The Persistent Percolation of Single-Stream Voids

TL;DR

This paper investigates the structure and percolation properties of single-stream voids in the cosmic web using ORIGAMI, revealing that these voids percolate and fill the volume despite being underdense and not bounded by shell-crossing.

Contribution

It introduces a parameter-free method to identify single-stream voids in cosmological simulations and analyzes their percolation behavior and volume characteristics.

Findings

Single-stream voids percolate and fill the simulation volume.

Multi-stream regions also percolate, contradicting traditional models.

Void volume fraction remains high even at high resolution.

Abstract

We study the nature of voids defined as single-stream regions that have not undergone shell-crossing. We use ORIGAMI to determine the cosmic web morphology of each dark matter particle in a suite of cosmological $N$-body simulations, which explicitly calculates whether a particle has crossed paths with others along multiple sets of axes and does not depend on a parameter or smoothing scale. The theoretical picture of voids is that of expanding underdensities with borders defined by shell-crossing. We find instead that locally underdense single-stream regions are not bounded on all sides by multi-stream regions, thus they percolate, filling the simulation volume; we show that the set of multi-stream particles also percolates. This percolation persists to high resolution, where the mass fraction of single-stream voids is low, because the volume fraction remains high; we speculate on the fraction of collapsed mass in the continuum limit of infinite resolution. By introducing a volume threshold parameter to define underdense void "cores", we create a catalog of ORIGAMI voids which consist entirely of single-stream particles and measure their percolation properties, volume functions, and average densities.