>GALACTIC DARK MATTER< Dark matter is a kind of matterhypothesized in astronomyand cosmologyto account for gravitational effects that appear to be the result of invisible mass. Dark matter cannot be seen directly with telescopes; evidently it neither emitsnor absorbs light or other electromagnetic radiationat any significant level. It is otherwise hypothesized to simply be matter that is not reactant to light. [ 1 ]Instead, the existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. According to the Planck mission team, and based on the standard model of cosmology, the total mass–energyof the known universecontains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. [ 2 ] [ 3 ]Thus, dark matter is estimated to constitute 84.5% of the total matter in the universe, while dark energy plus dark matter constitute 95.1% of the total content of the universe. [ 4 ] [ 5 ] Astrophysicists hypothesized dark matter because of discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the luminous matter they contain: stars, gas, and dust. It was first postulated by Jan Oortin 1932 to account for the orbital velocities of stars in the Milky Wayand by Fritz Zwickyin 1933 to account for evidence of missing mass in the orbital velocities of galaxiesin clusters. Subsequently, many other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxiesby Vera Rubin [ 6 ]in the 1960s–1970s, gravitational lensingof background objects by galaxy clusters such as the Bullet Cluster, the temperature distribution of hot gas in galaxies and clusters of galaxies, and more recently the pattern of anisotropies in the cosmic microwave background. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle. [ 7 ] [ 8 ]The search for this particle, by a variety of means, is one of the major efforts in particle physicstoday. [ 9 ] Although the existence of dark matter is generally accepted by the mainstream scientific community, some alternative theories of gravity have been proposed, such as MONDand TeVeS, which try to account for the anomalous observations without requiring additional matter.
Posted on: Wed, 10 Sep 2014 08:33:00 +0000
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