Cosmological Voids: Observational Evidence and Models

The first observation of a large region of universe not containing any visible matter was made in 1981 by Kirshner et al.1. Starting from the discovery of this huge void (about 80Mpc in diameter) of matter in Boötes, galaxy surveys analysis showed that a considerable fraction of the volume of the universe is occupied by empty regions containing only few visible galaxies. These regions have been named Cosmological Voids. Studying the characteristic features of these structures is surely relevant to a deeper understanding of the large scale structure of the universe. Nevertheless, present observational data allow to outline some general features only: cosmological voids seem to occupy about 40% in volume of the universe; they have quasi-spherical simmetry; they have typical diameters of the order of 10÷20Mpc; the morphological properties of galaxies located inside or nearby a void are not considerably different from the cosmological background. The most detailed observational studies on the subject have been made by a research group working in USA at the Departement of Physics of Drexel University in Philadelphia (see, for example, Hoyle et al.2). These studies are based on an automated alghorithm developed by El-Ad et al.3 and named Voidfinder, which allows to construct a void survey starting from a galaxy survey. Let’s briefly outline how Voidfinder works. The analysis of the galaxy survey starts by dividing the galaxies in two subgroups: void galaxies and boundary galaxies. Void galaxies are those having not more than two neighbour galaxies in a sphere of radius r centered on them (usually r is fixed to 5Mpc). All the remaining galaxies in the survey are boundary galaxies. The algorithm then divides the volume occupied by the galaxy survey in cubical cells of appropriate volume (chosen by user) and search for those cells not containing boundary galaxies. Now an approximated