Lemaître is credited with proposing the Big Bang theory of the origin of the universe, although he called it his 'hypothesis of the primeval atom'. He based his theory, published between 1927 and 1933, on the work of Einstein, among others. Einstein, however, believed in a steady-state model of the universe. Lemaître took cosmic rays to be the remnants of the event, although it now known that they originate within the local galaxy. He estimated the age of the universe to be between 10 and 20 billion years ago, which agrees with modern opinion.
At seventeen years old, after studying humanities at a Jesuit school, he entered the civil engineering school of the Catholic University of Leuven. In 1914, at the beginning of World War I, he paused his studies to engage as a volunteer in the Belgian army. At the end of hostilities, he received the Military Cross with palms.
After the war, he undertook studies in physics and mathematics and began to prepare for 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.
The tragedy of the war in which he took part deeply marked him: he entered the Malines seminary and was ordained as a priest in 1923. However, neither the war nor his studies nor his vocation dried up his curiosity: since 1920, he had learnt the theory of relativity and perfectly mastered it.
In 1923, he visited the University of Cambridge where the astronomer Arthur Eddington initiated him into modern stellar astronomy and numerical analysis. He spent the following year at Harvard College Observatory in Cambridge, Massachusetts with Harlow Shapley, who had just gained a name for his work on nebulae, and to the Massachusetts Institute of Technology, where he registered for the doctorate in sciences.
In 1925, on his return to Belgium, he became a part-time lecturer at the University of Leuven. He then began the report which would bring him international notoriety and which was published in 1927 in the Annales de la Société Scientifique de Bruxelles (Annals of the Scientific Society of Brussels), 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 the new idea of an expanding Universe.
At this time, Einstein, whilst approving of the mathematics of Lemaître's theory, refused to accept the idea of an expanding universe. He believed it immutable, but would later recognize that it was the greatest error of his life.
Not very concerned with honors, Lemaître did not think it desirable to become famous, nor to publicize his article. In fact, he was already concentrating on a new challenge: to solve the problem of the origin of the Universe. The same year, he returned to MIT to present his doctoral thesis on The gravitational field in a fluid sphere of uniform invariant density according to the theory of relativity. He obtained a PhD and was then named ordinary Professor at the University of Leuven.
In 1931, Eddington published an English translation of the 1927 article with a long commentary. Lemaître was then invited in London in order to take part in a meeting of the British Association on the relation between the physical universe and spirituality. It is there that he proposed an expanding universe which started with an initial singularity, and the idea of the Primeval Atom which he developed in a report published in the Monthly Notices of the Royal Astronomical Society. Himself liked to describe his theory as "the Cosmic Egg exploding at the moment of the creation", which was later to be coined by his critics as the Big Bang theory.
This proposal caused a sharp reaction from the scientific community of the time. Eddington found Lemaitre's notion unpleasant. As for Einstein, he found it suspect, because, according to him, it was too strongly reminiscent of the Christian dogma of creation and was unjustifiable from a physical point of view. The debate between cosmology and religion took the form of a polemic that would last several decades. In this debate, Lemaître would be a fundamental actor who unceasingly tried to separate science from faith.
However, in January 1933, Lemaitre and Einstein, who had met on several occassions - in 1927 in Brussels, at the time of a Solvay congress, in 1932 in Belgium, at the time of a cycle of conferences in Brussels and lastly in 1935 at Princeton - traveled together to California for a series of seminars. After the Belgian detailed his theory, Einstein stood up, applauded, and said, "This is the most beautiful and satisfactory explanation of creation to which I have ever listened".
In 1933, when he resumed his theory of the expanding universe and published a more detailed version in the Annals of the Scientific Society of Brussels, Lemaître would achieve his greatest glory. The American newspapers called him a famous Belgian scientist and described him as the leader of the new cosmological physics.
On March 17, 1934, Lemaître received the Francqui Prize, the highest Belgian scientific distinction, from King Léopold III. His proposers were Albert Einstein, Charles de la Vallée-Poussin and Alexandre de Hemptinne. The members of the international jury were Eddington, Langevin and Théophile de Donder. Another distinction that the Belgian government reserves for exceptional scientists was allotted to him in 1950: the decennial prize for applied sciences for the period 1933-1942.
In 1936, he was elected member of the Pontifical Academy of Sciences. He took an active role there, became the president in March 1960 and remaining so until his death. He was also named prelate in 1960.
In 1941, he was elected member of the Royal Academy of Sciences and Arts of Belgium.
During the 1950s, he gradually gave up part of his teaching workload, ending it completely with his éméritat in 1964.
At the end of his life, he was devoted more and more to numerical calculation. He was in fact a remarkable algebraicist and arithmetical calculator. Since 1930, he used the most powerful calculating machines of the time like the Mercedes. In 1958, he introduced at the University a Burroughs E 101, the University's first electronic computer. Lemaître kept a strong interest in the development of computers and, even more, in the problems of language and programming. With age, this interest grew until it absorbed him almost completely.
Sociable, devoted to his students and collaborators, he remained however an isolated researcher and one finds only few correspondences and scientific exchanges with his peers.
If this undeniable precursor of modern cosmology remains in the shade of the great names of the 20th century (Einstein, Eddington, Hubble and Gamow in particular), it is probably because he was a priest (Fred Hoyle, who coined the name Big Bang, never forgave him!) and because of the ambiguity of his character, at the same time modest and full of himself. Modest, because he neither pursued honors nor sought at all costs to be recognized. Full of himself, in his manner of affirming, at least in private, his capacities as a mathematician and the originality of his ideas. But that did not prevent him from being open, frank, merry, optimistic, jovial, and always remarkably flexible of mind.