Zel'dovich approximation

Summary

The Zel'dovich approximation describes the growth of cosmic structure by moving each particle of matter along a straight line in comoving coordinates. The displacement of each particle is fixed by the initial density field, and its size grows with the linear growth factor $D(t)$. It extends linear perturbation theory into the mildly nonlinear regime and predicts that matter first collapses into flat sheets. It is a basic tool of structure formation and is used to set up the initial conditions of cosmological N-body simulations.