Clustering in gravitating N -body systems

We study gravitational clustering of mass points in three dimensions with random initial positions and periodic boundary conditions (no expansion) by numerical simulations. Correlation properties are well-defined in the system and a sort of thermodynamic limit can be defined for the transient regime of clustering. Structure formation proceeds along two paths: i) fluid-like evolution of density perturbations at large scales and ii) shift of the granular (non-fluid) properties from small to large scales. The latter mechanism finally dominates at all scales and it is responsible for the self-similar characteristics of the clustering.