  An initially homogeneous freely evolving fluid of inelastic hard spheres
develops inhomogeneities in the flow field (vortices) and in the density field
(clusters), driven by unstable fluctuations. Their spatial correlations, as
measured in molecular dynamics simulations, exhibit long range correlations;
the mean vortex diameter grows as the square root of time; there occur
transitions to macroscopic shearing states, etc.
  The Cahn--Hilliard theory of spinodal decomposition offers a qualitative
understanding and quantitative estimates of the observed phenomena. When
intrinsic length scales are of the order of the system size, effects of
physical boundaries and periodic boundaries (finite size effects in
simulations) are important.
