The Persistent Percolation of Single-Stream Voids

Understand

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.