Cosmic voids detection without density measurements
  Cosmic voids are effective cosmological probes to discriminate among
competing world models. Their identification is generally based on density or
geometry criteria that, because of their very nature, are prone to shot noise.
We propose two void finders that are based on dynamical criterion to select
voids in Lagrangian coordinates and minimise the impact of sparse sampling. The
first approach exploits the Zel'dovich approximation to trace back in time the
orbits of galaxies located in voids and their surroundings, the second uses the
observed galaxy-galaxy correlation function to relax the objects' spatial
distribution to homogeneity and isotropy. In both cases voids are defined as
regions of the negative velocity divergence, that can be regarded as sinks of
the back-in-time streamlines of the mass tracers. To assess the performance of
our methods we used a dark matter halo mock catalogue CoDECS, and compared the
results with those obtained with the ZOBOV void finder. We find that the void
divergence profiles are less scattered than the density ones and, therefore,
their stacking constitutes a more accurate cosmological probe. The significance
of the divergence signal in the central part of voids obtained from both our
finders is 60% higher than for overdensity profiles in the ZOBOV case. The
ellipticity of the stacked void measured in the divergence field is closer to
unity, as expected, than what is found when using halo positions. Therefore our
void finders are complementary to the existing methods, that should contribute
to improve the accuracy of void-based cosmological tests.
