After the war, he studied physics and mathematics, and began to prepare for diocesan priesthood. He obtained his doctorate in 1920 with a thesis entitled l’Approximation des fonctions de plusieurs variables réelles (Approximation of functions of several real variables), written under the direction of Charles de la Vallée-Poussin. He was ordained a priest in 1923 and became a graduate student in astronomy at the University of Cambridge, spending a year at St Edmund’s House (now St Edmund’s College, Cambridge). Arthur Eddington taught him modern cosmology, stellar astronomy, and numerical analysis. He spent the next year at Harvard College Observatory in Cambridge, Massachusetts, with Harlow Shapley, who had just gained renown for his work on nebulae, and at the Massachusetts Institute of Technology (MIT), where he registered for the doctoral program in sciences.
In 1925, on his return to Belgium, he became a part-time lecturer at the Catholic University of Leuven. In 1927, he published an article in the little-known journal, Annales de la Société Scientifique de Bruxelles, under the title “Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques” (“A homogeneous Universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae”). In this report, he presented his a family of solutions to Einstein’s field equations that described an expanding universe, derived from General Relativity and later known as Hubble’s law, and provided the first observational estimation of the Hubble constant. While the article was not widely noticed, Arthur Eddington reportedly helped translate it into English in 1931, unfortunately omitting the article’s estimation of the “Hubble constant”. Lemaître returned to MIT to present his doctoral thesis. Upon obtaining what was now his second Ph.D., he was named ordinary professor at the Catholic University of Leuven.
In 1930, Eddington published in the Monthly Notices of the Royal Astronomical Society a long commentary on Lemaître’s 1927 article, in which he described the latter as a “brilliant solution” to the outstanding problems of cosmology. The original paper was published in an abbreviated English translation in 1931, along with a sequel by Lemaître responding to Eddington’s comments. Lemaître was then invited to London to participate in a meeting of the British Association on the relation between the physical universe and spirituality. There he proposed that the universe expanded from an initial point, which he called the “Primeval Atom”. He developed this idea in a report published in Nature. Lemaître himself also described his theory as “the Cosmic Egg exploding at the moment of the creation”; it became better known as the “Big Bang theory,” a pejorative term coined during a 1949 BBC radio broadcast by the astronomer and atheist Fred Hoyle, who was then still a proponent of the steady state universe and remained so until his death in 2001. Hoyle would later convert to theism as a result of his own astrophysical work.
In 1931, Lemaitre was the first scientist to propose the expansion of the universe was actually accelerating which was confirmed observationally in the 1990s through observations of very distant Type IA supernova with the Hubble Space Telescope. In 1933, Lemaître found an important inhomogeneous solution of Einstein’s field equations describing a spherical dust cloud, the Lemaître–Tolman metric. Lemaître was also an early adopter of computers for cosmological calculations. He introduced the first computer to his university (a Burroughs E101) in 1958 and was one of the inventors of the Fast Fourier transform algorithm. Among his many awards for outstanding science, he was given the inaugural Eddington Medal awarded by the Royal Astronomical Society. He died on 20 June 1966, shortly after having learned of the discovery of cosmic microwave background radiation, which provided further evidence for his proposal about the birth of the universe. The fifth Automated Transfer Vehicle to the International Space Station was named Georges Lemaitre in his honor. He is the Father of Cosmology.