  We derive the propagator for a massless, minimally coupled scalar on a
$D$-dimensional, spatially flat, homogeneous and isotropic background with
arbitrary constant deceleration parameter. Our construction uses the operator
formalism, by integrating the Fourier mode sum. We give special attention to
infrared corrections from the nonzero lower limit associated with working on
finite spatial sections. These corrections eliminate infrared divergences that
would otherwise be incorrectly treated by dimensional regularization, resulting
in off-coincidence divergences for those special values of the deceleration
parameter at which the infrared divergence is logarithmic. As an application we
compute the expectation value of the scalar stress-energy tensor.
