Brans-Dicke Theory as a Unified Model for Dark Matter - Dark Energy

The Brans-Dicke (BD) theory of gravity is taken as a possible theory of k-essence coupled to gravity. It then has been realized that the BD scalar field does indeed play a role of a k-essence, but in a very unique way which distinguishes it from other k-essence fields studied in the literature. That is, first in the BD scalar field-dominated era when the contribution from this k-essence overwhelms those from other types of matter, the BD theory predicts the emergence of a yet-unknown {\it zero acceleration} epoch which is an intermediate stage acting as a ``crossing bridge'' between the decelerating matter-dominated era and the accelerating phase. Upon realizing this, next, closer study of the effects of this k-essence on the evolutionary behavior of the matter-dominated and the accelerating eras has been performed. The result of the study indicates that the BD scalar field appears to interpolate {\it smoothly} between these two late-time stages by speeding up the expansion rate of the matter-dominated era somewhat while slowing down that of the accelerating phase to some degree. Thus with the newly found BD scalar field-dominated era in between these two, the late-time of the universe evolution appears to be mixed sequence of the three stages.