Power-law solutions and accelerated expansion in scalar-tensor theories
  We find exact power-law solutions for scalar-tensor theories and clarify the
conditions under which they can account for an accelerated expansion of the
Universe. These solutions have the property that the signs of both the Hubble
rate and the deceleration parameter in the Jordan frame may be different from
the signs of their Einstein-frame counterparts. For special parameter
combinations we identify these solutions with asymptotic attractors that have
been obtained in the literature through dynamical-system analysis. We establish
an effective general-relativistic description for which the geometrical
equivalent of dark energy is associated with a time dependent equation of
state. The present value of the latter is consistent with the observed
cosmological ``constant". We demonstrate that this type of power-law solutions
for accelerated expansion cannot be realized in f(R) theories.
