Weakly Interacting Massive Particles
Weakly interacting massive particles, or WIMPs, are a theoretical construct which was proposed to account for apparently missing mass in the galaxy. These particles would have to have much more mass than traditional hard-to-detect particles like neutrinos, so that they could account for enough of the mass of the galaxy. However, they would have to interact exclusively with the weak force, making it difficult for them to be observed by other, more traditional means. They could not emit light or otherwise interact with the electromagnetic strong forces, making them almost impossible to detect directly.
The rate of the universe's expansion can be measured, and by the current measurements, it is much slower than it ought to be for the amount of matter detected by current standards. Also, the observed movements of galaxies and galactic clusters is not consistent with the gravitational forces due to the visible matter. That is, there must be a great deal of unseen mass in the universe that is affecting the movements of the galaxies. The missing amount of gravitational matter is sometimes called "missing matter," or more commonly "dark matter." It is simply matter that does not emit electromagnetic radiation and thus is not visible to our astronomical instruments. The WIMPs would have to have a spherically symmetric distribution throughout the galaxy--that is, they would have to exist in about equal numbers in every direction--in order to explain the observed galactic dynamics.
Some physicists think that neutrinos may account for the weakly interacting massive particle mass. However, there is a known upper bound on the neutrino mass. Even with the most massive possible neutrino, they could only account for about 10% of the dark matter. The missing matter would have to be composed of elementary particles similar to neutrinos, however. Usually WIMP candidates are considered to be nonbaryonic, that is, not composed of quarks like the proton and neutron. Baryonic candidates are separated into a different category of matter and are not properly WIMPs since they can undergo strong and electromagnetic interactions.
Many different experimental searches for WIMPS have been carried out. Because WIMPs are massive, they can collect at the center of massive bodies such as the Earth or Sun. Some experimental searches have concentrated on looking for WIMP decay products which come from the Earth's core or the direction of the Sun. It is also possible for WIMPs to change the temperature profile of the Sun's interior by carrying energy out of the core. This, in turn, would affect the neutrino flux coming from the Sun. Other searches have looked for evidence of WIMPs moving through their experimental detectors. To date, no clear evidence of WIMPs has been uncovered.
Given the continuing developments in astrophysics detector technology, the search for WIMPs shows promise. However, these particles are still only theoretical, their properties not completely agreed upon. There are even some physicists who think that a whole new view of gravity is more appropriate than new exotic particles to explain the behavior of our galaxy. The existence of WIMPs is one of the most interesting problems to combine astrophysical observations and particle physical theory at the beginning of the twenty-first century.
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