Perturbations to the Cosmological Expansion in a Grainy Universe

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.