Void Dynamics
  Cosmic voids are becoming key players in testing the physics of our Universe.
Here we concentrate on the abundances and the dynamics of voids as these are
among the best candidates to provide information on cosmological parameters.
Cai, Padilla \& Li (2014) use the abundance of voids to tell apart Hu \&
Sawicki $f(R)$ models from General Relativity. An interesting result is that
even though, as expected, voids in the dark matter field are emptier in $f(R)$
gravity due to the fifth force expelling away from the void centres, this
result is reversed when haloes are used to find voids. The abundance of voids
in this case becomes even lower in $f(R)$ compared to GR for large voids.
Still, the differences are significant and this provides a way to tell apart
these models. The velocity field differences between $f(R)$ and GR, on the
other hand, are the same for halo voids and for dark matter voids. Paz et al.
(2013), concentrate on the velocity profiles around voids. First they show the
necessity of four parameters to describe the density profiles around voids
given two distinct void populations, voids-in-voids and voids-in-clouds. This
profile is used to predict peculiar velocities around voids, and the
combination of the latter with void density profiles allows the construction of
model void-galaxy cross-correlation functions with redshift space distortions.
When these models are tuned to fit the measured correlation functions for voids
and galaxies in the Sloan Digital Sky Survey, small voids are found to be of
the void-in-cloud type, whereas larger ones are consistent with being
void-in-void. This is a novel result that is obtained directly from redshift
space data around voids. These profiles can be used to remove systematics on
void-galaxy Alcock-Pacinsky tests coming from redshift-space distortions.
