Large Scale Structures
Since the 1970s large scale structure surveys have provided a picture of the superclusters and voids in the universe. All major known clusters of galaxies and at least 95% of all galaxies are found in superclusters. The voids between superclusters may contain faint galaxies but no bright galaxies. The voids tend to be spherical but superclusters are not. Superclusters have long filamentary or sheet-like structures that provide the boundaries for the voids.
Our galaxy is located at one end of the Local Group—a small cluster of galaxies with the Andromeda galaxy at the other end. The Local Group is near the outskirts of the Local Supercluster which has a diameter of 100 million light-years. This somewhat flattened supercluster consists of two major hot dog-shaped groups of galaxies. It contains a total of 1015 times the mass of the Sun, most of which is concentrated into 5% of the volume of the supercluster.
The biggest supercluster is the Perseus-Pegasus Filament discovered by David Batuski and Jack Burns of New Mexico State University. This filament contains the Perseus supercluster and stretches for roughly a billion light years. It is currently the largest known structure in the universe.
Margaret Geller and John Huchra have mapped a region that is 500 million light-years long but only 15 million light years thick, but this area, the Great Wall, has not been completely mapped. They may find it extends longer than 500 million light-years when the mapping is complete.
A number of other superclusters and voids are known but astronomers have only mapped the large scale structure of a small part of the universe. They need to do much more work before we completely understand the structure of superclusters and voids. There may even be larger scale structures.
One of the pressing issues related to superclusters is how such structures formed in the early universe. This question remains unanswered but observations of the cosmic background radiation indicate that its beginnings developed even before galaxy formation.
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Paul A. Heckert