Chameleon Brans-Dicke cosmology

We consider a generalized Brans-Dicke model in which the scalar field has a potential function and is also allowed to couple nonminimally with the matter sector. We assume a power-law form for the potential and the coupling functions as the inputs of the model and show that acceleration of the Universe can be realized for a constrained range of exponents of the potential function. We also argue that this accelerating phase is consistent with a large and positive Brans-Dicke parameter. In our analysis, the potential plays a more important role with respect to the coupling function in the dynamics of the Universe as the latter does not contribute to any of the relations characterizing evolution of the scale factor of the Universe and the scalar field. However, we will show that the coupling function is closely related to magnitude and direction of the energy transfer between matter and the scale field. We use this fact and some thermodynamic aspects of the model to put some constraints on the coupling function. In particular, we argue that the second law of thermodynamics constrains direction of the overall energy transfer. DOI: 10.1103/PhysRevD.86.127503