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Walls within Voids

Astrophysicists are using increasingly sophisticated arrays of ground-based and space-borne instruments to look out and back to the most distant (and oldest!) regions of the universe. They're constructing a map of the large-scale organization of the universe that is nothing short of amazing.

Astrophysicists at the Harvard-Smithsonian Center for Astrophysics are undertaking large-scale surveys of part of the universe. Their goal is to construct three-dimensional maps by measuring the cosmological redshifts of galaxies that appear to lie within thin, pie-shaped, adjoining "slices" of space.

Thus far, about 100,000 galaxies have been mapped (by the Center for Astrophysics team and others) from both the Northern and Southern hemispheres, to distances greater than 500 million light years. They already knew two of the galaxy's coordinates just by locating them on the sky with their telescopes. The measured redshift for each galaxy was converted to distance, with increasingly distant galaxies having greater redshifts. This allowed the scientists to plot the third coordinate.

North and South, Sheets and Voids

The first maps, published in 1986, were a great surprise to the astrophysicists. They had expected to find relative uniformity above the scale of the already-familiar galaxy clusters. Instead, the first surveys showed--and subsequent surveys have confirmed--that great clusters of galaxies are arranged in thin sheets or long filaments. The longest sheet detected, called the "Great Wall," extends hundreds of millions of light years across the maps.

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Interspersed amongst the sheets are great holes, ranging in size from 100 million to 400 million light years in diameter, almost devoid of galaxies. Margaret Geller and John Huchra, two of the scientists on the Center for Astrophysics team, likened the structures to a collection of giant soap bubbles, with the galaxies residing on the surface of the bubble.

Recent observations show, however, that the voids may not be completely empty after all--at least not all of them. Using the Hubble Space Telescope's Goddard high-resolution spectrograph, researchers from the University of Colorado, Boulder have detected clouds of hydrogen gas in nearby voids. Astronomers have detected hydrogen gas clouds before, but they've been in such remote parts of the universe that it was difficult to tell whether they lay in voids.

By extrapolation, researchers have estimated that the gas clouds contained within voids could collectively contain as much mass as there is in the visible galaxies--yet another contributer to the "missing" dark matter. The question is whether the gas clouds are comprised of primordial hydrogen, or are the halos of faint dwarf galaxies.

Scientists have actually peered even further out into the cosmos--to about 2,500 million light years. And, although they haven't actually mapped that part of the universe, what they see are not ever larger walls and voids, but more of the same. There are more powerful telescopes under construction that will allow the astrophysicists to probe even deeper into the universe. Will they see the same structures in the more distant--and younger--universe? Will they detect less structure, or simply different kinds of structures? Time, and the heavens, will tell.

Michael Norman, NCSA/Univ. of Illinois, on-camera
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The X-ray perspective

There's another way of looking at large-scale structures--by the X-rays they emit. Cosmologists believe that gases gravitationally pulled into high density regions underwent tremendous collisions that produced shock waves, causing the gases to heat up in many regions to as much as 1-10 million degrees Kelvin. The super-hot gases emit large amounts of energy in the form of X-rays, detectable through satellite-based instruments.

The Coma Cluster

Astrophysicists have targeted their sights on giant clusters of galaxies, particulary the Coma Cluster which lies more than 5 times farther away from us than the Virgo Cluster.
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Coma Cluster X Ray

The first instrument able to image the X-ray sky, launched in 1978, was the Einstein Observatory. Einstein detected only the densest (and therefore hottest) clusters of X-rays at the intersections of galaxy filaments.
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Since that time, the European Space Agency (ESA) and NASA have launched a more sensitive but less far-reaching instrument, ROSAT (ROentgen SATellite).
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Advanced X-Ray Facility

NASA plans to launch an instrument called the Advanced X-ray Astronomical Facility (AXAF) in the late 1990s. AXAF will be capable of detecting the faint X-rays from distant galaxies far beyond the reach of either ROSAT or the Einstein Observatory and with much greater sensitivity. Cosmologists anticipate a wealth of X-ray data.
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The formation of large-scale structures

How did those super-structures evolve from a universe that was initially uniform (or nearly so)?

Two theories involve dark matter. If neutrinos (hot dark matter) indeed have mass, they would tend to accumulate at the sites of excess density, left over from the density fluctuations of the Big Bang. The gravitational pull of the neutrinos may have pulled in surrounding matter, leading first to the formation of galaxy clusters and superclusters, and later to the formation of galaxies as these larger structures fragmented (the so-called "Top Down" model).

Now, it may seem that the nearly massless neutrinos could not exert much gravitational force, but remember that there are about 100 million neutrinos per cubic meter. Like Senator Everett Dirksen's famous quote about the federal budget ("A billion here and a billion there and pretty soon you're talking big money."), a little mass adds up!

A universe dominated by cold dark matter (weakly interacting massive particles, or WIMPS), on the other hand, suggests that there was a whole spectrum of density fluctuations in the early universe. Under this theory (the "Bottom Up" theory) galaxies form first only at the extreme and rare density peaks, and later the galaxies cluster into larger structures--clusters and superclusters. Most of the cold dark matter would be concentrated in the great voids, outside of the galaxy superclusters.

Theory meets data

Of course, cosmologists can't very well test their theories in the laboratory, so they turn to computers instead. Observations in both the visible and the X-ray portions of the spectrum give the cosmologists plenty of hard data with which to compare the predictions of their computer models. Pure hot dark matter models, it turned out, were a wash. That scenario predicted that galaxies formed much later than they really do. The pure cold dark matter models, while not perfect (they tend to produce larger and more elaborate superclusters than we observe), are closer to reality.

Michael Norman, an astrophysicist at the University of Illinois at Urbana-Champaign has constructed a computer model that incorporates both cold and hot dark matter, an approach that seems to more nearly approximate the observed structure of the universe.

Jeremiah Ostriker, Princeton University, on-camera
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Norman and Ostriker are part of an NSF-funded HPCC grand challenge project that aims to simulate the evolution of the X-ray universe on a large scale, in anticipation of the AXAF observations to come a few years from now.

Thanks to advances in both instrumentation and computation, closer matching of observation and theory is not only becoming possible, but crucial for further progress in solving the major mysteries of the origins, evolution and fate of the universe.

Cosmology Goes Digital
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Copyright, (c) 1995: Board of Trustees, University of Illinois

NCSA. Last modified 11/2/95.