A `bubble bath' theory of the universe. Latest evidence suggests that mammoth clusters of galaxies surround voids in space
ASTRONOMERS knew little about the overall structure and substance of the universe when the 20th century began. By the 1950s, they thought they grasped its essence. They believed the cosmos to be uniform on the large scale so that any one part of it would look the same as any other part. They believed it to consist primarily of hydrogen - the principal element by which stars and galaxies shine. But as the 20th century nears its end, they confront their earlier ignorance. Recent discoveries suggest the universe has lumps and bubbles of mammoth scale. Last fall, R.Brent Tully of the University of Hawaii reported finding what may be the largest such irregularity yet known. It's an association of galaxy clusters a billion light-years long. A light-year, or the distance light travels in a year, is 6 million, million miles. It would take light a billion years - roughly one-tenth of the age of the universe - to travel the length of this structure.Skip to next paragraph
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If the universe is homogeneous in the large, astronomers have yet to prove it. Dr. Tully notes that observers expected to begin to see this smoothness as their studies embraced larger volumes. Instead, he says, ``We see more inhomogeneity.''
Also, hydrogen no longer seems to be the primary cosmic substance. A great deal of mass - some experts suggest as much as 90 percent of the universe - is invisible. Astronomers detect this so-called dark mass - when they can do so at all - only by gravitational influence. Some theorists believe the bulk of it has yet to be found and that it may exist as new forms of matter.
Thus astronomers once again must confess they don't know what the universe is made of. Referring to this burgeoning ignorance, Yale University astrophysicist Lawrence M. Krauss has observed that ``cosmology is in many ways in its infancy.''
As astronomers wrestle with this ancient question of what our universe is really like, they point out that they expect some clumpiness. It's the vast scale of these irregularities that surprises them.
A completely smoothed out and featureless universe would be unstable. Small differences in density would develop. The denser clumps would grow as their stronger gravity pulled in material from their surroundings. Indeed, galaxies, clusters of galaxies, and gravitationally bound associations of clusters called superclusters may have formed at least partly in this way. But astronomers expected such structures to be more or less randomly distributed. Instead, they are finding a kind of Swiss cheese or bubble-bath arrangement. Vast associations of galaxies seem to spread around the bubble walls, leaving relatively empty voids inside.
Five years ago, Tully was pointing out that evidence available at that time showed that ``clumps and holes can exist on scales as large as 300 million light-years.'' Shortly thereafter, Riccardo Giovanelli of the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico, and Martha P. Haynes of the National Radio Astronomy Observatory at Green Bank, W.Va., mapped part of the sky at radio wavelengths and found a band of associated galaxies 700 million light-years long. It was the largest continuous structure charted on the sky up to that time. Dust had hidden it in surveys made with optical telescopes. But it showed up in the radio maps as a continuous band of galaxies linking two previously known superclusters - one in the constellations Perseus and Pisces and the other in Ursa Major and Lynx.
Then, about two years ago, John Huchra, Margaret Geller, and Valerie d'e Lapparent of the Harvard-Smithsonian Center for Astrophysics made a three-dimensional map of a slice of sky that contains nearly 1,100 galaxies. In the angular measure astronomers use in sky mapping, the slice was 6 degrees thick by 117 degrees wide and extended 450 to 900 million light-years in depth. Although that's a small part of the cosmos, it is big enough to suggest how galaxies are distributed. Instead of the expected random scattering, galaxies appeared to lie on the surface of voids 90 to 150 million light-years in diameter. They appeared like flecks of foam on the surface of soap bubbles.