Observational constraints on sign-changeable interaction models and alleviation of the $H_0$ tension

TL;DR

The paper investigates sign-changeable interactions between dark matter and dark energy, introducing two models (IDE1 and IDE2) that allow the energy transfer to reverse sign during cosmic evolution and confronts them with Planck 2015 CMB, BAO, Pantheon SNIa, and cosmic chronometer data.Background and perturbation equations are derived for each model, and a Bayesian framework is used to compare their viability against the ΛCDM paradigm.Across datasets, the dark-energy EOS favors phantom values (w_x < -1) at >2σ, while the coupling parameters remain consistent with zero within 1σ; the models mildly alleviate the H0 tension but fail to resolve the σ8 tension, and residual CMB/matter power spectra reveal distinguishable features not evident in the background evolution alone.Bayesian evidence generally prefers ΛCDM over the sign-changeable models, with stronger penalties for IDE2 due to its extra parameter, though the thermodynamic analysis shows the generalized second law holds and the universe tends toward thermodynamic equilibrium.Overall, sign-changeable interactions offer modest phenomenological differences and thermodynamic consistency, but do not supersede ΛCDM as the favored description of late-time cosmology.

Abstract

We investigate various scenarios which include interaction forms between dark matter and dark energy that exhibit sign reverse, namely where the transfer of energy between the dark fluids changes sign during evolution. We study the large-scale inhomogeneities in such interacting scenarios and we confront them with the latest astronomical data. Our analysis shows that the sign-changeable interaction models are able to produce stable perturbations. Additionally, the data seem to slightly favor a non-zero interaction, however, within $1σ$ confidence level (CL) the scenarios cannot be distinguished from non-interacting cosmologies. We find that the best-fit value of the dark-energy equation-of-state parameter lies in the phantom regime, while the quintessence region is also allowed nevertheless at more than 2$σ$ CL. Examining the effect of the interaction on the CMB TT and matter power spectra we show that while from the simple spectra it is hard to distinguish the interacting case from $Λ$CDM scenario, in the residual graphs the interaction is indeed traceable. Moreover, we find that sign-changeable interaction models can reconcile the $H_0$ tension, however the $σ_8$ tension is still persisting. Finally, we examine the validity of the laws of thermodynamics and we show that the generalized second law is always satisfied, while the second derivative of the total entropy becomes negative at late times which implies that the universe tends towards thermodynamic equilibrium.

Figures (15)

• Figure 1: The evolution of the normalized interaction function $Q/Q_0$, where $Q_0 = H_0 \rho_{t0}$ (with $H_0$ and $\rho_{t0}$ the present values of the Hubble parameter and the total energy density $\rho_t$ respectively), for the interaction model IDE1 of (\ref{['IDE1']}), for various values of the coupling parameter $\xi$ in units where $8\pi G=1$, for $w_x =-0.98$ (upper graph) and for $w_x = -1.01$ (lower graph).
• Figure 2: The evolution of the deceleration parameter as a function of the redshift, for the interaction model IDE1 of (\ref{['IDE1']}), for $w_x =-0.98$ (upper graph) and for $w_x = -1.01$ (lower graph), for various values of the coupling parameter $\xi$ in units where $8\pi G=1$. We have set $\Omega_{x0}\approx0.69$, $\Omega_{c0}\approx0.25$, $\Omega_{b0}\approx0.05$ and $\Omega_{r0}\approx10^{-4}$ in agreement with observations.
• Figure 3: The evolution of the coincidence parameter $r = \rho_c/\rho_x$, for the interaction model IDE1 of (\ref{['IDE1']}), for $w_x =-0.98$ (upper graph) and for $w_x = -1.01$ (lower graph), setting a typical value of $\xi = 0.01$ in units where $8\pi G=1$. For comparison we additionally depict the results for some well known interaction scenarios, namely $Q = 3H \xi \rho_x$, $Q = 3H \xi \rho_c$ and $Q = 3 H \xi (\rho_c+\rho_x)$.
• Figure 4: The evolution of the normalized interaction function $Q/Q_0$, where $Q_0 = H_0 \rho_{t0}$ (with $H_0$ and $\rho_{t0}$ the present values of the Hubble parameter and the total energy density $\rho_t$ respectively), for the interaction model IDE2 of (\ref{['IDE2']}), for various values of the coupling parameters $\alpha$ and $\beta$ in units where $8\pi G=1$, for $w_x =-0.98$ (upper graphs) and for $w_x = -1.01$ (lower graphs).
• Figure 5: The evolution of the deceleration parameter as a function of the redshift, for the interaction model IDE2 of (\ref{['IDE2']}), for $w_x =-0.98$ (upper graphs) and for $w_x = -1.01$ (lower graphs), for various values of the coupling parameters $\alpha$ and $\beta$ in units where $8\pi G=1$. We have set $\Omega_{x0}\approx0.69$, $\Omega_{c0}\approx0.25$, $\Omega_{b0}\approx0.05$ and $\Omega_{r0}\approx10^{-4}$ in agreement with observations.