Great wall not so great?

They call it the Great Wall-a thin, curving sheet of galaxies stretching half a billion light-years across the sky (SN: 11/25/89, p. 340). When astronomers map the location of the Great Wall and the Pisces-Perseus chain, these two superclusters together seem to form giant arcs that encircle Earth. The existence of such large-scale clusters and the patterns they form has stymied theorists struggling to explain the evolution of structure in the universe. Now, researchers suggest that these superclusters don't arrange themselves in a concentric pattern and that individual structures like the Great Wall may consist of a looser grouping of galaxies than they appear to. Theory and observation are closer together than previously thought, says Elizabeth A. Praton of Grinnell College in Grinnell, Iowa.

She notes that astronomers had found evidence of clustering when they mapped galaxies according to the rate at which they are speeding away from Earth. Because the universe is expanding, a galaxy with a higher velocity is considered more distant.

However, this assumption does not always hold true. A distant galaxy's total velocity has two parts-a large component caused by cosmological expansion and a smaller "peculiar velocity" generated, for example, by the tug of neighboring galaxies. Peculiar velocity can either add or subtract from the expansion velocity. Thus, a galaxy with a slightly smaller recession velocity may actually lie farther away than a galaxy with a slightly higher speed. Praton's computer simulations, which include peculiar velocity, show that the groups of galaxies aligned perpendicular to the line of sight from Earth are not as tightly packed as previously thought. J. Richard Gott of Princeton University says he agrees with the findings. Praton, Adrian L. Melott of the University of Kansas in Lawrence, and Grinnell colleague Margaret McKee presented them June 10 at a meeting of the American Astronomical Society in Madison, Wis.