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

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(0)a + b(e)(1 - a) in which the early-time coupling is given by a constant b(e), while to-day 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 sigma 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.