Evolution mapping – II. Describing statistics of the non-linear cosmic velocity field

Abstract

We extend the evolution–mapping approach, introduced in the first paper of this series to describe non-linear matter density fluctuations, to statistics of the cosmic velocity field. This framework classifies cosmological parameters into shape parameters, which determine the shape of the linear matter power spectrum, 𝑃L(𝑘,𝑧)⁠, and evolution parameters, which control its amplitude at any redshift. Evolution–mapping leverages the fact that density fluctuations in cosmologies with identical shape parameters but different evolution parameters exhibit similar non-linear evolutions when expressed as a function of clustering amplitude. We analyse a suite of N-body simulations sharing identical shape parameters but spanning a wide range of evolution parameters. Using a method for estimating the volume-weighted velocity field based on the Voronoi tessellation of simulation particles, we study the non-linear evolution of the velocity divergence power spectrum, 𝑃𝜃𝜃(𝑘)⁠, and its cross-power spectrum with the density field, 𝑃𝛿𝜃(𝑘)⁠. We demonstrate that the evolution–mapping relation applies accurately to 𝑃𝜃𝜃(𝑘) and 𝑃𝛿𝜃(𝑘)⁠. While this breaks down in the strongly non-linear regime, deviations can be modelled in terms of differences in the suppression factor, 𝑔(𝑎)=𝐷(𝑎)/𝑎⁠, similar to those for the density field. Such modelling describes the differences in 𝑃𝜃𝜃(𝑘) between models with the same linear clustering amplitude to better than 1 per cent accuracy at all scales and redshifts considered. Evolution–mapping simplifies the description of the cosmological dependence of non-linear density and velocity statistics, streamlining the sampling of large cosmological parameter spaces for cosmological analysis.