ONCE again, astronomers are hailing discovery of a supergiant. This time it's the most massive cosmic structure yet known. But the significance of the aggregation of galaxy clusters Brent Tully of the University of Hawaii is studying lies less in the fact that it spans one-tenth of the known universe than in the way it challenges conventional notions of how that universe has evolved.
Cosmologists like to keep their theories simple. One of their basic assumptions is that the universe has little large-scale structure. In other words, they assume that, on the largest scales, the universe is smooth and homogeneous. It looks more or less the same in all directions.
That assumption seems increasingly flaky as astronomers discover distinctive cosmic structures on ever larger scales. They have long known that galaxies group together in clusters, bound loosely by their mutual gravitational attraction, and that clusters group into superclusters. Our own Milky Way galaxy is part of a supercluster named after the constellation Virgo.
In recent years, however, observers have found even larger galaxy associations that stretch in long filaments and sheetlike structures over many hundreds of millions of light-years. In fact, the universe seems to have a Swiss cheese structure, with filaments of galactic material surrounding relatively empty regions.
The largest such ``void'' - a volume some 300 million light-years in diameter - lies in the constellation Bo"otes. Last March, Robert P. Kirshner and colleagues at the Harvard-Smithsonian Center for Astrophysics reported that an extensive survey failed to locate any bright galaxy within that volume. A similar region of ``normal'' space would have more than a thousand such objects.
Also, over the past decade, astronomers have convinced themselves that the Virgo supercluster, in which we are embedded, and a couple of nearby superclusters are sliding in the direction of the Southern Cross at some 2 million kilometers an hour. It looks as though an as yet unobserved mass is pulling these galaxies aside. If what astronomers call the great attracter exists, it would be an enormous concentration of mass - perhaps several million light-years long by several hundred thousand light-years wide.
Cosmologists were digesting such findings when the US National Science Foundation announced Tully's discovery last week. It's an aggregate of galactic superclusters that forms a relatively flat structure on the order of a billion light-years long by 150 light-years across. It contains some 100 times the mass of any other distinctive cosmic structure yet found.
Cosmologists aren't ready to give up their assumption that, viewed as a whole, the universe is homogeneous. Among other things, the ubiquitous low-frequency radio radiation left over from the birth of the cosmos is uniform across the sky. But cosmologists are hard pressed to explain how lumpiness on the scale of Tully's structure could arise.
In the conventional view, such vast structures would have evolved from equally vast primitive concentrations of mass that appeared shortly after the universe itself emerged in the Big Bang explosion. As Tully notes, the early appearance of such enormous mass concentrations should have caused irregularities in the background radiation itself. Yet no such irregularities are observed.
Thus cosmologists may be at a crossroads in their science. The universe is turning out to be quite different from what they had imagined.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.