One of the main aims in the next generation of precision cosmology experiments will be an accurate determination of the equation of state (EOS) for the dark energy (DE). If the latter is dynamical, the resulting barotropic index ω should exhibit a nontrivial evolution with the redshift. This is usually interpreted as a sign that the mechanism responsible for the DE is related to some dynamical scalar field, and in some cases this field may behave non-canonically (phantom field). Present observations seem to favor an evolving DE with a potential phantom phase near our time. In the literature, there is a plethora of dynamical models trying to describe this behavior. Here we show that the simplest option, namely a model with a variable cosmological term, Λ = Λ(t), leads in general to a nontrivial effective EOS, with index ωe, which may naturally account for these data features. We prove that in this case there is always a "crossing" of the ωe = -1 barrier near our time. We also show how this effect is modulated (or even completely controlled) by a variable Newton's gravitational coupling G = G(t).
Evolution of the scale factor with a variable cosmological term