On his return to Belgium in 1925, he became a part-time lecturer at the Catholic University of Louvain.
He began the report which brought him international fame when it 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 his new idea that the universe is expanding, which he derived from General Relativity; this later became known as Hubble's law, even though Lemaître was the first to provide an observational estimate of the Hubble constant.
The initial state he proposed was taken to be Einstein's own model of a finitely sized static universe.
The paper had little impact because the journal in which it was published was not widely read by astronomers outside Belgium; Arthur Eddington reportedly helped translate the article into English in 1931, but the part of it pertaining to the estimation of the "Hubble constant" was not included in the translation for reasons that remained unknown for a long time.
This issue was clarified in 2011 by Mario Livio; Lemaître omitted the paragraphs written by himself upon initially translating the paper for the Royal Astronomical Society, in favour of reports of new work on the subject, since by that time Hubble's calculations had already improved on his earlier ones.

At this time, Einstein, while not taking exception to the mathematics of Lemaître's theory, refused to accept that the universe was expanding; Lemaître recalled his commenting "Vos calculs sont corrects, mais votre physique est abominable" ("Your calculations are correct, but your physics is atrocious").
In the same year, Lemaître 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.
Upon obtaining his PhD, he was named ordinary professor at the Catholic University of Louvain.

In 1931, Arthur 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 later on 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's theory appeared for the first time in an article for the general reader on science and technology subjects in the December 1932 issue of Popular Science.
Lemaître's theory became better known as the "Big Bang theory," a picturesque term playfully coined during a 1949 BBC radio broadcast by the astronomer Fred Hoyle, who was a proponent of the steady state universe and remained so until his death in 2001.

Lemaître's proposal met with skepticism from his fellow scientists.
Eddington found Lemaître's notion unpleasant.
Einstein thought it unjustifiable from a physical point of view, although he encouraged Lemaître to look into the possibility of models of non-isotropic expansion, so it is clear he was not altogether dismissive of the concept.
Einstein also appreciated Lemaître's argument that Einstein's model of a static universe could not be sustained into the infinite past.

With Manuel Sandoval Vallarta, Lemaître discovered that the intensity of cosmic rays varied with latitude because these charged particles are interacting with the Earth's magnetic field.
In their calculations, Lemaître and Vallarta made use of MIT's differential analyzer computer developed by Vannevar Bus
On his return to Belgium in 1925, he became a part-time lecturer at the Catholic University of Louvain. He began the report which brought him international fame when it 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 his new idea that the universe is expanding, which he derived from General Relativity; this later became known as Hubble's law, even though Lemaître was the first to provide an observational estimate of the Hubble constant. The initial state he proposed was taken to be Einstein's own model of a finitely sized static universe. The paper had little impact because the journal in which it was published was not widely read by astronomers outside Belgium; Arthur Eddington reportedly helped translate the article into English in 1931, but the part of it pertaining to the estimation of the "Hubble constant" was not included in the translation for reasons that remained unknown for a long time. This issue was clarified in 2011 by Mario Livio; Lemaître omitted the paragraphs written by himself upon initially translating the paper for the Royal Astronomical Society, in favour of reports of new work on the subject, since by that time Hubble's calculations had already improved on his earlier ones.
At this time, Einstein, while not taking exception to the mathematics of Lemaître's theory, refused to accept that the universe was expanding; Lemaître recalled his commenting "Vos calculs sont corrects, mais votre physique est abominable" ("Your calculations are correct, but your physics is atrocious"). In the same year, Lemaître 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. Upon obtaining his PhD, he was named ordinary professor at the Catholic University of Louvain.
In 1931, Arthur 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 later on 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's theory appeared for the first time in an article for the general reader on science and technology subjects in the December 1932 issue of Popular Science. Lemaître's theory became better known as the "Big Bang theory," a picturesque term playfully coined during a 1949 BBC radio broadcast by the astronomer Fred Hoyle, who was a proponent of the steady state universe and remained so until his death in 2001.
Lemaître's proposal met with skepticism from his fellow scientists. Eddington found Lemaître's notion unpleasant. Einstein thought it unjustifiable from a physical point of view, although he encouraged Lemaître to look into the possibility of models of non-isotropic expansion, so it is clear he was not altogether dismissive of the concept. Einstein also appreciated Lemaître's argument that Einstein's model of a static universe could not be sustained into the infinite past.
With Manuel Sandoval Vallarta, Lemaître discovered that the intensity of cosmic rays varied with latitude because these charged particles are interacting with the Earth's magnetic field. In their calculations, Lemaître and Vallarta made use of MIT's differential analyzer computer developed by Vannevar Bus
Georges Lemaître