Quintessence, superquintessence, and observable quantities in Brans-Dicke and nonminimally coupled theories

The different definitions for the equation of state of a nonminimally coupled scalar field that have been introduced in the literature are analyzed. Particular emphasis is made upon those features that could yield an observable way of distinguishing nonminimally coupled theories from general relativity, with the same or with alternate potentials. It is found that some earlier claims that superquintessence, a stage of superaccelerated expansion of the universe, is possible within realistic nonminimally coupled theories are the result of an arguable definition of the equation of state. In particular, it is shown that these previous results do not import any observable consequence, i.e., that the theories are observationally identical to general relativity models and that superquintessence is not more than a mathematical outcome. Finally, in the case of nonminimally coupled theories with coupling $F=1+\xi {\phi }^2$ and tracking potentials, it is shown that no superquintessence is possible.