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. He was the first to argue that the recession of galaxies is evidence of an expanding universe and to connect the observational Hubble–Lemaître law with the solution to the Einstein field equations in the general theory of relativity for a homogenous and isotropic universe. 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.

Lemaître studied engineering, mathematics, physics, and philosophy at the Catholic University of Louvain and was ordained as a priest of the Archdiocese of Mechelen in 1923. His ecclesiastical superior and mentor, Cardinal Désiré-Joseph Mercier, encouraged and supported his scientific work, allowing Lemaître to travel to England, where he worked with the astrophysicist Arthur Eddington at the University of Cambridge in 1923–1924, and to the United States, where he worked with Harlow Shapley at the Harvard College Observatory and at the Massachusetts Institute of Technology (MIT) in 1924–1925.

Lemaître was a professor of physics at Louvain from 1927 until his retirement in 1964. 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. Lemaître also argued in favor of including a positive cosmological constant in the Einstein field equations, both for conceptual reasons and to help reconcile the age of the universe inferred from the Hubble–Lemaître law with the ages of the oldest stars and the abundances of radionuclides.

Father Lemaître remained until his death a secular priest of the Archdiocese of Mechelen (after 1961, the "Archdiocese of Mechelen-Brussels"). In 1935, he was made an honorary canon of St. Rumbold's Cathedral. In 1960, Pope John XXIII appointed him as Domestic Prelate entitling him to be addressed as "Monsignor". In that same year he was appointed as president of the Pontifical Academy of Sciences, a post that he occupied until his death. Among other awards, Lemaître received the first Eddington Medal of the Royal Astronomical Society in 1953, "for his work on the expansion of the universe."

Georges Lemaître was born in Charleroi, Belgium, the eldest of four children of Joseph Lemaître, a prosperous industrialist who owned a glassworks factory, and Marguerite née Lannoy, who was the daughter of a brewer. Georges was educated at the Collège du Sacré-Cœur, a grammar school in Charleroi run by the Jesuits. In 1910, after a fire destroyed the glassworks, the family moved to Brussels, where Joseph had found a new position as manager for the French bank Société Générale. Georges then became a pupil at another Jesuit school, St. Michael's College. Although he had expressed his interest in pursuing a religious vocation, his father convinced him to attend university first and to train as a mining engineer.^{[citation needed]}

In 1911, Lemaître began to study engineering at the Catholic University of Louvain. In 1914, after the outbreak of World War I, Lemaître interrupted his studies to volunteer for the Belgian army. He participated in the Battle of the Yser, in which the Belgians succeeded in halting the German advance. When the army transferred him from the infantry to artillery, Lemaître was sent to complete a course on ballistics. 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. However, at the end of hostilities he received the Belgian War Cross with bronze palm, one of only five rank-and-file troops to receive that award from the hands of King Albert I.

Lemaître was an admirer of the French Catholic writer Léon Bloy. During a leave from his military service in World War I, Lemaître visited Bloy in Bourg-la-Reine, near Paris, where Bloy was living in a house that had belonged to his late friend and fellow writer Charles Péguy. On that occasion, Lemaître shared with Bloy an essay entitled Les trois premières paroles de Dieu ("The First Three Words of God"), in which he attempted to reconcile the Genesis creation narrative with modern science. Bloy, however, was unimpressed and advised Lemaître to grow more familiar with the works of the Church Fathers. This experience may have contributed to Lemaître's abandonment of the "concordist" effort to reconcile theological and scientific knowledge at a common intellectual level.

After the war, Lemaître abandoned engineering for the study of physics and mathematics. In 1919 he also completed the course taught at the Higher Institute of Philosophy, established by Cardinal Désiré-Joseph Mercier to promote neo-Thomism. Lemaître obtained his doctorate in science in 1920 with a thesis entitled L'approximation des fonctions de plusieurs variables réelles ("The approximation of functions of several real variables"), written under the direction of mathematician Charles de la Vallée-Poussin.