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Nico Schuster, Cosmologist

Simple dynamics inside the emptiest places

Why cosmic voids matter: nonlinear structure & linear dynamics

Nico Schuster, Nico Hamaus, Klaus Dolag & Jochen Weller
Key figure for Simple dynamics inside the emptiest places

While denser regions of the cosmic web, like massive galaxy clusters, are dominated by complex gravitational forces, gravity behaves more gently in the emptiest regions of space, offering cleaner physical environments. Cosmic voids are actively expanding, pushing matter outward in a highly organized way that avoids the messy physics found elsewhere in the universe.

Using state-of-the-art hydrodynamical simulations (which include additional physics alongside standard gravity) across a vast range of scales, we investigated exactly how matter and galaxies move around these underdense regions. Surprisingly, we discovered that these motions can be described exquisitely well by the simple equations of linear mass conservation.

As shown in the image above, this holds true even for incredibly small, individual voids on scales of just a few megaparsecs, a regime where we would normally expect the physics to become highly nonlinear. Yet, the measured velocities of galaxies (dotted lines) match the simple theoretical predictions (solid lines) very well.

This confirms that despite the complexity of the surrounding cosmic web, voids provide a uniquely clean and mathematically predictable laboratory for precision cosmology.

Official abstract
We use the Magneticum suite of state-of-the-art hydrodynamical simulations to identify cosmic voids based on the watershed technique and investigate their most fundamental properties across different resolutions in mass and scale. This encompasses the distributions of void sizes, shapes, and content, as well as their radial density and velocity profiles traced by the distribution of cold dark matter particles and halos. We also study the impact of various tracer properties, such as their sparsity and mass, and the influence of void merging on these summary statistics. Our results reveal that all of the analyzed void properties are physically related to each other and describe universal characteristics that are largely independent of tracer type and resolution. Most notably, we find that the motion of tracers around void centers is perfectly consistent with linear dynamics, both for individual, as well as stacked voids. Despite the large range of scales accessible in our simulations, we are unable to identify the occurrence of nonlinear dynamics even inside voids of only a few Mpc in size. This suggests voids to be among the most pristine probes of cosmology down to scales that are commonly referred to as highly nonlinear in the field of large-scale structure.

Read on arXiv →

Published version: JCAP (2023) →