Credited to Belgium. Britannica notes that Aleksandr Friedmann of Russia proposed an expanding universe in the 1920s as well, and that the modern version was developed by George Gamow in the 1940s.
How it began
Georges Lemaître, born in Belgium in 1894, was a priest, mathematician and physicist who became professor at the Catholic University of Louvain in 1927. That year he found a solution to Einstein's field equations describing an expanding universe and published it in French in the Annales de la Société Scientifique de Bruxelles. Using available data on galaxies, he derived the proportionality between distance and recession velocity later called Hubble's law.
In May 1931 he went further in a short note in Nature, arguing that an expanding universe implies a single origin: all the energy of the universe packed into one quantum, which he later called the primeval atom.
Before it existed
Until about 1930 the prevailing model was a static universe, the form Einstein had given his own cosmology. Alexander Friedmann, a Russian mathematician, had shown in papers of 1922 and 1924 that general relativity allows a universe that expands or contracts, but he died in 1925 and Lemaître did not know of his work. Edwin Hubble's distance measurements, published in 1925, established the vast scale of the universe, and in 1929 he showed that galaxies appear to recede at velocities proportional to their distance. Hubble himself did not interpret this as expanding space.
How it works
Lemaître's 1927 paper read the redshifts of galaxies as an effect of space itself expanding, rather than of galaxies moving through space. His 1931 proposal ran that expansion backward. Going back in time there would be fewer and fewer quanta, until all energy sat in a single unstable quantum that broke apart in a kind of super-radioactive process, decaying into smaller atoms and launching the expansion.
He grounded the idea in thermodynamics and quantum theory, and wrote that the notions of space and time would have no meaning at the very beginning. He compared the evolution of the world to a fireworks display that has just ended.
Who pushed back
The 1927 paper went largely unnoticed. Lemaître recalled that Einstein, at the 1927 Solvay conference, found the calculations sound but the physics "abominable". The primeval atom met stronger objections. Arthur Eddington called the notion of a beginning "repugnant", and Einstein reportedly said it "suggests creation too much". Many scientists felt that discussing a cosmic beginning lay outside science, a suspicion that Lemaître's priesthood may have reinforced. He insisted that science and religion were separate domains. Fred Hoyle, who coined the term "Big Bang" in a 1949 BBC broadcast, remained an opponent and defended a steady-state universe.
What changed first
In 1930 Eddington, reminded by Lemaître of the 1927 paper, promoted it, and an English translation appeared in the Monthly Notices of the Royal Astronomical Society in March 1931. Together with Hubble's observations, it helped convince most astronomers that the universe is expanding. The primeval atom reached general readers in a 1932 magazine article by the astronomer Donald Menzel, and Lemaître lectured in the United States in 1933 and 1934, where newspapers presented him as a leader of the new cosmology. He published a book on the hypothesis in 1946.
What lasted
George Gamow and his colleagues carried the idea from the primeval atom toward the modern Big Bang theory. In 1948 Ralph Alpher, Robert Herman and Gamow predicted a relic radiation left from the hot, dense early universe. In 1965 Arno Penzias and Robert Wilson announced the discovery of the cosmic microwave background, which convinced most experts that the Big Bang model was valid. Lemaître learned of the discovery shortly before his death in 1966. He had received the first Eddington Medal of the Royal Astronomical Society in 1953 for his work on the expansion of the universe.
Where it stands today
The Big Bang model is the accepted account of the universe's origin, in which the universe emerged from an extremely hot, dense state and space has been expanding since. In 2018 the International Astronomical Union recommended that the distance and velocity relation be called the Hubble-Lemaître law, recognizing his 1927 derivation. Lemaître also argued for keeping a positive cosmological constant and in 1934 interpreted it as an energy of the vacuum, an idea that is now one of the main approaches to what cosmologists call the dark energy problem.
