Ellipsoidal collapse and previrialization

We study the nonlinear evolution of a dust ellipsoid embedded in a flat Fri edmann background universe, in order to determine the evolution of the dens ity of the ellipsoid as the perturbation associated with it detaches from t he general expansion and begins to collapse. We show that while the growth rate of the density contrast of a mass element is enhanced by shear, in agr eement with Hoffman's 1986 result, the angular momentum acquired by the ell ipsoid has the right magnitude to counterbalance the effect of the shear. T his result confirms the previrialization conjecture by showing that initial asphericities and tidal interactions begin to slow the collapse after the system has broken away from the general expansion.