An Overview of the Theories on Dark Matter
History
It was always known that there was matter in the sky that we couldn’t see. Dark matter makes up 90% of the universe. It cannot emit, absorb or reflect light, which makes it truly black. It is also easy to confuse dark matter with antimatter. If dark matter is made out of antimatter, then all matter in our universe would have already been annihilated. It is pretty safe to assume that dark matter is not made out of antimatter (5, 6). The first evidence that there was a significant amount of matter that we couldn’t see was from examining clusters of galaxies. In 1933, the astronomer Fritz Zwicky was studying a couple clusters of distant galaxies. He estimated the total mass of a group of galaxies by measuring their brightness.
When he used a different method to calculate the mass again, he came up with a number that was 400 times his original estimation. This inconsistency and difference in masses is now known as “the missing mass problem.” Though, that is a misleading term, since the mass is there, but rather it is light that is missing (2). Zwicky said, “Matter, although invisible, can still be demonstrated through its gravitational clout.” He was the first to suggest that invisible matter could be detected indirectly by its gravitational effects (7). He was the first to pointed out in the 1930s that dark matter must exist. The reason for this, he claimed, was that the mass of known matter in galaxies is not great enough to hold a cluster of galaxies together. Each independent galaxy moves at too great a speed for the galaxies to remain in a cluster. Yet the galaxies were not spinning away from each other. They had to be held together by a gravitational field created by undetected mass (8).
About the same time, Jan Oort measured the speed and direction of stars in our galaxy. Oort is probably best known for the theory of “differential galactic rotation”. An astromomer by the name of B. Lindblad developed a theory of galactic structure that had the sun quite a distance from the center, and the rest of the galaxy rotated around that center. The objects farther from the center moved slower than the ones close. Oort was available to prove that the theory was correct (1).
Nobody did much with Zwicky’s finding until the 1970’s when scientists realized that the existence of unseen masses would support their theories of universe structure (2). More than fourty years later, astronomer Vera Rubin found that the same principle is true within a single galaxy. The mass of the stars alone do not exert enough gravitational force to hold the galaxy together. She discovered that stars in the far reaches of a galaxy rotate about the galactic center at the same speed as stars that are close-in. Rubin concluded that some invisible, massive substance surrounds a galaxy, exerting gravitational force on all the stars (9).
Theory
There are many theories on what dark matter is. There are two common theories, called MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). Although these acronyms are amusing, they can help you remember which is which. MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes. MACHOs are made of baryonic matter, which is just ordinary matter made of protons and neutrons(10). WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made up of other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs (2). Non-Baryonic is also broken down into Hot Dark Matter and Cold Dark Matter. Hot dark matter is smaller and moves rapidly, while cold dark matter is massive and slow. (3)
There are also interesting theories on dark matter. Some scientists believe that WIMPS (along with the number 42, of course) could be the answer to life, the universe and everything. Since WIMPS are made of non-baryonic matter, if it is discovered that they played an important role in the creation of our universe, it would be quite a shock. If non-baryonic matter exists, “it could mean not only are we not at the center of the universe, but we aren’t even made up of the same stuff as most of the universe. We are just this small excess, an insignificant phenomenon, and the universe is something completely different” (3).
The amount of dark matter is measured by watching the movement of the stars. Stars move like a horse on a carousel. They bob up and down as they circle around the center of the galaxy. They move above a galactic disk, as they are pulled down by the disk, and then back up again. The measurements of the distance that the pendulum swings are used to determine the amount of matter in the galaxy (8).
Also, the level of dark matter is directly related to whether the universe is closed, open or flat. When “Closed”, which can only occur with colossal amounts of dark matter, the universe slowly pulls back into one point, and in a bang it becomes nothing. If there is no dark matter, then the universe is “Open”, and will never stop expanding. When the perfect amount of dark matter is achieved, then the universe is “Flat” and will not expand nor shrink, and everything (i.e. time, growth) will stop (4).
1) VanDenBergh, Sydney. “The Early History of Dark Matter”. The Astrophysical
Journal, 19 April 1999.
2) Miller, Chris. Cosmic Hide and Seek: the Search. 1995. May 30 2004 .
3) Guidry, Mike. A Primer on Dark Matter. May 30 2004 .
4) The Shadows of Creation by Michael Riordan & David N Schramm 1991 W.H. Freeman and Company New York
5) Antimatter: Mirror of the Universe. CERN. May 30 2004
6) Yost, Scott A.. Dark Matter. May 30 2004 .
7) Bartusiak, Marcia. Through a Universe Darkly. Harper Collins, 1993
8) Rubin, Vera. Bright Galaxies, Dark Matters. The American Institute of Physics,
1997.
9) Harvard-SmithsonianCenter for Astrophysics. Dark Matter Mystery. March 22 2004.
10) Roberts, John . Does Dark Matter Matter?. BBC. May 30 2004 .
