The sparkling Universe: a scenario for cosmic void motions
  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, defined by the mean velocity of haloes in
the surrounding shells in the numerical simulation, and by galaxies in the
Sloan Digital Sky Survey Data Release 7. We find void mean bulk velocities
close to 400 km/s, comparable to those of haloes (~ 500-600 km/s), 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 analyze 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. This
contrasts with massive cluster motions who are mainly governed by the pull of
the large-scale overdense regions. Our analysis of void pairwise velocities
shows how their relative motions are generated by 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 behavior of void motions. In order to
compare the theoretical results and the observations we have inferred void
motions in the SDSS using linear theory, finding that the estimated
observational void motions are in statisticalagreement with the results of the
simulation. Regarding large-scale flows, our results suggest a scenario of
galaxies and galaxy systems flowing away from void centers with the additional,
and morerelevant, contribution of the void bulk motion to the total velocity.
