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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