Mapping the Universe - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 67

Fig. 2.19 Large-scale structures in the Universe (Reprinted from Scientific American, June 1999)

uniformity. But scientists face a paradox: how can the uniformity on the ultimate
scale be reconciled with the clumpy distributions on smaller scales? Mapping the
universe may provide vital clues.
In the late 1970s and early 1980s, cosmologists began to systematically map
galaxies (Gregory and Thompson 1982 ). Cosmo-cartographers discovered that on
scales of up to 100 million light-years, galaxies are distributed as a fractal with a
dimension of between one and two. The fractal distribution of matter would be a
severe problem for the cosmological principle because a fractal distribution is never
homogeneous and uniform. However, subsequent surveys indicated that on scales of
hundreds of millions of light-years, the fractal nature broke down. The distributions
of galaxies appeared to be random on these scales. The cosmological principle was
saved just before it ran into its next challenge.
Astronomer John Huchra at the Harvard-Smithsonian Center for Astrophysics
(CfA) is well known for his work on mapping the Universe. Between 1985 and 1995,
John Huchra, Margaret Geller and others measured relative distances via redshifts
for about 18,000 bright galaxies in the northern sky to make maps of the distribution
of galaxies around us. The CfA used redshift as the measure of the radial coordinate
in a spherical coordinate system centered on the Milky Way. This initial map was
quite surprising; the distribution of galaxies in space was not random, with galaxies
actually appearing to be distributed on surfaces, almost bubble like, surrounding
large empty regions, or “voids.” Great voids and elongated structures are clearly
indicating organized structure of matter on large-scales. Any cosmological theory
must explain how these structures evolved from an almost uniform universe.
CfA's redshift survey revealed a “Great Wall” of galaxies 750 million light-years
long, more than 250 million light-years wide and 20 million light-years thick (See
Fig. 2.20 ). This Great Wall is now called the CfA Great Wall to differentiate it from