New Wrinkles for Maps of Cosmos
BOSTON — ASTRONOMERS trying to chart the universe have a plateful of new findings to digest. Some of the results reported over the past few months reinforce what Margaret Geller and John Huchra of the Harvard Smithsonian Center for Astrophysics call ``one of the most sobering results'' of their early attempts at cosmic mapping. This is the realization that the matter of the universe is organized into structures whose size is larger than astronomers had ever imagined.
In fact, they say, ``We do not yet have a sample of the universe large enough to determine the typical quantitative characteristics of [these structures].'' Matter seems to be concentrated in thin extended sheets along the edges of large voids that contain little detectable material at all.
Astronomers are puzzled as to how the universe got this way. Other recently reported findings, however, suggest that the force of gravity, acting over the 10 billion to 20 billion-year age of the universe, could do the job.
These studies involve the early use of two new tools that give astronomers unprecedented cosmic perspectives.
One such tool is the supercomputer with the power to simulate the evolution of structure in a universe containing millions of material elements acting under the influence of their mutual gravitational attraction. The other tool is a method by which astronomers can at last begin to ``see'' the invisible so-called cold, dark matter that they believe makes up more than 90 percent of the universe's mass.
When astronomers speak of ``mapping'' the universe, they mean making plots of the positions of known galaxies as these are measured relative to Earth and doing it on larger scales than have been covered before. Several such studies have been made over small sections of the sky. These show the well-known groupings of galaxies into clusters of galaxies that are bound together by their mutual gravity. And they show clusters of these clusters.
But as the scale of the studies has expanded, they have begun to show the larger structures into which these clusters are organized. They congregate in thin sheets surrounding voids to suggest a bubble-bath or sponge-like organization of cosmic matter.
This has confounded the astronomers' expectations. Traditional theory held that, while matter forms galaxy clusters, the universe is homogenous in the large. This is why Drs. Geller and Huchra called the discovery of massive structure ``sobering'' in reviewing their own work in the journal Science.
Their goal is to map the galaxy distribution over most of the sky. They still have some way to go. But their latest progress report last November revealed what they dubbed the ``Great Wall.'' It is a thin sheet of galaxies 200 million to 300 million light-years from Earth that's at least 500 million light-years long by 200 million light-years wide and about 1.5 million light-years thick. Actually, as Geller and Huchra note, astronomers will not know the full extent of such structures until they have mapped a far larger volume of the cosmos.
The full picture may be even more complex than even the Geller-Huchra mapping suggests. At a meeting of the American Astronomical Society last month, Alex Szalay of Johns Hopkins University and David Koo of Lick Observatory described a different kind of survey. It is actually a compilation of four surveys at the north and south galactic poles - that is, in directions where there's least interference from matter within this galaxy.
Although the compilation covers a very narrow sky area, it reaches to very faint galaxies to give a kind of core sample through much of the universe. This sample indicates a near periodic occurrence of galaxy structures like the Great Wall spaced about 400 million light-years apart going outward from Earth.
While astronomers have no ready explanation for how the universe became structured in this way, the new computer simulations suggest nothing more exotic than gravity has been at work.
Physics Prof. Edmund Bertschinger and graduate student James Gelb at the Massachusetts Institute of Technology (MIT) showed the Astronomical Society a six-minute computer generated film of galaxy evolution that produced patterns reminiscent of those actually seen in nature. Earlier this month, astrophysicist J. Richard Gott and graduate student Changbom Park of Princeton University reported a more-sophisticated simulation along these lines.
Both of these studies assume that the invisible cold dark matter is the key material with which gravity works to build the cosmic architecture. Studies of how galaxies rotate and how they move in relation to one another within clusters show that they are influenced by 10 times as much mass as astronomers can detect. Astronomers do not know what this matter is. But they think it must be different from the ordinary matter they see in stars and cosmic dust and gas.
In the MIT simulation, the computer tracked 2 million ``particles,'' each of which represented a dark matter mass equivalent to 4 billion suns. As these particles moved under their mutual gravity, 2,000 galaxies formed and grouped into larger structures.
The Princeton simulation involved 4 million particles - 2 million representing dark matter and 2 million representing luminous galaxies. Here again, gravity moved mass around to produce intricate large-scale structures. Both the MIT and Princeton experimenters emphasize that their results are preliminary. Their studies are small-scale compared even to the Geller-Huchra maps, let alone the universe itself. But they show the potential of computer simulation to explore cosmic evolution.
Meanwhile, other scientists are testing a method that may let astronomers finally ``see'' the dark matter, at least indirectly. J. Anthony Tyson and Richard Wenk of AT&T Bell Laboratories and Francisco Valdes of the (United States) National Optical Astronomy Observatories are using the dark matter's own gravity to reveal its presence.
Some 20 billion very distant and very faint galaxies cover the entire sky. They form a background against which to ``see'' concentrations of mass on the intervening space. The gravity of a concentration of dark matter acts like a lens to bend the light from these distant galaxies. Astronomers can detect this distortion of the images of the background galaxies and, hence, can detect the presence of the otherwise invisible dark matter that causes the distortion.
Tracing the distribution of dark matter in this way could ultimately yield a map to complement the charts of visible matter and give a more complete overview of the universe than astronomers have ever had.