Oscillating scalar fields in extended quintessence
  We study a rapidly-oscillating scalar field with potential $V(\phi) =
k|\phi|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form
$(1- 8 \pi G_0 \xi \phi^2) R$ in the action. In the weak coupling limit, we
calculate the effect of the nonminimal coupling on the time-averaged equation
of state parameter $\gamma = (p + \rho)/\rho$. The change in $\langle \gamma
\rangle$ is always negative for $n \ge 2$ and always positive for $n < 0.71$
(which includes the case where the oscillating scalar field could serve as dark
energy), while it can be either positive or negative for intermediate values of
$n$. Constraints on the time-variation of $G$ force this change to be
infinitesimally small at the present time whenever the scalar field dominates
the expansion, but constraints in the early universe are not as stringent. The
rapid oscillation induced in $G$ also produces an additional contribution to
the Friedman equation that behaves like an effective energy density with a
stiff equation of state, but we show that, under reasonable assumptions, this
effective energy density is always smaller than the density of the scalar field
itself.
