Truthing the stretch: Non-perturbative cosmological realizations with multiscale spherical collapse

Abstract

Here we present a simple, parameter-free, non-perturbative algorithm that gives low-redshift cosmological particle realizations accurate to few-Megaparsec scales, called MUSCLE (MUltiscale Spherical ColLapse Evolution). It has virtually the same cost as producing N-body-simulation initial conditions, since it works with the 'stretch' parameter {\psi}, the Lagrangian divergence of the displacement field. It promises to be useful in quickly producing mock catalogs, and to simplify computationally intensive reconstructions of galaxy surveys. MUSCLE applies a spherical-collapse prescription on multiple Gaussian-smoothed scales. It achieves higher accuracy than perturbative schemes (Zel'dovich and 2LPT), and, by including the void-in-cloud process (voids in large-scale collapsing regions), solves problems with a single-scale spherical-collapse scheme. Slight further improvement is possible by mixing in the 2LPT estimate on large scales. Additionally, we show the behavior of {\psi} for different morphologies (voids, walls, filaments, and haloes). A Python code to produce these realizations is available at http://skysrv.pha.jhu.edu/~neyrinck/muscle.html.