  The matter content of the Universe is generally regarded as a perfect fluid
on sufficiently large scales, for all epochs. But the recent cosmological
matter distribution, consisting of an (ideally) random distribution of
gravitationally collapsed structures, is more accurately described as a
collection of discrete grains, than as a fluid. It is well known that granular
materials may have very different macroscopic properties than fluids;
analogously, we investigate the possibility that pervasive small-scale
inhomogeneities in the recent Universe may lead to perturbations of the
cosmological expansion on intermediate and/or large scales.
