Small groups of Galaxies as Cosmological Clocks
  We study the formation and evolution of small groups of galaxies using new Monte Carlo simulations. These are directly based on the random walk approach to the statistics of condensations collapsing by gravitational instability, and the results are consistent with the Press and Schechter formula. We stress how observational features of groups, such as galaxy membership, depend on the global dynamics of the Universe. This is because the amount of aggregations between infalling galaxies is governed by the ratio of the group age to the time scale for dynamical friction. This ratio strongly depends on cosmology. Thus in low-density models, even if flat, collapse of group sized perturbations followed by fast mergers, would often originate a single massive remnant. This is contrast to observed associations as compact groups of galaxies. 
P. Tozzi, F. Governato and A. Cavaliere