Who first proposed the Big Bang Theory?

Georges Henri Joseph Édouard Lemaître was a Belgian Catholic priest, mathematician, astronomer, and professor of physics at the Catholic University of Louvain. He was the first to theorize that the recession of nearby galaxies can be explained by an expanding universe, which was observationally confirmed soon afterward by Edwin Hubble. He first derived "Hubble's law", now called the Hubble–Lemaître law by the IAU, and published the first estimation of the Hubble constant in 1927, two years before Hubble's article. Lemaître also proposed the "Big Bang Theory" of the origin of the universe, calling it the "hypothesis of the primeval atom", and later calling it "the beginning of the world".