Georges Henri Joseph Édouard Lemaître ( ; ; 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."
Early life Georges Henri Joseph Édouard 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 , 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.
University studies and military service
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. Years later, Einstein questioned Lemaître on the idea of concordism. Lemaître opposed the idea that faith and science are opposed, but also acknowledged that concordism was invalid. He argued, "Should a priest reject relativity because it contains no authoritative exposition on the doctrine of the Trinity? Once you realize that the Bible does not purport to be a textbook of science, the old controversy between religion and science vanishes"
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 titled ("The approximation of functions of several real variables"), written under the direction of mathematician Charles de la Vallée-Poussin.
Religious training Lemaître had considered joining the Jesuits or the Benedictines, but finally decided to prepare instead for the diocesan priesthood. Between 1920 and 1923 he was a student at the , the seminary for "late vocations" (i.e., mature students for the priesthood) of the Archdiocese of Mechelen. It was during his spare time at the seminary that Lemaître learned the general theory of relativity. He was ordained as a priest on 22 September 1923 by Cardinal Mercier. As a diocesan priest in French-speaking Belgium, he was known as "Abbé Lemaître".
At the seminary, Lemaître joined the ("Priestly fraternity of the Friends of Jesus"), which had been created by Cardinal Mercier to promote the spiritual life of select diocesan priests and which was established canonically by his successor, Cardinal Jozef-Ernest van Roey. As a member of the fraternity, Lemaître took vows of chastity, poverty, and obedience, as well as special ("vow of immolation") promising complete submission to the person of Christ. 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 ("Queen of Peace") in Schilde, near Antwerp, and also undertook translations of the mystical works of John of Ruusbroec.
Voyage to Britain and the US
In 1922, Lemaître applied to the Belgian Ministry of Sciences and Arts for a travel bursary. As part of that application, he submitted a thesis on the astronomical implications of general relativity that included a demonstration that the most general form of the Einstein field equations included a cosmological constant term. The jury awarded Lemaître a prize of 8,000 Belgian francs.
Cardinal Mercier supported Lemaître's scientific work and helped him to obtain further financial support for a two-year visit to Great Britain and the United States. Only ten days after his ordination, Lemaître left Belgium to take up residence at St Edmund's House, then a community of Catholic priests studying for degrees at the University of Cambridge and which would later become St Edmund's College. At Cambridge, Lemaître was a research associate in astronomy and worked with the eminent astrophysicist Arthur Eddington, who introduced Lemaître to modern cosmology, stellar astronomy, and numerical analysis.
Lemaître then spent the following year at the Harvard College Observatory, in Cambridge, Massachusetts, working with Harlow Shapley, a leading expert in the study of what were then called "spiral nebulae" (now identified as spiral galaxies). 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.
Work on cosmology
On his return to Belgium in 1925, Lemaître became a part-time lecturer at the Catholic University of Louvain and began working on a report that was finally published in 1927 in the ("Annals of the Scientific Society of Brussels"), under the title ("A homogeneous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae"). There he developed (independently of the earlier work of Alexander Friedmann) the argument that the equations of Albert Einstein's general theory of relativity implied that the Universe is not static (see Friedmann equations). Lemaître connected this prediction to what he argued was a simple relation of proportionality between the average recessional velocity of galaxies and their distance to the Earth.
This 1927 paper had little impact because the were not widely read by astronomers or physicists outside of Belgium. Moreover, in this 1927 paper Lemaître assumed a universe with a positive cosmological constant, but at this time, Einstein insisted that only a universe in which the cosmological constant had the precise value to make it static was physically acceptable. Lemaître later recalled Einstein saying to him: "" ("your calculations are correct, but your physics is abominable").
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. Upon obtaining that second doctorate, Lemaître was appointed ordinary professor at the Catholic University of Louvain.
Hubble-Lemaître law
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. Although Hubble himself did not interpret that result in terms of an expanding Universe, his work attracted widespread attention and soon convinced many experts, including Einstein, that the Universe is not static. The proportionality between distance and recessional velocity for galaxies has since been commonly known as "Hubble's law", but in 2018 the International Astronomical Union (IAU) adopted a resolution recommending that it be referred to as the "Hubble-Lemaître law".
In 1931, an English translation of Lemaître's 1927 report appeared in the Monthly Notices of the Royal Astronomical Society, with a commentary by Arthur Eddington that characterized Lemaître's work as a "brilliant solution" to the outstanding problems of cosmology and a response by Lemaître to Eddington's comments. This English translation, however, omitted Lemaître's estimate of the "Hubble constant" for reasons that remained unclear for many years. 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.
Hypothesis of the primeval atom