Georges Henri Joseph Édouard Lemaître (July 17, 1894 – June 20, 1966) was a Belgian Roman Catholic priest, honorary prelate, professor of physics and astronomer at the Université catholique de Louvain. He sometimes used the title Abbé or Monsignor.
Lemaître proposed what became known as the Big Bang theory of the origin of the Universe, which he called his 'hypothesis of the primeval atom'
Lemaitre was a pioneer in applying Einstein's theory of general relativity to cosmology. He suggested a precursor of Hubble's law in 1927. In 1931, he published his primeval atom theory in Nature. At the time, Einstein believed in a static universe and had expressed skepticism about Lemaître's 1927 paper. A similar solution to Einstein's equations, implying a changing radius of the universe, had been proposed in 1922 by Alexander Friedman, as Einstein told Lemaître when he approached him with the theory at the 1927 Solvay Conference. (Einstein had also criticized Friedman's theory.) But it is Lemaître's theory that changed the course of science, for the following reasons:
-Friedman was a mathematician who neither worked with astronomical data r cared whether his theory was a description of physical reality.
-Friedman died in 1925, soon after first proposing his theory.
-Lemaître worked with astronomers and designed his theory to have testable implications, and to be in accord with observations of the time.
-Arthur Eddington made sure that Lemaître got a hearing in the scientific community.
Lemaître proposed his theory at an opportune time, since Edwin Hubble would soon publish his velocity-distance relation that strongly supported an expanding universe and, consequently, the Big Bang theory. In fact, Lemaître's 1927 paper derived what became known as Hubble's Law, two years before Hubble did so. Because Lemaître spent his entire career in Europe, his contributions are not as well known in the United States (USA) as those of Hubble or Einstein, men well known in the USA by virtue of residing there.
Both Friedman and Lemaître concluded that the universe must be expanding. Lemaître further concluded that an initial "creation-like" event must have occurred. This is the Big Bang theory as we know it today, and why he is credited with its discovery.
Einstein at first dismissed Friedman and then (privately) Lemaître out of hand, saying that not all mathematics leads to correct theories. After Hubble's discovery was published, Einstein quickly and publicly endorsed Lemaître's theory, helping both the theory and its proposer get fast recognition.
In 1933, Lemaître found an important inhomogeneous solution of Einstein's field equations describing a spherical dust cloud, the Lemaitre-Tolman metric.
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