Running coupling: Does the coupling between dark energy and dark matter change sign during the cosmological evolution?

TL;DR

This work investigates whether the coupling between dark energy and dark matter can change sign during cosmic evolution by introducing a running coupling $Q(a)=3 b(a) H_0 \rho_0$ with $b(a)=b_0 a + b_e(1-a)$. It analyzes three dark energy EOS scenarios—\LambdaCDM ($w=-1$), XCDM ($w_0$), and CPL ($w(a)=w_0+w_1(1-a)$)—fitting to Union2 SNIa, BAO, CMB, $H(z)$, and $f_{gas}$ data via MCMC. The results favor a time-varying vacuum with a coupling that crosses $b=0$ at $z\approx 0.2$–$0.3$ (roughly $1\sigma$), implying DM decays to DE at early times and DE decays to DM at late times. This sign-changeable interaction could play a significant role in addressing dark sector phenomenology, motivating more general and oscillatory interaction models and improved treatment of DE perturbations in future work.

Abstract

In this paper we put forward a running coupling scenario for describing the interaction between dark energy and dark matter. The dark sector interaction in our scenario is free of the assumption that the interaction term $Q$ is proportional to the Hubble expansion rate and the energy densities of dark sectors. We only use a time-variable coupling $b(a)$ (with $a$ the scale factor of the universe) to characterize the interaction $Q$. We propose a parametrization form for the running coupling $b(a)=b_0a+b_e(1-a)$ in which the early-time coupling is given by a constant $b_e$, while today the coupling is given by another constant, $b_0$. For investigating the feature of the running coupling, we employ three dark energy models, namely, the cosmological constant model ($w=-1$), the constant $w$ model ($w=w_0$), and the time-dependent $w$ model ($w(a)=w_0+w_1(1-a)$). We constrain the models with the current observational data, including the type Ia supernova, the baryon acoustic oscillation, the cosmic microwave background, the Hubble expansion rate, and the X-ray gas mass fraction data. The fitting results indicate that a time-varying vacuum scenario is favored, in which the coupling $b(z)$ crosses the noninteracting line ($b=0$) during the cosmological evolution and the sign changes from negative to positive. The crossing of the noninteracting line happens at around $z=0.2-0.3$, and the crossing behavior is favored at about 1$σ$ confidence level. Our work implies that we should pay more attention to the time-varying vacuum model and seriously consider the phenomenological construction of a sign-changeable or oscillatory interaction between dark sectors.

Figures (5)

• Figure 1: The probability contours at $1\sigma$ and $2\sigma$ confidence levels in the parameter planes for the interacting $\Lambda$CDM model.
• Figure 2: The probability contours at $1\sigma$ and $2\sigma$ confidence levels in the parameter planes for the interacting XCDM model.
• Figure 3: The probability contours at $1\sigma$ and $2\sigma$ confidence levels in the parameter planes for the interacting CPL model.
• Figure 4: The reconstructed evolutionary history for $w(z)$ of the interacting CPL model.
• Figure 5: The reconstructed evolutionary histories for $b(z)$ in the three interacting models. The dashed line in each plot represents the noninteracting line.