Georges Henri Joseph Édouard Lemaître (/ləˈmɛtrə/ lə-MET-rə; French: [ʒɔʁʒ ləmɛːtʁ] ⓘ; 17 July 1894 – 20 June 1966) was a Belgian Catholic priest, theoretical physicist, and mathematician who made major contributions to cosmology and astrophysics.

[3][4][5] That work led Lemaître to propose what he called the "hypothesis of the primeval atom", now regarded as the first formulation of the Big Bang theory of the origin of the universe.

A pioneer in the use of computers in physics research, in the 1930s he showed, with Manuel Sandoval Vallarta of MIT, that cosmic rays are deflected by the Earth's magnetic field and must therefore carry electric charge.

His prospects of promotion to officer rank were dashed after he was marked down for insubordination as a result of pointing out to the instructor a mathematical error in the official artillery manual.

In the spirit of the fraternity, Lemaître did not discuss his involvement with the Amis de Jésus outside of the group, but he regularly made silent retreats in a house called Regina Pacis ("Queen of Peace") in Schilde, near Antwerp, and also undertook translations of the mystical works of John of Ruusbroec.

[9] Lemaître also registered at that time in the doctoral program in science at the Massachusetts Institute of Technology (MIT), with the Belgian engineer Paul Heymans as his official advisor.

[19] Also in 1927, Lemaître returned to MIT to defend his doctoral dissertation on The gravitational field in a fluid sphere of uniform invariant density according to the theory of relativity.

[21] In 1929, the US astronomer Edwin Hubble published a paper in the Proceedings of the National Academy of Sciences of the United States of America showing, based on better and more abundant data than what Lemaître had had at his disposal in 1927, that, in the average, galaxies recede at a velocity proportional to their distance from the observer.

[23] The issue was clarified in 2011 by Mario Livio: Lemaître himself removed those paragraphs when he prepared the English translation, opting instead to cite the stronger results that Hubble had published in 1929.

This was published in Nature,[6] and later that year Lemaître participated in a public colloquium on "The Evolution of the Universe" held in London on 29 September 1931 to mark the centenary of the British Association for the Advancement of Science.

[24] Lemaître's theory was first presented to a general audience in the December 1932 issue of Popular Science, in a piece written by the astronomer Donald Howard Menzel of Harvard University.

In 1933 at the California Institute of Technology, after Lemaître presented his theory, Einstein stood up, applauded, and is reported to have said, "This is the most beautiful and satisfactory explanation of creation to which I have ever listened.

In 1946, Lemaître published his book on L'Hypothèse de l'Atome Primitif ("The Primeval Atom Hypothesis"), which was translated into Spanish in the same year and into English in 1950.

The astronomer Fred Hoyle introduced the term "Big Bang" in a 1949 BBC radio broadcast to refer to cosmological theories such as Lemaître's, according to which the Universe has a beginning in time.

In 1948, theoreticians Ralph Alpher, Robert Herman, and George Gamow predicted a different form of "fossil radiation" based on the Big Bang model, now known as the cosmic microwave background (CMB).

In 1965, shortly before his death, Lemaître learned from his assistant Odon Godart of the recent discovery of the CMB by radio astronomers Arno Penzias and Robert Wilson.

Lemaître viewed his work as a scientist as neither supporting nor contradicting any truths of the Catholic faith, and he was strongly opposed to making any arguments that mixed science with religion,[16] although he held that the two were not in conflict.

[34]Lemaître was reportedly horrified by that intervention and was later able, with the assistance of Father Daniel O’Connell, the director of the Vatican Observatory, to convince the Pope not make any further public statements on religious or philosophical interpretations of matters concerning physical cosmology.

[36]With Manuel Sandoval Vallarta, whom he had met at MIT, Lemaître showed that the intensity of cosmic rays varies with latitude because they are composed of charged particles and therefore are deflected by the Earth's magnetic field.

Lemaître and Vallarta also worked on a theory of primary cosmic radiation and applied it to their investigations of the Sun's magnetic field and the effects of the galaxy's rotation.

In his only work in physical chemistry, Lemaître collaborated in the numerical calculation of the energy levels of monodeuteroethyelene (a molecule of ethylene with one of its hydrogen atoms replaced by deuterium).

[45] Lemaître died on 20 June 1966, shortly after having learned of the discovery of cosmic microwave background radiation, which provided solid experimental support for his theory of the Big Bang.

[50] In 1936, Lemaître received the Prix Jules Janssen, the highest award of the Société astronomique de France, the French astronomical society.