STRUCTURE OF THE UNIVERSE
Large-Scale
Structures Large-scale structures of the universe did not become a preoccupation
of cosmology until the mid-1980s. At that time astronomers Margaret Geller and John Huchra of Harvard University surveyed the northern skies. Their galaxy maps revealed unmistakable patterns that Geller described as looking like "a kitchen sink full of soapsuds," with bubblelike spheres spreading 300 million light-years across. One particularly huge string of galaxies was dubbed the Great Wall. It also became known as the Great Attractor, because the Milky Way and surrounding galaxies seemed to be streaming toward it. Yet only about 10 percent of the observed gravitational attraction could be accounted for by visible matter.
Theories describing the evolution of such
structures can be roughly grouped into two categories, "top down" and "bottom up." The first category has large structures forming early in the history of the universe and then breaking into smaller and smaller ones. Russian theorist Yakov Zeldovich is a leading proponent of the view that huge, flat clouds of gas emerged shortly after the big bang, perhaps because ripples from the blast produced density fluctuations in the primeval gas. These supercluster-size "pancakes" would each stretch over tens of millions of light-years and contain enough matter to form one thousand trillion stars. "Bottom up" theories argue instead that galaxies must have formed first, then grouped into clusters of galaxies, and later congealed into superclusters.
Small-Scale Structures
Evidence for both kinds of theories, perhaps ironically, comes from study of the so-called cosmic microwave
background. The discovery of this leftover glow from the big bang in 1965 spawned a new subfield of astronomy dedicated to exploring the structure of this fossil radiation that comes from all directions, appears to be homogenous in all directions, and is perfectly smooth. This smoothness poses an obvious problem. If the microwave background is indeed the fossil relic of the beginning of the universe, and that universe was completely smooth, how did large-scale structures arise?
The background radiation has been traveling through space-time since radiation first separated from matter. For the first 300,000 years or so after the big bang, matter and energy percolated together, with particles popping into existence only to transform back into radiation. Only when the universe was cool enough so that matter could congeal did it suddenly become transparent, with matter and radiation clearly separated. It is at this point that the microwave radiation seen today, now cooled to a frigid three degrees above absolute zero, would have started its journey through space and time. In those first 300,000 years, gravity was still far too weak to overpower the enormous energies at work. Anything that clumped together would immediately have been ripped apart. Yet if gravity started its work only after the universe had cooled sufficiently, there would not have been enough time in the following 15 billion years or so for huge structures such as the Great Wall to have formed. Clumping works far more readily if there is some kind of density fluctuation, some "seed" of a small concentration of matter or energy to attract more matter to it and continue to grow into galaxies and clusters of galaxies. Try as they might, however, scientists studying the microwave background found no density or temperature fluctuations of any kind.
Then, in 1992, astrophysicist George Smoot announced the results of an experiment performed aboard a satellite named the Cosmic Background Explorer (see radio astronomy). The experiment revealed tiny irregularities in the temperature of the background radiation, suggesting that the density of the early universe was not uniform after all. That is, less-dense areas would have been penetrated by radiation more easily, so temperatures would be higher there, and vice versa. These COBE results required great sensitivity, measuring temperature differences of less than a millionth of a degree. Moreover, a great deal of interference had to be accounted forÑincluding, for example, the effect of the Milky Way's motion on the background radiation. Finally, the scale of the differences COBE recorded was still too large to account for galaxies and clusters. Finer measurements at smaller scales have already been taken, confirming the COBE results, and a second-generation microwave background explorer is already under construction.
Another explanation for clumping might lie with dark matter. Dark matter that does not interact with other matter except gravitationally would not have been affected by the turbulence of the early universe. It could therefore have started producing structures even in the first 300,000 years.