On Local Approximations to the Nonlinear Evolution of Large-Scale Structure

We present a comparative analysis of several methods, known as local Lagrangian approximations, which are aimed to the description of the nonlinear evolution of large-scale structure. We have investigated various aspects of these approximations, such as the evolution of a homogeneous ellipsoid, collapse time as a function of initial conditions, and asymptotic behavior. As one of the common features of the local approximations, we found that the calculated collapse time decreases asymptotically with the inverse of the initial shear. Using these approximations, we have computed the cosmological mass function, finding reasonable agreement with N-body simulations and the Press-Schechter formula.Comment: revised version with color figures, minor changes, accepted for publication in the Astrophysical Journal, 30 pages, 13 figure