The Evolution of Voids in the Adhesion Approximation

We apply the adhesion approximation to study the formation and evolution of voids in the Universe. Our simulations – carried out using 128 particles in a cubical box with side 128 Mpc – indicate that the void spectrum evolves with time and that the mean void size in the standard COBE-normalised Cold Dark Matter (hereafter CDM) model with h50 = 1, scales approximately as D̄(z) = D̄0 √ 1+z , where D̄0 ≃ 10.5 Mpc. Interestingly, we find a strong correlation between the sizes of voids and the value of the primordial gravitational potential at void centers. This observation 1 could in principle, pave the way towards reconstructing the form of the primordial potential from a knowledge of the observed void spectrum. Studying the void spectrum at different cosmological epochs, for spectra with a built in k-space cutoff we find that, the number of voids in a representative volume evolves with time. The mean number of voids first increases until a maximum value is reached (indicating that the formation of cellular structure is complete), and then begins to decrease as clumps and filaments merge leading to hierarchical clustering and the subsequent elimination of small voids. The cosmological epoch characterizing the completion of cellular structure occurs when the length scale going nonlinear approaches the mean distance between peaks of the gravitational potential. A central result of this paper is that voids can be populated by substructure such as mini-sheets and filaments, which run through voids. The number of such mini-pancakes which pass through a given void, can be measured by the genus characteristic of an individual void which is an indicator of the topology of a given void in initial (Lagrangian) space. Large voids have on an average a larger genus measure than smaller voids indicating more substructure within larger voids relative to smaller ones. We find that the topology of individual voids is strongly epoch dependent, with void topologies generally simplifying with time. This means that as voids grow older they become progressively more empty and have less substructure within them. We evaluate the genus measure both for individual voids as well as for the entire ensemble of voids predicted by the CDM model. As a result we find that the topology of voids when taken together with the void spectrum, is a very useful statistical indicator of the evolution of the structure of the Universe on large scales. 2