| Dark Matter Essay, Research Paper
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, to name only a few. The range of particles that could constitute nonbaryonic dark matter is limited only slightly by theorists’ imaginations. The particles include photinos, neutrinos, gravitinos, axions, and magnetic monopoles, among many others. Of these, researchers have detected only neutrinos — and whether neutrinos have any mass remains unknown. Experiments are under way to detect other exotic particles. If they exist, and if one has a mass in the correct range, then that particle might pervade the universe and constitute dark matter.
The MACHO Project is a collaboration between scientists at the Mt. Stromlo & Siding Spring Observatories, the Center for Particle Astrophysics at the Santa Barbara, San Diego, & Berkeley campuses of the University of California, and the Lawrence Livermore National Laboratory. Their primary aim is to test the hypothesis that a significant fraction of the dark matter in the halo of the Milky Way is made up of objects like brown dwarfs or planets: these objects have come to be known as MACHOs, for MAssive Compact Halo Objects. The signature of these objects is the occasional amplification of the light from extragalactic stars by the gravitational lens effect. The amplification can be large, but events are extremely rare: it is necessary to monitor photometrically several million stars for a period of years in order to obtain a useful detection rate. For this purpose they must have built a two-channel system that employs eight CCDs, mounted on the 50 inch telescope at Mt. Stromlo. The high data rate (several GBytes per night) is accommodated by custom electronics and on-line data reduction.
The group has taken ~27,000 images with this system since June 1992. Analysis of a subset of these data has yielded databases containing light curves in two colors ten million in the bulge of the Milky Way. A search for microlensing has turned up four candidates toward the Large Magellanic Cloud and 45 toward the Galactic Bulge.
Another explanation for the missing mass is WIMPs, which stands for Weakly Interacting Massive Particles. Bernard Sadoulet, a UC Berkeley astrophysicist, and Blas Cabrera of Stanford head one of several projects looking at this potential explanation. If WIMPs do exist, they would be exotic but massive subnuclear particles such as photinos, far more massive than protons or neutrons. And they would interact so rarely with ordinary matter that billions of them could pass entirely through a human body or even through the earth without being detected.
Sadoulet and Cabrera are 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.
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