The Motion of Emptiness

The distribution of galaxies at large scales reveals a complex structure where the almost empty and nearly spherical regions, dubbed as cosmic voids, arise as matter flows away from primordial underdense perturbations toward filaments and walls. Thus, cosmic voids encode relevant information of the growth and evolution of structure through their dynamics. We perform a statistical study of the global motion of cosmic voids using both a numerical simulation and observational data. We analyse their relation to large--scale mass flows and the physical effects that drive those motions. We analyse the bulk motions of voids and find mean velocities in the range 300-400 km/s, not far from the expected mean peculiar velocities of groups and galaxy clusters, depending on void size and the large--scale environment. Statistically, small voids move faster than large ones, and voids in relatively higher density environments have higher bulk velocities than those placed in large underdense regions. Also, we examine the mean mass density around voids finding, as expected, large--scale overdensities (underdensities) along (opposite to) the void motion direction, suggesting that void motions respond to a pull--push mechanism. We analyse the distribution of the pairwise relative velocities of voids and find a remarkable bimodal behaviour consistent with an excess of both systematically approaching and receding voids. The magnitude of these systematic relative velocities account for more than 100 km/s, reaching large coherence lengths of up to 200 Mpc/h. We determine that the origin of this bimodality resides in the large--scale density fluctuations. In agreement with linear theory, voids embedded in low (high) density regions mutually recede (attract) each other, providing the general mechanism to understand the bimodal behaviour of void motions. The relative motion of cosmic voids suggests a scenario of a sparkling Universe, with approaching and receding voids according to their local environment. In order to compare the theoretical results and the observations we have used samples of voids from the Sloan Digital Sky Survey and the peculiar velocity field inferred from linear theory finding consistent results with the simulation predictions. Regarding large--scale flows, our results suggest a scenario of galaxies and galaxy systems flowing away from void centres with the additional, and more relevant, contribution of the void bulk motion to the total velocity.