  It has recently been suggested that the acceleration of the Universe can be
explained as the backreaction effect of superhorizon perturbations using second
order perturbation theory. If this mechanism is correct, it should also apply
to a hypothetical, gedanken universe in which the subhorizon perturbations are
absent. In such a gedanken universe it is possible to compute the deceleration
parameter $q_0$ measured by comoving observers using local covariant Taylor
expansions rather than using second order perturbation theory. The result
indicates that second order corrections to $q_0$ are present, but shows that if
$q_0$ is negative then its magnitude is constrained to be less than or of the
order of the square of the peculiar velocity on Hubble scales today. We argue
that since this quantity is constrained by observations to be small compared to
unity, superhorizon perturbations cannot be responsible for the acceleration of
the Universe.
