The sparkling Universe: the coherent motions of cosmic voids
  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.
