Dark Matter Essay -- essays research papers

There is perhaps no current problem of greater importance to astrophysics and cosmology than that of "dark matter". The controversy, as the name implies, is centered on the notion that there may exist an enormous amount of matter in the Universe that cannot be detected from the light that it emits. The evidence of dark matter is from the motions of astronomical objects, specifically stellar, galactic, and galaxy cluster/supercluster observations. The basic argument is that if we measure velocities in some region, then there has to be enough mass there for gravity to stop all the objects from flying apart. When such velocity measurements are done on large scales, it turns out that the amount of inferred mass is much more than can be explained by the luminous mass. Hence we infer that there is non-luminous matter in the Universe, i.e. there is dark matter. Dark matter has important consequences for the evolution of the Universe. According to standard cosmological theory, the Universe must conform to one of three possible types: open, flat, or closed. A parameter known as the "mass density" - that is, how much matter per unit volume is contained in the Universe - determines which of the three possibilities applies to the Universe. In the case of an open Universe, the mass density (denoted by the Greek letter Omega) is less than unity, and the Universe is predicted to expand forever. If the Universe is closed, Omega is greater than unity, and the Universe will eventually stop its expansion and recollapse back upon itself. For the case where Omega is exactly equal to one, the Universe is delicately balanced between the two states, and is said to be "flat". Dark matter candidates are usually split into two broad categories, with the second category being further sub-divided: baryonic and bon-baryonic. Then, under non-baryonic, hot dark matter (HDM) and cold dark matter (CDM) are its types. Depending on their respective masses and speeds, CDM candidates have relatively large mass and travel at slow speeds (hence "cold"), while HDM candidates include minute-mass, rapidly moving (hence "hot") particles. As leading possible candidates for baryonic dark matter, there are black holes (large and small), brown dwarfs (stars too cold and faint to radiate), sun-size MACHOs, cold gas, dark galaxies and dark clusters, ... ...e seeking them in a Stanford laboratory by watching for radiation as they excite crystals of germanium in a detector, but they haven't detected any yet. Soon they will try an even more exotic search by moving their equipment to an old iron mine 2,400 feet deep in northern Minnesota where one or two WIMPs, if they really exist, might very occasionally make their existence known. The detectors are hockey puck-sized superconducting crystals of germanium and silicon. These pure crystals are cooled to about 500 degrees below zero. A particle hitting a detector disturbs the molecular structure of the crystal and registers as a slight temperature increase. Because WIMPs easily pass through most matter, they can pass through the shields and register a signal. To date, the detectors at Stanford have registered a handful of signals, but an analysis suggests that these were caused by stray particles that originally came from cosmic rays and managed to penetrate the 35 feet of rock over the detectors. The ever-so important question what is dark matter will not be answered tomorrow. More data has to be taken, the theories have to tweaked, and many physicists must continue to work together.