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Just What is Dark Matter?
Throughout the years, scientists here on Earth have made many observations concerning the nature and size of planets, stars, solar systems, galaxies, and even the entire universe. Scientists have shown that heavenly bodies affect each others movement through gravity. However, there are many things in the universe whose movements and natures cannot be explained as gravitational effects of other bodies that have been observed. In fact, scientists are unable to account for at least 90% of the matter that gravitationally affects what they can see (4).
Before 1932, no one thought that there may be "missing" mass in the universe. However, in that year, Jan Oort conducted an experiment, in which he hoped to determined the mass of a given volume of the Milky Way Galaxy. Like many other scientists of his day, Oort knew how to measure the mass of stars. He discovered that if he measured the velocity of the oscillitory motion of the stars in the area, he could determine the force acting upon them, and therefore, the mass of the entire region. Surprisingly, though, "he found that there appeared to be considerably more mass in our region of the galaxy than the sum of the masses of the individual stars indicated" (6). Where did this mass come from?
Skeptics would conclude that the experiment was flawed. However, over recent years, scientists have conducted many other different experiments, and most of them reach the conclusion that the mass of objects we can see is not equal to the mass of objects we *should* see.
For example, in 1993, NASA announced that the shape of one observed galaxy is, according to their X-ray pictures of gasses at its edge, elliptical. However, the observeble mass in this galaxy, stars, are located at the center of the galaxy. If they were the only mass in the galaxy, the halo of gasses observed would be round, since those stars are so far from the gasses. This is not so. (8)
In order to gain a better perspective pretend that the background of this page is your view of the universe (If you're in a text-only browser or mode, you'll have to think a little bit harder than everyone else). You can see white and blue stars, a few clusters, and even purple nebulae. These are all beautiful, but is it right to conclude that that's all there is to the universe? What's all that black stuff in the back, that we can't see? Is it just empty space? Doubtful. Scientists call the things we cannot see but can measure the effects of in the universe, "Dark Matter." The biggest question: What *is* it?
"We know practically nothing about the physical nature of dark
matter."
-I.D. Novikov, 1995 (7)
"Massive Compact Halo Objects [MACHOs] are non-luminous objects that make up the halos around galaxies" (4). MACHOs are regarded as the most likely candidates for dark matter, though there is still not conlusive evidence. MACHOs are thing which we can detect, such as dim stars and black holes. If MACHOs are what make up dark matter, all scientists have to do is locate all of them, and the puzzle will be solved. The fact that MACHOs are detectable and known makes them attractive candidates for scientists looking for a simple, feasible answer.
John Bachall, a professor at Princeton University, says, "the most plausible form in which the unseen... matter could reside is faint stars," (2). Maybe Dwarf stars can account for dark matter. Red dwarfs are tiny little stars which are very dim, and therefore hard to detect. The fact that they are so small limits their effects gravitationally, so that a very large number of them would be necessary to amount to the "missing" mass (6). It is unlikely that red dwarfs account for all of the dark matter in the universe.
The same problem arises, with brown dwarfs, which aren't big enough to make a difference either (5). The bigger problem with them, though, is that they are theoretical bodies, gaseous clouds that never became hot enough for nuclear reactions, die out, and now exist without emitting light, is that not one of them has ever been identified (6).
Black holes are the other MACHOs which are most likely to be dark matter candidates. Black holes contain so much matter that they collapse in on themselves. They are so dense, and their gravity is so strong, that light cannot escape from them. (4) Black holes are difficult to detect, and, if found to be more common than they are thought to be, could serve as likely dark matter candidates.
"How am I betting? At least fifty-fifty on protons and neutrons all the
way."
-Viginia Trimble (6)
"Thus we may be on the threshold of a wonderful new discovery,
unprecedented in its importance for both cosmology and astrophysics. Or
else, all of the premises on which these experiments are based are
wrong..."
-Lawrence M. Krauss, 1988 (1)
Technically, Neutrinos are MACHOs, but since there is debate over the question of whether they have mass or not, they deserve their own special category, near the bottom.
Neutrinos, discovered in 1956, are particles which cause free-neutrons to decay. They are known to exist and have been detected. Neutrinos are so small that some scientists argue that they may possibly have no mass at all. Assuming that neutrinos have a mass, and may therefore interact gravitationally with other matter, scientists believe that more of them should be detected. However, three types of neutrinos are known to exist, and detectors can only find one at a time, so if the neutrinos fluctuate from form to form, it is possible that they exist undetected and have a gravitational affect on the matter around them. The problem is that, according to the Big Bang Theory, neutrinos would have had to move at near light speed at the begining of the universe, and only when they slowed down would the observable bodies we see today have formed, leaving absolutely nothing in the areas between galaxies. Neutrinos should be, therefore, discounted as dark matter, because there is matter between galaxies. (3)
So what is dark matter? I don't know. Dark matter is something we can't see directly, but whose effects we can measure. If Occam's Razor is applied, and the simplest explanation is sought, MACHOs like red dwarfs and black holes are the best candidates, we only have to look a little bit harder to find them. Whatever the case may be, it seems evident that something *is* out there...
Works Cited
(1) Audouze, J. and J. Tran Thanh Van, eds. Dark Matter. Cedex, France: Editions Frontieres, 1988.
(2) Bahcall, J., T. Piran and S. Weinberg, eds. Dark Matter in the Universe. Singapore: World Scientific Publishing Co Pte Ltd., 1987.
(3) "Dark Matter" [http://physics7.berkeley.edu/darkmat/dm.html]
(4) "Dark Matter" [http://w3.gti.net/cmmiller/drkmttr.html]
(6) Parker, Barry. Invisible Matter and the Fate of the Universe. New York: Plenum Press, 1989.
(7) Sanchez, N. and A. Zichichi, eds. Current Topics in Astrofundamental Physics: The Early Universe. Boston: Kluwer Academic Publishers, 1995.
(8) "Scientists Find Galactic Evidence of Dark Matter" [http://the-tech.mit.edu/TechTalk/31325.TXT.html]