Dark Matter and Ripples at the Beginning of Time:
As complicated and complete as this history of the early universe may seem, significant gaps in our understanding of the evolution of the universe remain. Some of these gaps were closed with the development of unified field theories and the inflationary scheme of the universe. The problem of explaining the existence of galaxies clusters, and superclusters, however, remains.It now appears that all the impressive luminous objects in the sky constitute less than 10% of the matter in the universe, perhaps a good deal less than 10%. The rest of the matter exists in forms that we cannot see, but whose effects we can measure. This mysterious new kind of material is called DARK MATTER.
The easiest place to see evidence for dark matter is in the galaxies such as our own Milky Way. Far out from the stars and spiral arms such as associate with galaxies, we can still see diffuse clouds of hydrogen gas. This gas gives off radio waves, so we can detect its presence and its motion. In particular, we can tell how fast it is rotating. When we do these sorts of measurements, a rather startling fact emerges. Kepler's law implies that any object orbiting around a central body under the influence of gravity will travel slower the father out it is. The distance planet Jupiter, for example, moves more slowly in orbit around the Sun than does the Earth. Similarly, you would expect that when hydrogen molecules are far enough away from the center of a galaxy, these more distance atoms would move more slowly than those closer in. Even though we can see these hydrogen atoms out to a distance three times and more the distance from the center of the Milky Way to the end of spiral arms, no one has ever seen the predicted slowing down.
The only way to explain this phenomenon is to say that those hydrogen atoms are still in the middle of the gravitational influence of the Galaxy. This means that luminous matter--the bright starts and spiral arms--is not the only thing that is exerting a gravitational force. Something else, something that makes up at least 90% of the mass of the galaxy and that extends far beyond the stars, exerts a gravitational force and effects the motion of the hydrogen we observe. Studies of many galaxies show the same effect, and scientists are now convinced that at least 90% of the universe is made up of this mysterious dark matter. Scientists also find evidence that dark matter exists in between galaxies, in clusters, and in other places in the universe.
Dark matter is strange, indeed. It does not interact through the electromagnetic force. If it did, it would absorb or emit photons and it would not be "dark" in the sense we were using the term here. Yet, because we know that it exerts gravitational attraction, we can conclude that this unseen "stuff" must be a form of matter--matter that interacts with ordinary matter only through the gravitational force. Detecting dark matter, and finding out what it is, remains a very active research field today.
The existence of dark matter might help us understand a key event in the early history of the universe, because dark matter could have formed into clumps before atoms formed.In the first several hundred thousand years after the big bang, photons blew apart collections of luminous matter that were trying to form galaxies, but light would not have affected the clumping of dark matter. Therefore, when atoms formed and luminous matter could clump together, it found itself in a universe in which large clustering of dark matter already existed. The luminous matter would simply have fallen into these clusters and would not have had to form under the influence of its own gravitational attraction. Thus, if dark matter exists, and if dark matter formed clumps early in the history of the universe, the problem of structure is solved.
In 1992, important new results from the COBE satellite supported the notion of dark matter. Examining the microwave background in great detail. astronomers found evidence that some regions in the sky emits microwave background at a slightly higher temperature than the surrounding regions. These so-called "ripples at the beginning of time" were not very large--they correspond to temperature increases of the tiniest fraction of a degree. Nevertheless, they could have been emitted only from regions that were more dense and slightly hotter than their neighbors. These ripples appear to mark the beginning of the collection of luminous matter immediately after the formation of atoms.
In 1993, astronomers announced the discovery of one kind of dark matter called MACHOS (massive compact halo objects). These bodies from a swarm of large objects, each a fraction of the size of the Sun, that orbit the Milky Way. Thus these recent data support both the existence of dark matter in the early universe and its role in producing the structure we see in the sky. This particular area of study will continue to be important in the coming years, and you will be sure to see information about it in the media.
