RECONSTRUCTION OF COSMOLOGICAL DENSITY AND VELOCITY-FIELDS IN THE LAGRANGIAN ZELDOVICH APPROXIMATION

We present a method for reconstructing cosmological density and veloci ty fields using the Lagrangian Zel'dovich formalism. The method involv es finding the least-action solution for straight-line particle paths in an evolving density field. Our starting point is the final, evolved density, so that we are in effect carrying out the standard Zel'dovic h approximation based process in reverse. Using a simple numerical alg orithm we are able to minimize the action for the trajectories of seve ral million particles. We apply our method to the evolved density take n from N-body simulations of different cold dark matter (CDM)-dominate d universes, testing both the prediction for the present-day velocity field and that for the initial density field. The method is easy to ap ply, reproduces the accuracy of the forward Zel'dovich approximation, and also works directly in redshift-space with minimal modification.