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Galaxy rotation problem
The galaxy rotation problem refers to the phenonemon of the discrepancy between the observed rotation speeds of matter in the disk portions of spiral galaxies and the predictions of Newtonian dynamics considering the luminous mass. The presence of this discrepancy is explained by astronomers as pointing to the existence of dark matter in the halo of the galaxy.
In the beginning of the 1980s, the first observational evidence was reported that spiral galaxies do not spin as expected according to Keplerian dynamics. Based on this model, matter (such as stars and gas) in the disk portion of a spiral should orbit the center of the galaxy similar to the way in which planets in the solar system orbit the sun, that is, according to Newtonian mechanics. Based on this, it would be expected that the average orbital speed of an object at a specified distance away from the majority of the mass distribution would decrease inversely with the square root of the radius of the orbit (the dashed line in Fig. 1). At the time of the discovery of the discrepancy, it was thought that most of the mass of the galaxy had to be in the galactic bulge, near the center.
Observations of the rotation curve of spirals, however, do not bear this out. Rather, the curves do not decrease in the expected inverse square root relationship but are "flat" -- outside of the central bulge the speed is nearly a constant function of radius (the solid line Fig. 1). The explanation that requires the least adjustment to the physical laws of the universe is that there is a substantial amount of matter far from the center of the galaxy that is not emitting light in the mass-to-light ratio of the central bulge. This extra mass is proposed by astronomers to be due to dark matter within the galactic halo, the existence of which was first posited by Fritz Zwicky some 40 years earlier in his studies of the masses of galaxy clusters. Presently, there are a large number of pieces of observational evidence that point to the presence of cold dark matter, and its existence is a major feature of the present Lambda-CDM model that describes the cosmology of the universe.
A small minority of amateurs and a few fringe astronomers have expressed discomfort with the dark matter theory and have offered alternative explanations of their own. One of the most popular alternative explanations that has been offered is the Modified Newtonian Dynamics (MOND) theory. MOND posits that the abberations are not due to problems with the mass-to-light ratios, but rather due to the actual physics of gravity changing at large scales. MOND has been moderately successful in reproducing a few key observations that are seen as evidence for dark matter, but there remains observations that point to dark matter such as anisotropies in the cosmic microwave background and mass distributions garnered from gravitational lensing studies that seem to contradict MOND.
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