Main properties

The fluid of stars in a galaxy closely resembles the fluid of molecules. The fluid of galaxies does not. Let us examine three main differences.

(a) The distribution of galaxies is not chaotic. There are groups, clusters, superclusters, and there is a large-scale structure, which we are now beginning to realize. The distinction between clusters and superclusters is not sharp, and as superclusters are elements of larger structures their limits and sizes are difficult to establish. The largest observed structures are as large as the limits of the deepest surveys. A continuous spectrum of inhomogeneities describes structures larger than a galaxy better than it describes discrete objects such as stars and galaxies.

The relative increase in the density with respect to the mean density δ is about 102 –103 for clusters. Typical intercluster distances of about 5 Mpc and inter-supercluster distances of about 25 Mpc are revealed by cross-correlation studies. The value of δ for a galaxy is about 105.

The large-scale structure of the Universe is complex. Many clusters are aligned in huge filaments. Others seem to form sheets. There are also voids which are apparently deplete of galaxies. A simplified picture of the large-scale structure might consist of an ensemble of large polyhedral voids. In the limiting sheets separating two adjacent voids there are clusters. Along the limiting line intersections there are more clusters. At the limiting vertices there are still more clusters. The linear dimensions of the voids are typically 20 – 50 Mpc.