Role of scalar fields in cosmological models

Recent astronomical observations of supernovae and cosmic microwave background indicate that the universe is accelerating. Scalar–tensor theories of gravity give rise to suitable cosmological models where a late-time accelerated expansion is naturally realized. In an alternative proposal the cosmic acceleration is generated by means of a scalar field (quintessence), in a way similar to the early-time inflation. In this paper, we consider two classes of cosmological models with scalar fields. The first one corresponds to the Jordan–Brans–Dicke tensor–scalar theory with a cosmological scalar and the second one contains a conformally coupled scalar field with quartic potential. In both type of models the cosmological dynamics is described and the deceleration parameter is evaluated. The values of the parameters are specified for which a late-time accelerated expansion is realized.