We present a model for the distribution of void sizes and its evolution
within the context of hierarchical scenarios of gravitational structure
formation. For a proper description of the hierarchical buildup of the system
of voids in the matter distribution, not only the "void-in-void" problem should
be taken into account, but also that of the "void-in-cloud" issue. Within the
context of the excursion set formulation of an evolving void hierarchy is one
involving a "two-barrier" excursion problem, unlike the "one-barrier" problem
for the dark halo evolution. This leads to voids having a peaked size
distribution at any cosmic epoch, centered on a characteristic void size that
evolves self-similarly in time, this in distinct contrast to the distribution
of virialized halo masses which do not have a small-scale cut-off.Comment: 6 pages, 4 figures, to appear in the Proceedings of IAU Colloquium
195: "Outskirts of Galaxy Clusters: intense life in the suburbs", Torino
Italy, March 200
