Georges-Henri Lemaître (July 17, 1894 – June 20, 1966) was a Belgian Roman Catholic priest and astronomer.

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, as well as ancient cosmological-philosophical traditions. Einstein, however, believed in a steady-state model of the universe.

Lemaître took cosmic rays to be the remnants of the event, although it is 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.

Biography

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 Mechelen 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. Lemaître 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 occasions - 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.