Encyclopedia of the History of Astronomy and Astrophysics by David Leverington

Author:David Leverington [Leverington]
Language: eng
Format: epub
Publisher: Cambridge University Press
Published: 2014-07-27T16:00:00+00:00

Large-scale structure of the universe

It gradually became clear in the first half of the twentieth century that galaxies were generally associated gravitationally with other galaxies as groups (like the Local Group) and/or as clusters of galaxies (see above). Groups typically contained a few dozen galaxies within a diameter of up to 2 Mpc (megaparsec), whilst clusters typically contained hundreds or thousands of galaxies within a diameter up to ∼10 Mpc. Then in the 1950s Gerard de Vaucouleurs found evidence of the first supercluster, now called the Local or Virgo supercluster, which included the Virgo cluster. Eventually more and more superclusters were found with a large range of sizes varying from about 10 to 100 Mpc.

In 1966 Elihu Boldt and colleagues, using a sounding rocket experiment, detected what appeared to be an extended source of X-rays from the Coma cluster of galaxies. James Felten and colleagues suggested that this could be the result of thermal bremsstrahlung radiation emitted by intergalactic gas at a temperature of ∼108 K. Subsequent investigators could find no evidence of this X-ray source until it was rediscovered in 1971 by John Meekins and colleagues, and Herbert Gursky and colleagues in parallel. In the same year Edwin Kellogg, Gursky and colleagues also found similar emission from the Virgo and Perseus clusters with the Uhuru spacecraft. Since then thermal bremsstrahlung radiation has been found to be a normal feature of clusters of galaxies, indicating that they are generally permeated by very hot gas.

Arno Penzias and Robert Wilson had discovered the microwave background radiation in 1965, which was the radiation left over from the Big Bang. Further work by others indicated that this radiation was extremely regular across the sky, supporting the idea, common at the time, that the universe on the largest scales would be essentially unstructured. But in 1981 Robert Kirshner and colleagues found evidence for a large void in Bootes. In the same year Gregory, Thompson and Tifft also found evidence of filamentary structure in the universe when they confirmed the existence of the Perseus-Pisces chain of galaxy clusters at a distance of ∼75 Mpc. Then in 1982 Zeldovich, Einasto and Shandarin identified a generally cell-like structure of the local universe of superclusters.

Compared to the size of the universe as a whole, the volume that had been examined by the early 1980s was very small. As a result John Huchra, Margaret Geller and colleagues began an investigation, as part of an extension to the Center for Astrophysics redshift survey, to try to map the universe as far out from the Milky Way as possible. In 1986 they announced their first results for a slice of the universe centred on the Milky Way extending for about 120° in right ascension and 6° in declination, down to a limiting magnitude of 15.5 (equivalent to a distance of ∼200 Mpc). In this they found a bubble-like structure, in which the clusters and superclusters of galaxies formed the surface of the bubbles. This created a real problem for the theoreticians who tried to explain it.

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