1989

Walls of Galaxies

Margaret Geller (b. 1948), John Huchra (1948–2010)

Advances in telescopes, spectrometers, and photographic plates allowed early-twentieth-century astronomers such as Vesto Slipher and Edwin Hubble to determine the Doppler Shifts of distant galaxies in our expanding universe. Using Hubble’s Law, the distances to those red-shifted galaxies could be estimated. It was slow going—only around 600 galaxies had estimated distances by the 1950s. By the 1970s and 1980s, however, larger telescopes, advances in digital detectors (such as charge-coupled devices, or CCDs), and the advent of dedicated all-sky galaxy surveys had enabled red shifts to be measured for more than 30,000 galaxies. Finally, the universe could begin to be mapped.

A pioneer in galaxy mapping has been the American astronomer Margaret Geller, who, along with fellow Harvard-Smithsonian Center for Astrophysics (CfA) astronomer John Huchra, has been using the results of several large-scale galaxy red-shift surveys to discover the structure of the cosmos. The first CfA red-shift survey ran from 1977 to 1982, and the second from 1985 to 1995. In 1989 Geller and Huchra reported that the distribution of galaxies is far from uniform. Instead, galaxies are clumped into enormous filamentary structures surrounding giant voids containing few galaxies, in a structure now called the cosmic web. One of the structures found by Geller and Huchra was called the “Great Wall”; at over 500 million light-years long and 300 million light-years wide, the Great Wall is one of the largest known structures in the universe.

Astrophysicists have a powerful working model for the formation of these vast patterns in the universe: galaxies and larger patterns grow from very small irregularities in the matter distribution in the early universe. In some versions of this model, these irregularities can arise from a very early (during the first 10^-32 seconds after the Big Bang!) epoch of inflation, which smeared matter into weblike filaments that may have clumped into galaxies.

Harvard astronomer Margaret Geller.

Since the initial CfA studies, other more ambitious galaxy surveys have followed, such as the Sloan Digital Sky Surveys (starting in 2000), which despite more than a decade of work have only mapped out 1/10,000th of the visible universe so far!

A slice through part of a 3-D map of the structure of distant galaxies, from the Sloan Digital Sky Survey. Earth is at the center right, and the outer circle at left is 2 billion light-years away. Each dot is a galaxy, with redder dots representing galaxies containing older stars.