Concept of an expanding universe whose average density remains constant, matter being continuously created throughout it to form new stars and galaxies at the same rate that old ones recede from sight. A steady-state universe has no beginning or end, and its average density and arrangement of galaxies are the same as seen from every point. Galaxies of all ages are intermingled. The theory was first put forward by William Macmillan (1861–1948) in the 1920s and modified by Fred Hoyle to deal with problems that had arisen in connection with the big-bang model. Much evidence obtained since the 1950s contradicts the steady-state theory and supports the big-bang model.
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In physics, the combination of two theories of particle physics into a single framework to describe all interactions of subatomic particles except those due to gravity (see gravitation). The two theories, the electroweak theory and the theory of quantum chromodynamics, describe the interactions between particles in terms of the exchange of intermediary particles. The model has proved highly accurate in predicting certain interactions, but it does not explain all aspects of subatomic particles. For example, it cannot say how many particles there should be or what their masses are. The search goes on for a more complete theory, and in particular a unified field theory describing the strong, weak, and electromagnetic forces.
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Automobile built by the Ford Motor Co. from 1908 until 1927, the first widely affordable mass-produced car. Assembly-line production methods introduced by Henry Ford in 1913 enabled the price of this five-seat touring car to drop from $850 in 1908 to $300 in 1925. Over 15 million Model T's were built. The car was offered in several body styles, all mounted on a standard chassis. Various colors were initially available, but after 1913 its sole color was black. It was replaced by the popular Model A in 1928.
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