Dynamical vacuum energy and phantom divide-line cross in scalar cosmology

Abstract

In this paper, we study cosmological models in scalar tensor theory of gravity a la Brans–Dicke with a dynamical equation of state parameter, a dynamical effective cosmological constant and a Hubble parameter varying like \( H = \varepsilon t^{s} + H_{0} \) with \( \varepsilon \) and \( H_{0} \) are real positive parameters and \( s \) is some pending power index. It is observed that the universe is accelerating in time and it is dominated by a decreasing energy density and a vacuum energy density. However the vacuum energy density is increasing logarithmically at early epoch of time and then decaying toward its present positive value at late-time epoch. For specific choices of the free parameters in the theory, it is observed that the phantom divide line is crossed in the presence of a cosmological constant and the universe expands asymptotically a la de-Sitter at very large time. Additional features are discussed as well.