131 years ago today, on July 17, 1894 (June 15, 1894 lunar calendar), the Belgian astronomer and cosmologist Lemaitre was born. Lemaitre Georges (1894.7.17-1966.6.20), Belgian priest, astronomer and cosmologist. During World War I, Lemaitre served as an artillery officer in the Belgian army as a civil engineer. After the war, he entered a seminary and was ordained in 1923 as a priest. He studied at the Solar Physics Laboratory at Cambridge University from 1923 to 1924, and later at the Massachusetts Institute of Technology in the United States, where he learned about the discoveries of the American astronomer E.P. Hubble and H. Shapley's research on the expansion of the universe. As a professor of astrophysics at the University of Louvain in 1927, Lemaitre proposed the Lemaitre model of the universe, in which the regression of galaxies could be explained within the framework of Einstein's general theory of relativity. Although the expansion model of the universe has been proposed for a long time, Lemaitre's theory, modified by Gamow, has dominated cosmology. Lemaitre uses the Schwarzschild metric (Schwarzschildvacuumsolution) to transform coordinates to obtain the Lemaitre metric, a free-falling metric under attractive force. This metric is the basis of the Lemaitre model of the universe. Later, Robertson and Walker developed the Lemaitre metric, resulting in the Robertson-Walker metric with higher dimensional (three-dimensional) symmetry, which served as the basis for Gamow's model of the Big Bang universe. Lemaitre also studied cosmic rays and the three-body problem, which is a mathematical method for describing the motion of three objects in space that attract each other. Lemaitre's main works are "On the Evolution of the Universe" (1933) and "The Primitive Atomic Hypothesis" (1946). Comment: He proposed the modern Big Bang theory~