  It has been suggested that the acceleration of the Universe may be due to the
backreaction of perturbations to the Friedmann-Robertson-Walker background. For
a Universe dominated by cold dark matter, it is known that the backreaction of
superhorizon perturbations can not drive acceleration. We extend this result to
models with cold dark matter together with a scalar field. We show that the
scalar field can drive acceleration only via the standard mechanism of a
constant or nearly constant piece of its potential (i.e., a cosmological
constant); there is no separate mechanism involving superhorizon backreaction.
This rules out some models which have been proposed in the literature.
