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

Here we present a simple, parameter-free, non-perturbative algorithm that gives lowredshift 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., the Lagrangian divergence of the displacement field. It promises to be useful in quickly producing mock catalogues, 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 secondorder Lagrangian perturbation theory - 2LPT), and, by including the void-in-cloud process ( voids in large-scale collapsing regions), solves problems with a single-scale sphericalcollapse scheme. Slight further improvement is possible by mixing in the 2LPT estimate on large scales. Additionally, we show the behaviour of. for different morphologies (voids, walls, filaments, and haloes). A PYTHON code to produce these realizations is available at http://skysrv.pha.jhu.edu/similar to neyrinck/muscle.html.