The sparkling Universe: the coherent motions of cosmic voids

TL;DR

This paper investigates the coherent motions of cosmic voids, revealing a bimodal velocity distribution linked to their large-scale environment, supported by simulations and SDSS-DR7 data, suggesting a dynamic, 'sparkling' universe.

Contribution

It provides the first detailed analysis of void bulk motions, demonstrating environment-dependent bimodal velocity distributions and coherence over large scales.

Findings

Void motions are around 300-400 km/s, similar to galaxy clusters.

Void pairs show a bimodal velocity distribution with approaching and receding motions.

Environmental classification explains the velocity bimodality.

Abstract

We compute the bulk motions of cosmic voids, using a $\Lambda$CDM numerical simulation considering the mean velocities of the dark matter inside the void itself and that of the haloes in the surrounding shell. We find coincident values of these two measures in the range $\sim$ 300-400 km/s, not far from the expected mean peculiar velocities of groups and galaxy clusters. When analysing the distribution of the pairwise relative velocities of voids, we find a remarkable bimodal behaviour consistent with an excess of both systematically approaching and receding voids. We determine that the origin of this bimodality resides in the void large scale environment, since once voids are classified into void-in-void (R-type) or void-in-cloud (S-type), R-types are found mutually receding away, while S-types approach each other. The magnitude of these systematic relative velocities account for more than 100 km/s, reaching large coherence lengths of up to 200 h$^{-1}$ Mpc . We have used samples of voids from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) and the peculiar velocity field inferred from linear theory, finding fully consistent results with the simulation predictions. Thus, their relative motion suggests a scenario of a sparkling Universe, with approaching and receding voids according to their local environment.