Similar to the population of dark halos, the population of voids in the cosmic matter distribution is evolving in a hierarchical fashion. Unlike their overdense peers evolving from the equivalent peaks in the primordial Gaussian density field, the hierarchical evolution of underdense troughs proceeds such that a peaked void size distribution emerges.
Two processes determine the fate of a primordial underdensity. When it is embedded with some of its peers in a larger scale underdensity, they will merge at a specific epoch into a larger void. In addition a second process occurs, that of “void collapse.” If a primordial underdensity finds itself embedded in a larger scale overdensity it will start to contract, collapse and disappear. This process explains the absence of small voids, and will naturally produce a peaked void size distribution.
We show that on the basis of an extension of the extended Press-Schechter excursion set formalism that at any cosmic time voids indeed predicts a peaked size distribution. The characteristic size of this void distribution evolves self-similarly in time, as larger voids form from mergers of voids present at an earlier cosmic epoch. By means of constrained N-body simulations of void evolution, we are trying to identify the population of collapsing voids. Most are small voids near the boundaries of large ones and get squeezed out of existence by large shear flows near the boundary.
This picture offers a tantalizing view of the formation of the cosmic web as the product of a merging void population whose current typical size is ∼15-20h-1 Mpc. The formalism is being extended towards predictions on e.g. the properties of a void galaxy population.