Georges Lemaître, (born July 17, 1894, Charleroi, Belgium—died June 20, 1966, Leuven), Belgian astronomer and cosmologist who formulated the modern big-bang theory, which holds that the universe began in a cataclysmic explosion of a small, primeval “super-atom.”

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A civil engineer, Lemaître served as an artillery officer in the Belgian Army during World War I. After the war he entered a seminary and in 1923 was ordained a priest. He studied at the University of Cambridge’s solar physics laboratory (1923–24) and then at the Massachusetts Institute of Technology, Cambridge (1925–27), where he became acquainted with the findings of the American astronomers Edwin P. Hubble and Harlow Shapley on the expanding universe. In 1927, the year he became professor of astrophysics at the Catholic University of Leuven (Louvain), he proposed his big-bang theory, which explained the recession of the galaxies within the framework of Albert Einstein’s theory of general relativity. Although expanding models of the universe had been considered earlier, notably by the Dutch astronomer Willem de Sitter, Lemaître’s theory, as modified by George Gamow, has become the leading theory of cosmology.

Lemaître also did research on cosmic rays and on the three-body problem, which concerns the mathematical description of the motion of three mutually attracting bodies in space. His works include Discussion sur l’évolution de l’univers (1933; “Discussion on the Evolution of the Universe”) and L’Hypothèse de l’atome primitif (1946; The Primeval Atom: An Essay on Cosmogony).