In 1989 astronomers discovered a dense band of galaxies arching some 500 million light-years across the sky. The size of this so-called Great Wall astonished observers, who were at a loss to explain how something so enormous could have formed during the lifetime of the universe. But after six years of theoretical gymnastics, it now appears there may be no Great Wall to explain away. Elizabeth Praton, a physicist at Grinnell College in Iowa, says the Great Wall is essentially a great cosmic optical illusion.
The illusion arises, says Praton, from the way astronomers measure distance to remote regions of the universe. For objects far beyond our galaxy, astronomers must rely on the phenomenon of redshift, which arises from the expansion of the universe. As other galaxies race away from our own, their light is redshifted--stretched toward longer, redder wavelengths. The farther a galaxy lies from Earth, the bigger its redshift. Astronomers relied primarily on redshift to map the size and distance of the Great Wall.
But galaxies also have their own intrinsic motion that can stretch or crunch emitted light depending on the galaxy’s direction of motion. So a galaxy moving away from Earth, in addition to being carried along by cosmic expansion, will have an extra redshift and appear farther from us than it actually is.
Praton worked with astronomer Adrian Melott and his colleagues at the University of Kansas to study a computer model in which most of the universe’s galaxies are distributed not in a baffling Great Wall but in relatively small clusters. In some of these clusters, gravity will be pulling galaxies toward some central point, so that galaxies on the side of the cluster nearest Earth will move away from us; galaxies on the farther side will move toward us. But combined with the redshift due to universal expansion, these galaxies, which are actually on opposite sides of the cluster, perhaps millions of light-years apart, will appear in redshift surveys to be about the same distance from Earth. Praton’s model shows that this can create the appearance of a huge connected wall from the motions of many smaller clusters that are not in fact bound together.
Not all astronomers accept Praton’s model, but she says an ambitious survey of galaxies could settle the issue. If her model is right, a survey would detect concentric rings of Great Walls in every direction around Earth, an illusion produced by innumerable distant small clusters appearing to coalesce on redshift maps. Praton’s model universe is filled with such small clusters, and some long, tenuous filaments of galaxies, but is completely devoid of Great Walls. Of her model cosmos she says: It looks very bland.