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For centuries, physicists and philosophers alike have wondered what makes up our universe. Aristotle thought that all matter came in one of four forms: Earth, Air, Fire, and Water. Since then we have come a long way, with the discovery of the atoms and the subatomic particles they are made of. We can even guess at what makes up protons and neutrons. We have since then discovered and predicted the existence of particles other than the atom, such as the photon, neutrino, axion, and many others.
Despite all our advances in particle physics and astrophysics, we still don't know what form of matter makes up 95% of the universe. Physicists have named this mysterious substance dark matter, for it can not be detected by observation (it does not emit visible or other frequency light waves). However, we know that dark matter must exist, following Newton's universal law of gravity.
There are two ways to prove the existence of dark matter. We know that the universe must have a certain mass in order for its attractive gravitational forces to slow the expansion of the universe which started at the big bang. We can precisely calculate the rate at which the universe is expanding currently, and how fast it has expanded in the past. From this we get the theoretical mass of the universe. This figure falls far short of the visible mass of the universe, which consists of stars, planets, and hot gas. This is how scientists are able to prove that we can only see about 5% of our universe.
We can also prove that dark matter exists in galaxies by examining how they spin. When an object rotates in a circular orbit, the object has a tendency to fly off in a path tangent to the orbit. If the stays within the orbit, it has a radial acceleration which is equal to its velocity squared over the radius of the orbit. The only force which is keeping the body in the orbit is the force of gravity, which is dependent on the mass of the system. Knowing this, physicists can calculate the mass of a galaxy by looking at how fast stars orbiting its center are moving. Physicists can also calculate where the highest percentage of dark matter should be in the galaxy. In most cases, it is located in a ring just outside the galaxy. In the case of the galaxy shown in the photo, dark matter must be present in the dark space between the nucleus of older yellow stars and the outer ring of young, blue stars.
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The question of dark matter is one that has plagued physicists for nearly a century. Lots of progress has been towards solving the problem, but physicists are still far from knowing exactly what makes up dark matter. The rest of this web page is an overview of the various theories behind dark matter and how they came about.
There are two very different theories among physicists on what makes up Dark Matter. Some physicists think that Dark Matter is made up of normal (baryonic) matter, in the form of what they call MACHOs (MAssive Compact Halo Object). Machos includes black holes and other dark, massive bodies. The other possible candidate for Dark Matter are WIMPs (Weakly Interacting Massive Particles). Wimps, such as neutrinos, are fundamental particles with little mass and very weak interactions on each other and the baryonic matter around them. While both theories are completely viable, current research seems to be favoring the Wimps.
Machos (MAssive Compact Halo Objects) include all bodies in space made up of baryonic matter (normal atoms) which are massive and relatively dense. The Halo in the name comes from the fact that most dark matter should exist in rings around galaxies. The Machos which could account for Dark Matter include stars which don't have enough mass to burn, small stars which have used up their fuel and collapsed to brown dwarfs, or large stars which have used up their fuel and collapsed to form black holes. There may be other anomalies taking the form of Machos which could account for dark matter, but we haven't been able to predict them with current theory.
It is much harder to prove the existence of Machos than WIMPs, because they are all far away and can't be seen. Wimps can't be seen either, but they are present nearly everywhere in the universe. Astronomers have proved the existence of black holes to a small degree of certainty, and even less for brown dwarfs. However, we are confident they exist and can prove it using Einstein's theory of relativity, but we also know that they could not make up for all the missing mass in the universe. By looking at the number of stars still burning and the age of the universe, physicists know approximately how many black holes and brown dwarfs there are, and their combined mass doesn't come close to the amount of dark matter which must exist.
When a star runs out of fuel, one of three things happens. If the star is large, it usually explodes and its gasses are spread across the universe. If the star is small, it collapses into a brown dwarf, where it begins fusion again, but on a much smaller scale. These stars burn very faintly and are hard to detect. The third possibility occurs when a star's mass is smaller than the star needs to explode, but larger than the mass needed to form a brown dwarf. In this case, the star collapses in on itself until all its mass is located at a single point of infinite density (a singularity). This infinitesimally small star cannot be seen, for even light cannot escape its gravitational pull. Everything within a certain radius (the event horizon) of a black hole is sucked in and from the point it crosses the event horizon we know nothing of what happens to it. Normal laws of physics (relativity and Newtonian mechanics) break down at singularities, but the gravitational force of the black hole's mass can still be felt by surrounding objects.
Black holes are intriguing ideas, but they are not likely to account for much dark matter. This is because most stars the size needed to make a black hole are not likely to have already run out of fuel since the big bang. However, we have seen evidence that they exist in the lensing effect of their gravity on the light from stars behind the black hole. Physicists are very certain that a black hole exists in each of the two galaxies below. We can predict black holes to exist in the universe, but their combined mass does not come close to accounting for dark matter.
A brown dwarf is born when a star of a certain mass burns up the last of its fuel and collapses. The act of collapsing increases the speed and pressure of the atoms, and because of this fusion starts up again when it has collapsed to a certain size. The heat of the fusion causes the star to stop collapsing, and the star can burn faintly like this for a long time. The radiation put off by brown dwarfs is very small, and is indistinguishable from, but probably contributes to, the background radiation from the universe. The light put off by these venerable stars is so faint we cannot see them with even the most powerful of telescopes.
Brown dwarfs are expected to be more numerous than black holes, but there are still very few in the universe. Their total mass can only account for a very small fraction of the dark matter that must be out there.
Many physicists studying dark matter think that it is made up primarily of Wimps (Weakly Interacting Massive Particles). Particles falling under this category have mass (albeit very little) and interact very weakly with other particles via the nuclear forces and the electromagnetic force. Of course, their gravitational force on other matter would still be dependent on their mass and the distance between them. Wimps include particles such as neutrinos and axions, both very weak massive particles. There are other Wimps which we know of, but none are as good candidates for dark matter as neutrinos and axions.
It is very likely that we will solve the problem of Dark Matter by studying Wimps and where they came from. We already know a lot about neutrinos (enough to know that they are not the sole source of dark matter), and nearly as much about axions. However, it will probably be the discovery of new kinds of Wimps that will make solving the Dark Matter problem possible.
Neutrinos are known, detectable WIMPs which are emitted by stars and other hot bodies. They are known to make up a small percentage of dark matter, but it is debatable how much. Most estimates fall around 0.1%. While this is a very small percentage, the study of neutrinos as dark matter is still invaluable. The fact that we can detect them makes them one of the most definite contributors to dark matter, and the further study of neutrinos may help us to discover what form other Wimps will take.
As of this year, many breakthroughs have been made in neutrino research. At the Sudbury Neutrino Observatory (SNO), physicists have proved that solar neutrinos take on three forms on their journey from the sun to the Earth. The new light capturing techniques implemented by SNO have made the detection of all three of these forms, the Tau, Muon, and Electron neutrino possible. A couple years ago the same team of scientists proved that the neutrino has mass, although very little. Their estimates put it at 1/60,000 of the mass of an electron.
Axions are hypothetical WIMPs which occur through a complex process involving the Peccei-Quinn symmetry. Axions could have been produced in great quantities during the big bang, but so far attempts to detect axions have been inconclusive. Theoretically, an axion could be detected through its weak coupling to electromagnetism. Two new experiments are currently underway to detect axions. Karl von Bibber at LLNL is using a cryogenically cooled detector which should be able to reach into parameter space. Matsuki in Kyoto are trying to detect axions using an atomic beam of Rydberg atoms. No breakthroughs have been made yet as far as I know. However, axions remain one of the top candidates for dark matter. If they do exist, they could be responsible for even more mass than the neutrino.
MOND stands for MOdified Newtonian Dynamics. It is an alteration to Newton's law of gravity, made to fit the observations of galactic movement and to do away with the dark matter problem. It was first proposed by Mordehai Milgrom in 1983. The theory was looked down on by most physicists because it went against Newton's laws, which have been proven to work in nearly every case imaginable. If proven to be true, it would take all the fun out of solving the dark matter mystery.
The way that MOND works is that for very small accelerations (about 10^-10 meters per second squared), force is proportional to acceleration squared (f=ma^2 instead of f=ma). The theory accurately predicts the movements of galaxies, doing away with the need for a halo of dark matter. The theory also eliminates the need for extra mass in the universe to slow the universal expansion.
The mechanics of nearly every galactic system found in the universe agree with MOND's predictions, with the exception of galaxy cluster cores. In order for cluster cores to fall in line with MOND, there must be some dark matter present. This fact makes MOND insufficient to explain dark matter, but there are arguments for why it is still a viable theory. For all we know, cluster cores could be more susceptible to the formation of black holes and other collapsing stars.
MOND can not be tested experimentally because the accelerations involved in the theory cannot be simulated here on Earth. The theory can only be compared with observations, and in that matter it performs incredibly well. However, many argue that MOND is not a true scientific theory. It only explains what it was meant to explain, and even then there are exceptions. The fact that it agrees with observations doesn't necessarily mean it is grounded in reality; rather, that it predicts what it was designed to predict and nothing more..
For more information on MOND, read Milgrom's article in the August 2002 issue of Scientific American.
Currently, researchers are literally shooting in the dark when trying to detect dark matter. Physicists have been looking less and less to MACHOs to explain dark matter. We know that MACHOs have a part to play, but that part is very small (only about 10% of dark matter could possible be from MACHOs). On the other hand, WIMPs are hard to predict theoretically, much less observe. So far, neutrinos are the only WIMPs we have proved to exist in quantities enough to make a significant difference in the mass of the universe. Physicists have great faith in the importance of WIMPs, and many experiments have been designed to detect them. Follow the links in the left frame for examples of current WIMP research.
DAMA (DArk MAtter) and CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) are two of the major dark matter detection experiments taking place in the Gran Sasso laboratory in Italy. DAMA was designed to search for particles with masses from few GeV to several hundreds GeV. It works by detecting the recoil of a nucleus when it is hit by a WIMP. The two different DAMA detectors use NaI and LXe. CRESST works through a cryogenically cooled thermometer made of sapphire. When a WIMP hits the nucleus of the supercooled sapphire, some of its energy is transferred to the nucleus and is detected by the thermometer. So far the two have yielded many results, all of which were above my ability to understand or explain in this webpage.
Chandra is an x-ray observatory run by NASA with the goal to accumulate clues to the form of dark matter by looking at its effects on galaxies and its gravitational lensing effect. Chandra recently made a breakthrough that further proves the existence of dark matter. An x-ray observation of galaxy NGC 720 showed that the galaxy was enveloped in a cloud of hot gas that had a different orientation than that of visual observations of the galaxy. The only way that the hot gas could be contained in the galaxy would be through the gravitational force exerted by dark matter. These results contradict the MOND theory, while reinforcing the idea of cold dark matter (WIMPs).
The Sudbury Neutrino Observatory was built over a mile underground in an old nickel mine in Ontario, Canada. The detection apparatus is a giant sphere filled with heavy water (water with a neutron in its nucleus) lined with extremely sensitive photon detectors. The idea is that when a neutrino collides with a deuterium nucleus, photons will be emitted and details about the collision can be obtained. The detector is located so far underground so that the earth's crust will filter out other radiation that might interfere with the experiment.
SNO's first major discovery was that neutrinos have mass. This had been predicted, and some claimed to have proved it, but SNO was the first to calculate the mass with some degree of accuracy. Later on they were able to prove that solar neutrinos take on three forms: the tau, muon, and electron neutrino. All have different characteristics that may provide insight into other forms of dark matter. Dr. Raymond Davis was awarded the Nobel prize in physics this year for this very discovery.