Dark Matter
What do scientists look for when they search for dark matter? We cannot see or touch it: its existence is implied. Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun (9). The two main categories that scientists consider as possible candidates for dark matter have been dubbed 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 (1). MACHOs are made of 'ordinary' matter, which is called baryonic matter. WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs.
Astronomers and particle physicists disagree about what they think dark matter is. Walter Stockwell, of the dark matter team at the Center for Particle Astrophysics at U.C. Berkeley, describes this difference. "The nature of what we find to be the dark matter will have a great effect on particle physics and astronomy. The controversy starts when people made theories of what this matter could be--and the first split is between ordinary baryonic matter and non-baryonic matter" (10). Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.
MACHOs
Massive Compact Halo Objects are non-luminous objects that make up the halos around galaxies. Machos are thought to be primarily brown dwarf stars and black holes (2). Like many astronomical objects, their existence had been predicted by theory long before there was any proof. The existence of brown dwarfs was predicted by theories that describe star formation (7). Black holes were predicted by Albert Einstein's General Theory of Relativity (11).
Brown Dwarfs. Brown dwarfs are made out of hydrogen--the same as our sun but they are typically much smaller. Stars like our sun form when a mass of hydrogen collapses under its own gravity and the intense pressure initiates a nuclear reaction, emitting light and energy. Brown dwarfs are different from normal stars. Because of their relatively low mass, brown dwarfs do not have enough gravity to ignite when they form (7). Thus, a brown dwarf is not a "real" star; it is an accumulation of hydrogen gas held together by gravity. Brown dwarfs give off some heat and a small amount of light (7).
Black Holes. Black holes, unlike brown dwarfs, have an over-abundance of matter. All that matter "collapses" under its own enormous gravity into a relatively small area. The black hole is so dense that anything that comes too close to it, even light, cannot escape the pull of its gravitational field (11). Stars at safe distance will circle around the black hole, much like the motion of the planets around the sun (7). Black holes emit no light; they are truly black.
Detecting MACHOs
Astronomers are faced with quite a challenge with detecting MACHOs. They must detect, over astronomical distances, things that give off little or no light. But the task is becoming easier as astronomers create more refined telescopes and techniques for detecting MACHOs.
Searching with Hubble. With the repair of the Hubble Space Telescope, astronomers can detect brown dwarfs in the halos of our own and nearby galaxies. Images produced by the Hubble Telescope, however, do not reveal the large numbers of brown dwarfs that astronomers hoped to find. "We expected [the Hubble images] to be covered wall to wall by faint, red stars," reported Francesco Paresce of the Johns Hopkins University Space Telescope Science Institute in the Chronicle of Higher Education (5). Research results are disappointing--calculations based on the Hubble research estimate that brown dwarfs constitute only 6% of galactic halo matter (12).
Gravitational Lensing. Astronomers use a technique called gravitational lensing in the search for dark matter halo objects. Gravitational lensing occurs when a brown dwarf or a black hole passes between a light source, such as a star or a galaxy, and an observer on the Earth. The object focuses the light rays, causing the light source to brighten (13). Astronomers diligently search photographs of the night sky for the telltale brightening that indicates the presence of a MACHO.
Wouldn't a MACHO block the light? How can dark matter act like a lens? The answer is gravity. Albert Einstein proved in 1919 that gravity bends light rays (13). He predicted that a star, which was positioned behind the sun, would be visible during a total eclipse. Einstein was right--the gravity of the sun bent the light rays coming from the star and made it appear next to the sun.
