The generalized Brans-Dicke theory and its cosmology

Abstract: A generalized Brans-Dicke (GBD) theory is proposed and studied in this paper. The interesting property has been found in the GBD theory, for example it can naturally solve the problem of \gamma value emerging in f(R) modified gravity without introducing the so-called chameleon mechanism. In addition, it can be found that the GBD theory could solve some problems existing in other theories. (1) The $f(R)$ theory is equivalent to the BD theory with a potential (abbreviated as BDV) for taking a specific value of the BD parameter \omega=0, where the specific choice: \omega=0 is quite exceptional, and it is hard to understand the corresponding absence of the kinetic-energy term for the field. However, fields in the GBD own the non-disappeared dynamical effect. (2) In the double scalar-fields quintom model, it is required to include both the canonical quintessence field and the non-canonical phantom field in order to make the state parameter to cross over w=-1, while several fundamental problems are associated with phantom field, such as the problem of negative kinetic term and the fine-tuning problem, etc. While, in the GBD model, the state parameter of geometrical dark energy can cross over the phantom boundary as achieved in the quintom model, without bearing the problems existing in the quintom model. (3) The GBD theory tends to investigate the physics from the viewpoint of geometry, while the BDV or the two scalar-fields quintom model tends to solve physical problems from the viewpoint of matter. It is possible that several special characteristics of scalar fields could be revealed through studies of geometrical gravity in the GBD. As an example, we investigate the potential V of the BD scalar field, and an effective form of V could be given by studying on the GBD theory. And, it seems that a viable condition for the BD theory could be found.