Another change in his life brought about by his wartime experiences came about when he enrolled at the Maison Saint Rombaut, a seminary of the Archdiocese of Malines, and was ordained in 1923, becoming Abbé Lemaître. Now, with the strong mathematical background obtained from his studies with de la Vallée Poussin, Lemaître turned towards mathematical astronomy and went to Cambridge in England where he studied with Eddington during the academic years 1923-24, then he went to the United States spending the next academic year at the Harvard College Observatory in Massachusetts. In 1925 he accepted a position as a part-time lecturer at the Catholic University of Louvain in Belgium but continued to spend time at Harvard and at the Massachusetts Institute of Technology in the United States. He was awarded a Ph.D. in 1927 for his thesis The gravitational field in a fluid submitted to MIT. His supervisor at MIT had been Harlow Shapley. The research he had undertaken, partly at Harvard, partly at MIT and partly at Louvain, was written up as Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques and published in the Annales de la Société Scientifique de Bruxelles in 1927.
In this groundbreaking paper Lemaître derived what is now known as Hubble's Law relating the speed with which a galaxy is moving away to its distance. In 1927 the famous Solvay Conference was held and most of the leading physicists attended. Einstein was at the conference and he spoke to Lemaître in Brussels telling him that the ideas in his 1927 paper had been presented by Friedmann in 1922, but he also said that although he thought Lemaître's solutions of the equations of general relativity were mathematically correct, they presented a solution which was not feasible physically. Einstein said:-
Your calculations are correct, but your grasp of physics is abominable.Einstein was not alone in finding Lemaître's ideas totally unacceptable; rather this was the opinion of almost all scientists. However in 1929 Hubble published work presenting considerably more evidence of an expanding universe, contradicting the then accepted theory of a static universe. Eddington and other members of the Royal Astronomical Society began to undertake work to try to solve the problem brought about by the discrepancy between theory and observation. Lemaître then sent a copy of his 1927 paper to Eddington who immediately saw that it provided an explanation. Eddington arranged for an English translation of Lemaître's paper to be published in the Monthly Notices of the Royal Astronomical Society and indeed it appeared there in March 1931. There was still a part of Lemaître's theory that scientists, including Eddington, found impossible to accept, namely the implication that the universe had a beginning at a finite time in the past. Almost all wanted to believe that the universe had always existed. We leave it to the reader to ponder the thought that perhaps Lemaître's deep Christian beliefs made the thought that the world began at a finite time in the past (as the book of Genesis claims) more easily accepted.
Lemaître responded to the objections against his theory in a paper published in Nature in May 1931. He wrote:-
If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.This was the first explicit formulation of the currently accepted 'big bang' theory. We should note that, although accepted by most scientists, Fred Hoyle did not accept this theory and the term 'big bang' was Hoyle's scornful description of Lemaître's theory in a 1950 radio broadcast. In 1933 Einstein and Lemaître gave a series of lectures in California. After listening to Lemaître explain his theory in one of these seminars, Einstein stood up and said:-
This is the most beautiful and satisfactory explanation of creation to which I have ever listened.Lemaître published a more detailed version of his theory in L'univers en expansion in 1933. An English translation of this paper by M A H MacCallum was published in 1997. The ideas presented in his 1933 paper reached the popular press who described him as the world's leading cosmologist. An article in the New York Times featured a photograph of Einstein and Lemaître with a caption:-
They have a profound respect and admiration for each other.Of course, the fact that Lemaître was both a leading scientist and a Catholic Priest was part of the fascination that the popular press had. In the same article, the author wrote:-
'There is no conflict between religion and science,' Lemaitre has been telling audiences over and over again in this country .... His view is interesting and important not because he is a Catholic priest, not because he is one of the leading mathematical physicists of our time, but because he is both.Honours from several different sources came his way such as the Francqui Prize in 1934. This prize, presented to Lemaître by King Léopold III, was the highest scientific honour that Belgium could bestow. In 1936 he was inducted into the Pontifical Academy of Science by Pope Pius XI. He was elected member of the Royal Academy of Sciences and Arts of Belgium in 1941, became the first to be awarded the Eddington Medal by the Royal Astronomical Society in 1951, and he served as President of the Pontifical Academy of Sciences from 1960 to 1966.
Lemaître had been appointed Professor at Louvain in 1927 and remained there for the rest of his career. Let us look at a few of his later publications, particularly those of a more mathematical nature. In 1942 he published L'itération rationnelle in which he discussed Gauss's method of successive approximations applied to a system of two equations in two unknowns to determine the orbit of a planet from three observations. Lemaître then applied these ideas to accelerate the orthodox process of iteration, taking the Picard iterative solution of first order differential equations as an example. He applied the same techniques in another paper published in the same year, namely Intégration d'une équation différentielle par itération rationnelle . In Sur un cas limite du problème de Stormer (1945) he studied trajectories of an electron in the neighborhood of lines of force of a magnetic dipole field, then returned to his study of numerical solutions to first order differential equations in Interpolation dans la méthode de Runge-Kutta (1947). In 1948 he published a paper applying mathematical techniques to a problem in astronomy publishing Modèles mécaniques d'amas de nébuleuses . B L J Bok writes in a review of this paper:-
The author is concerned with the problem posed by the existence of large random velocities for the individual members of clusters of galaxies. Is it possible to account for the existence of more or less permanent concentrations of galaxies in which no single galaxy remains long in the same place? The two-fold purpose of the paper is to delineate the underlying mechanical model and to write down the fundamental equations of the problem. It is shown how these equations can be applied toward the solution of the well-known problem of uniform distribution in a homogeneous, expanding universe.Another paper published in the same year Modèles de nébuleuses à vitesses radiales , this time jointly with R Vander Borght, is concerned with the study of equilibrium configurations for stellar systems with radial symmetry and in which only radial and no transverse velocities are permitted. In 1949 he returned to his study of an expanding universe in Cosmological application of relativity. H P Robertson writes:-
The paper opens with a rapid expository review of the general relativity theory of gravitation, including discussion of kinematics, conservation laws, spherical symmetry, and the solutions of Schwarzschild and de Sitter in terms of comoving coordinates. There follows an account of the homogeneous expanding universe models of Friedmann, with specialization to the type demanded by red-shift and time-scale observations. The remaining third of the paper is concerned with effects of inhomogeneities in the model, with a brief account of author's hypotheses and predictions concerning the origin of cosmic rays, their condensation into clouds, formation of nebulae and clusters of nebulae, and offers an explanation of the prevalence of hydrogen and helium as materialization of kinetic energy.Lemaître taught less through the 1950s but continued to publish on the same topics that had interested him in the 1940s. His papers include Application des méthodes de la mécanique céleste au problème de Stormer (1950), Modèles mécaniques d'amas de nébuleuses (1951), Coordonnées symétriques dans le problème des trois corps (1952), and Régularisation dans le problème des trois corps (1954). However, a new interest came into Lemaître's research related to the introduction of computers into mathematical research. He developed computer languages, proposed new calculating techniques, and continued his interests in computational mathematics. Some papers on calculating are Comment calculer? (1954) in which he proposes that paper and pencil computing be replaced by paper and typewriter. Instead of using the Arabic numerals 0 - 9 he proposes that the four letters i, j, k, l which lie within easy reach of the right hand be used to give a peculiar binary coded decimal representation. In Pourquoi de nouveaux chiffres? (1955) he discusses the shortcomings of the decimal system and even the Arabic digits. Certain advantages of the binary system are stressed such as the fact that it takes the fun out of playing Nim. His argument against the Arabic numerals is given again in Why new digits? (1955). He stresses the advantages of the binary system in Le calcul élémentaire (1956). His arithmetical architecture is discussed by Lipnik in .
Lemaître retired in 1964 when he was made professor emeritus. He continued to publish interesting papers, such as The expansion of the Universe (1967), and The principle of continuity according to Jean-Victor Poncelet (1967). We end this biography by quoting from a review by R N Tiwari of  since in many ways it provides both a summing up of Lemaître's contributions, but poses some fascinating questions for the reader to consider:-
Lemaître graduated in engineering, and his studies were interrupted due to World War I. He joined the army and, to quote from the author's statement, "after 53 months of war ordeals and military camps, he lost interest in a professional career and decided to become a priest"; this ultimately resulted in a change from engineering to the mathematical sciences, particularly to general relativity, which marked a very notable turning point in Lemaître's life. Whether this change of mind was due to war or the seed of such a thought was inherent in him and germinated at the appropriate time, and whether the environment of the Church and its a priori principles had any influence on his scientific discoveries (such as the evolution of the universe from a primordial atom, the existence of singularity at the initial epoch, etc.) cannot be inferred conclusively. The sequence of events narrated by the author shows, however, that time and again Lemaître was accused (especially by Einstein) of using scientific reasonings "to defend a (religious) dogma of the Church". Was it really so? Was Lemaître the scientist being guided by Lemaître the Catholic priest? The author leaves these points for readers to decide for themselves. However, he remarks that "for modern scientific cosmologists, although they may feel uneasy about this primordial singularity, the objectivity of the thinking of its initiator is beyond doubt".
Article by: J J O'Connor and E F Robertson