COMPOSITION OF THE UNIVERSE
The kinds and amounts of matter that populate the universe are of intense interest because,
in the end, the density of the universe will determine its fate. At present, dark matter appears to account for the greatest percentage of matter. The evidence for this conclusion has come from several different fields.
Galaxy Rotation
The existence of dark matter was first proposed in 1933 by astronomer Fritz Zwicky. He discovered that galaxies on the outer edges of the Coma cluster were orbiting so rapidly that they should have flown out of the system. Something unseen appeared to be providing the gravitational pull needed to keep the group together. At the time Zwicky's observation was not taken seriously. In the 1970s, however, astronomer Vera Rubin made a similar observation when tracking the spiral motion of galaxies. It had been thought that a galaxy's mass would be concentrated in the bright central portions where most of the visible stars appear. In her study of the Andromeda galaxy, however, Rubin found that objects on the galaxy's outer rim were orbiting just as fast as, if not faster than, materials near the center. For several years Rubin and her colleagues measured the relative red shifts and blue shifts of stars in hundreds of galaxies in order to determine how rapidly stars were moving in various parts of a galaxy. They concluded that 90 percent of the amount of mass needed to provide sufficient gravitational pull to keep the galaxies from flying apart was not visible or otherwise detectable.
This signaled an enormous change in the way cosmologists think about matter. Visible matter was no longer the main constituent of the universe, but merely the snow on huge mountains of unseen matter. By the late 1970s, dark matter had become one of the most important issues in cosmology.
Nucleosynthesis
At the same time, astrophysicists were also producing evidence that as much as 99 percent of the matter in the universe could not be accounted for by conventional theories as to how matter and energy came into being. Nucleosynthesis, the study of how matter is forged into atomic nuclei, has made remarkably accurate predictions about the proportions and quantity of matter that should exist in the universe today. Essentially, astrophysicists treat the universe as a problem in physics. If one extrapolates back to the big bang, and one knows the physical conditions involved, it should be a relatively straightforward process to figure out how much matter now exists, and in what proportions. These "recipes" produce a universe composed of 75 percent hydrogen, 24 percent helium, and 1 percent trace elementsÑjust what astronomers have actually measured, and in exactly the right amounts. The same recipes, however, produce only 10 percent of the "normal" matter that would be required to produce the present universe.
Physicists have also been very successful in explaining the evolution of matter in the cores of stars and supernovas, but they do not have a successful theory to explain why the universe appears to be composed almost entirely of matter and not antimatter. When energy transforms into matter in particle accelerators, every matter particle produced is accompanied by an antimatter particle. Conversely, when matter and antimatter meet, they annihilate each other. The net amount of matter stays the same. Why, then, did not all the matter and antimatter particles produced in the big bang simply annihilate each other? A small imbalance in favor of matter particles must have existed during the first milliseconds of time to produce the present universe. Various forms of grand unification theories have attempted to explain this small surplus, but they have not yet succeeded.
Dark Matter Species
Almost all astrophysicists believe that the missing mass of the universe takes the form of one of two classes of matter or some combination of both. One class is ordinary but invisible matter. These MACHOmassive compact halo objects) would be objects such as burned-out stars, brown dwarf stars too small to ignite, or even black holes. Studies reported in 1996 suggest that such matter may indeed be more abundant than had at first been assumed. The other class of dark matter candidates is more exotic. For example, theoretical particles such as axions, neutralinos, and photinosÑcollectively known as WIMPs (weakly interacting massive particles)Ñare the subject of intense experimental searches. Other theories suggest that fast-moving elementary particles such as neutrinos might account for the missing mass. While neutrinos are insubstantial (these elusive entities have been called "spinning nothings"), they are so abundant that even a tiny mass would make them a major contributor to the total matter in the unverse. In 1998, experiments conducted in a zinc mine deep beneath the Japanese Alps strongly suggested that neutrinos may indeed have mass. If so, they could make up a substantial percentage of cosmic dark matter. Still, it is believed that neutrinos are too "hot" to clump into structures.
In addition to dark matter the universe is populated by scores of objects still poorly understood, including black holes and quasars. In the next few years gravity-wave telescopes are expected to join the current array of telescopes now observing the skies. When this happens, such objects will undoubtedly become better understood, further enhancing knowledge of the composition of the universe.