ROMANIAN REPORTS IN PHYSICS, vol.73, no.2, 2021 (SCI-Expanded)
Brans-Dicke theory in its original form can explain the radiation and matter dominated eras with a constant Brans-Dicke scalar field and the explanation of dark energy requires a nonvanishing potential term for the scalar field in the action. Within the framework of Jordan-Brans-Dicke theory where we take the scalar field as the Jordan field with a standard kinetic term and no mass term, we find the same radiation dominated solution as given by Einstein cosmology. On the other hand, by adding a mass term for the scalar field to the action, the radiation dominated solution is modified. We make an expansion in increasing powers of the scale size. This is valid for small scale sizes. We show that this expansion describing the early radiation dominated era also gives the late time dark energy dominated era in the Friedmann equation.