The Texture of The Universe

Despite Guth's prediction, astronomical observations continue to suggest that the universe is open, without boundaries. As David Koo from the University of California, Santa Cruz, concluded, "If omega is less than one, the universe will not collapse. Presumably, we will die by ice rather than by fire." Yet astrophysicists look eagerly for hints of further amounts of dark matter by studying the various structures observed in the celestial sky. How the universe forged its present structures—galaxies grouping to form clusters, and the clusters themselves aggregating into superclusters—is significantly dependent on both the nature and amounts of dark matter filling the universe. The dark matter provides vital information, said Edmund Bertschinger of the Massachusetts Institute of Technology, "for the reigning cosmological paradigm of gravitational instability." Had there not been matter of a certain mass several hundred thousand years after the Big Bang, the universe would have been smooth rather than "clumpy," and the interactions between aggregations of particles necessary to form structures would not have taken place. S. George Djorgovski from the California Institute of Technology agreed that the ''grand unifying theme of the discussion at the symposium—and for that matter in most of the cosmology today—is the origin, the formation, and the evolution of the large-scale structure of the universe,'' which, he said, "clearly must be dominated by the dark matter."

Geller and another guest at the symposium, Simon White from Cambridge University in Great Britain, have for years been involved in pioneering research to elucidate this large-scale structure. Until the 1980s, the best map of the universe at large was based on a vast catalog of galaxy counts, around 1 million, published in 1967 by Donald Shane and Carl Wirtanen from the Lick Observatory in California. They compiled their data from an extensive set of photographs taken of the entire celestial sky as seen from the Lick Observatory. Shane and Wirtanen spent 12 years counting each and every galaxy on the plates, and from their catalog P. James E. Peebles and his colleagues at Princeton University later derived the first major map of the universe, albeit a two-dimensional one. Peebles' 1-million-galaxy map was striking: The universe appeared to be filled with a network of knotlike galaxy clusters, filamentary superclusters, and vast regions devoid of galaxies. But was that only an illusion?

The raw input for Peebles' map did not include redshift data—data for establishing the third point of reference, the absolute distances of the various galaxies. How does one know, from a two-dimensional view alone, that the apparent shape of a group of stars (or galaxies) is not merely an accidental juxtaposition, the superposition of widely scattered points of light (along that particular line of sight) into a pattern discernible only because a human being is stationed at the apex? Without a parallax or triangulated view, or some other indicator of the depth of the picture being sketched, many unsupported suppositions were made, abetted perhaps by the inherent tendency of the human brain to impose a pattern on the heterogeneous input it receives from the eye.

The universe does display inhomogeneity; the question is how much. Soon after galaxies were discovered by Hubble in the 1920s, the world's leading extragalactic surveyors noticed that many of the galaxies gathered, in proximity and movement, into small groups and even larger clusters. The Milky Way and another large spiraling galaxy called Andromeda serve as gravitational anchors for about 20 other smaller galaxies that form an association called the Local Group, some 4 million light-years in width (a light-year being the distance light travels in a year, about 6 trillion miles). Other clusters are dramatically larger than ours, with over 1000 distinct galaxies moving about together. Furthermore, our Local Group is caught at the edge of an even larger assembly of galaxies, known as the Local Supercluster. Superclusters, which can span some 108 light-years across, constitute one of the largest structures discernible in astronomical surveys to date.

In mapping specific regions of the celestial sky, astronomers in the 1970s began to report that many galaxies and clusters appeared to be strung out along lengthy curved chains separated by vast regions of galaxyless space called voids. Hints of such structures also emerged when Marc Davis and John Huchra at the Harvard-Smithsonian Center for Astrophysics completed the first comprehensive red-shift survey of the heavens in 1981, which suggested a "frothiness" to the universe's structure, a pattern that dramatically came into focus when Geller and Huchra extended the redshift survey, starting in 1985. Probing nearly two times farther into space than the first survey, the second effort has now pegged the locations of thousands of additional galaxies in a series of narrow wedges, each 6 degrees thick, that stretch across the celestial sky. Geller and her associates found, to their surprise, that galaxies were not linked with one another to form lacy threads, as previous evidence had been suggesting. Rather, galaxies appear to congregate along the surfaces of gigantic, nested bubbles, which Geller immediately likened to a "kitchen sink full of soapsuds." The huge voids that astronomers had been sighting were simply the interiors of these immense, sharply defined spherical shells of galaxies. Each bubble stretches several hundreds of millions of light-years across.

White and his collaborators, hunting constantly for clues to this bubbly large-scale structure, have developed a powerful computer simulation of its formation. This domain allows them to probe the nonequilibrium, nonlinear dynamics of gravitating systems. They have applied a great deal of theoretical work to the dark matter problem, helping to develop current ideas about the collapse of protogalaxies, how filaments and voids form in the large-scale distribution of clusters, and how galaxy collisions and mergers may have contributed to the evolution of structure in the present universe. They have used their numerical simulation technique to explore the types of dark matter that might give rise to such structures, such as neutrinos (stable, elementary particles with possibly a small rest mass, no charge, and an extreme tendency to avoid detection or interaction with matter) and other kinds of weakly interacting particles that have been hypothesized but not yet discovered. They link various explanations of dark-matter formation and distribution to the large-scale galaxy clustering that seems—as astrophysicists reach farther into deep space—to be ubiquitous at all scales.

But cosmologists look at the universe from many different perspectives, and many astrophysicists interested in galaxy formation and structure energetically search the skies in hopes of finding a galaxy at its initial stage of creation. Djorgovski goes so far as to call the search for a new galaxy aborning "the holy grail of modern cosmology. Though I suspect the situation is far more complicated, and I am not expecting the discovery of primeval galaxies." Rather, he predicted, "we will just learn slowly how they form," by uncovering pieces of the puzzle from various observations. One such effort he calls "paleontocosmology: since we can bring much more detail to our studies of nearby [and therefore older] galaxies than those far away, we can try to deduce from the systematics of their observed properties how they may have been created."

This ability to look back in time is possible because of the vast distances the radiation from the galaxies under observation must travel to reach earthbound observers. All electromagnetic radiation travels at a speed of approximately 300,000 kilometers per second (in a year about 9 trillion kilometers or 6 trillion miles, 1 light-year). Actually, the distances are more often described by astrophysicists with another measure, the parallax-second, or parsec, which equals 3.26 light-years. This is the distance at which a star would have a parallax equal to 1 second of arc on the sky. Thus 1 megaparsec is a convenient way of referring to 3.26 million light-years.

The laws of physics, as far as is known, limit the velocity of visible-light photons or any other electromagnetic radiation that is emitted in the universe. By the time that radiation has traveled a certain distance, e.g., 150 million light-years, into the range of our detection, it has taken 150 million years—as time is measured on Earth—to do so. Thus this radiation represents information about the state of its emitting source that far back in time. The limit of this view obviously is set at the distance light can have traveled in the time elapsed since the Big Bang. Astrophysicists refer to this limit as the observable universe, which provides a different prospect from any given point in the universe but emanates to a horizon from that point in all directions. Cosmologists study the stages of cosmic evolution by looking at the state of matter throughout the universe as the photons radiating from that matter arrive at Earth. As they look farther away, what they tend to see is hotter and is moving faster, both characteristics indicative of closer proximity to the initial event. The Big Bang paradigm allows cosmologists to establish points of reference in order to measure distances in deep space. As the universe ages, the horizon as viewed from Earth expands because light that was emitted earlier in time will have traveled the necessary greater distance to reach our view. At present, the horizon extends out to a distance of about 15 billion light-years. This represents roughly 50 times the average span of the voids situated between the richest superclusters recently observed. These superclusters are several tens of millions of light-years in extent, nearly 1000 times the size of our Milky Way galaxy. Our star, the Sun, is about 2 million light-years from its galactic neighbor, Andromeda. Nine orders of magnitude closer are the planets of its own solar system, and the Sun's light takes a mere 8 minutes to reach Earth.

Tyson's surveys excite astrophysicists because they provide data for galaxies more distant than those previously imaged, ergo, a window further back in time. Nonetheless, Jill Bechtold from the University of Arizona reminded symposium participants that "quasars are the most luminous objects we know, and we can see them at much greater distances than any of the galaxies in the pictures that Tony Tyson showed. Thus, you can use them to probe the distribution of the universe in retrospect," what astrophysicists call "look-back" time. Quasar is the term coined to indicate a quasistellar radio source, an object first recognized nearly 30 years ago. Quasars are thought to be the result of high-energy events in the nuclei of distant galaxies, and thus produce a luminance that can be seen much further than any individual star or galaxy.

Tyson and his fellow scientists took the symposium on a journey into deep space, showing pictures of some of the farthest galaxies ever seen. While these are the newest galaxies to be observed, their greater distance from Earth also means they are among the oldest ever seen, reckoned by cosmological, or lookback time. The astrophysicists described and explained the pictures and the myriad other data they are collecting with powerful new imaging tools, and also talked about the simulations and analyses they are performing on all of these with powerful new computing approaches. Along the way, the history of modern cosmology was sketched in: exploratory efforts to reach out into space, and back in time, toward the moment of creation some 10 billion to 20 billion years ago, and to build a set of cosmological theories that will explain how the universe got from there to here, and where—perhaps—it may be destined. In less than two decades, scientists have extended their probes to detect the radiation lying beyond the comparatively narrow spectrum of light, the only medium available to optical astronomers for centuries. The pace of theoretical and experimental advance is accelerating dramatically. The sense of more major discoveries is imminent.