_a(1-a)$, allowing time dependence in the interaction strength.
• Background and perturbation equations are solved numerically, incorporating energy exchange between dark matter and dark energy sectors via the interaction term $Q$.
• Cosmological constraints are derived using Markov Chain Monte Carlo (MCMC) sampling with observational datasets: CMB (Planck), BAO, and Pantheon SNe Ia.
• Bayesian evidence analysis is performed to compare models with constant vs. dynamical coupling and equation of state, using the Bayes factor for model selection.
• The parameter space is divided into phantom ($w < -1$) and quintessence ($w > -1$) regions, with stability criteria applied to ensure physical consistency.
• The analysis includes constraints on $\Omega_{m0}$, $\sigma_8$, $H_0$, and $S_8$, with shifts due to energy flow between dark sectors explicitly tracked.

Experimental results

Research questions

• RQ1 Can a time-varying coupling between dark matter and dark energy alleviate the $H_0$ and $S_8$ tensions simultaneously?
• RQ2 What is the Bayesian evidence for dynamical coupling compared to constant coupling in interacting dark energy models?
• RQ3 How do evolving dark energy equation of state and coupling parameters affect the current values of $\Omega_{m0}$ and $\sigma_8$?
• RQ4 In which regions of the parameter space (phantom or quintessence) do we find the strongest evidence for non-zero coupling and early dark energy?
• RQ5 To what extent do future weak lensing surveys provide unique sensitivity to the nature and strength of dark sector interactions?

Key findings

• For the most general model with both dynamical coupling and dynamical dark energy equation of state, the CMB+BAO+Pantheon dataset yields 95% CL evidence for an early dark energy component and slightly milder evidence for a dynamical coupling.
• The model $\xi_0\xi_a w_0^p w_e^p$ IDE $Q_A$ shows significant alleviation of both $H_0$ and $S_8$ tensions, with non-zero coupling and early dark energy components favored.
• In the phantom region ($w_0^p < -1$), the model $\xi_0\xi_a w_0^p w_e^p$ IDE $Q_A$ finds 95% CL evidence for a dynamical coupling and early dark energy, while the quintessence region shows no evidence for interaction.
• For model B in the phantom region, $w < -1$ is strongly favored at 95% CL, and $\xi_a \neq 0$ is indicated at 68% CL when combining CMB and Pantheon data.
• In the quintessence region, $\Omega_{m0}$ is reduced and $\sigma_8$ is increased due to energy flow from dark energy to dark matter, but no evidence for interaction is found.
• CMB alone data shows slightly higher $\Omega_{m0}$ and lower $\sigma_8$ in the $\xi_0\xi_a w_0^q w_e^q$ IDE $Q_B$ model, indicating subtle shifts from $\Lambda$CDM due to energy exchange.