May 1931: Georges Lemaître proposes that the universe has a beginning

In 1931, physicist, mathematician, and ordained priest Georges Lemaître proposed a paradigm-changing cosmological theory: a universe with a singular beginning.

His brief note in the May 9 issue of Nature — with fewer than 475 words and no equations — can be considered “the true ‘Charter’ of the modern Big Bang theory,” cosmologist Jean-Pierre Luminet wrote in the July 2011 issue of General Relativity and Gravitation.

By 1930, the once-prevailing model of a static universe was floundering. Edwin Hubble’s extragalactic distance measurements, published in 1925, helped establish the vast scale of the universe. Then, in 1929, he showed that galaxies seemed to be receding at velocities proportional to their distance. This left Albert Einstein, Willem de Sitter, Arthur Eddington, and their contemporaries grappling with how to reconcile observation and cosmological theory.

In a January 1930 meeting of the Royal Astronomical Society, Eddington proposed looking beyond static models of the universe to see whether other solutions to Einstein’s field equations fit observations. Lemaître, a professor at Catholic University of Louvain in Belgium who had studied under Eddington, read the meeting notes and reminded Eddington that he (Lemaître) had, in fact, done this two years earlier. He’d even sent Eddington the paper.

Lemaître was born in Belgium in 1894. After he earned a Ph.D. in mathematics in 1920, he went to seminary school; it was during this time that he began to study general relativity. Once ordained in 1923, he spent a year studying astrophysics and general relativity under Eddington at Cambridge University, followed by a year in North America, during which he enrolled in the physics Ph.D. program at Massachusetts Institute of Technology and began working with Harlow Shapley at the Harvard College Observatory. Lemaître defended his dissertation in 1927.

That same year, before attending the storied 1927 Solvay Congress on Physics, Lemaître calculated a solution to Einstein’s field equations that took the form of an expanding universe.

Lemaître was not the first scientist to do so: In papers published in 1922 and 1924, Alexander Friedmann, a relatively unknown Russian mathematician, had used Einstein’s general theory of relativity to propose a universe that could expand or contract. Lemaître was unaware of Friedmann’s work, and Friedmann tragically died in 1925 at just 37 years old.

But in 1927, Lemaître had evidence that Friedmann did not — Hubble’s 1925 data. An expanding universe, Lemaître noted, was compatible with Hubble’s observations. He also derived the velocity-distance relation that would later be deemed “Hubble’s law.” He published the paper in French in the Annales de la Société Scientifique de Bruxelles.

“The great novelty [of this paper] was that Lemaître provided the first interpretation of cosmological redshifts in terms of space expansion, instead of a real motion of galaxies,” Luminet wrote more than 80 years later. “This idea proved to be one of the most significant discoveries of the century.”

At the time, however, the paper flew under the radar. “As this was long before the insistent demand for an explanation of the red-shift arose and before the discovery of the velocity distance relation, and as the paper was buried in this little read publication, it entirely escaped attention,” noted astronomer John Stanley Plaskett in a 1932 review of the progress on astronomy for Publications of the Astronomical Society of the Pacific.

Lemaître later recalled meeting Einstein at the 1927 Solvay Congress and discussing his findings. “After some favorable technical observations, [Einstein] concluded by saying that from the point of view of physics it seemed to him absolutely abominable,” he wrote in a 1957 piece about his encounters with Einstein.

By 1930, the community was primed for Lemaître’s research. Eddington elevated the paper’s status and it was published in English in the March 1931 issue of Monthly Notices of the Royal Astronomical Society. Many considered Lemaître’s model of the expanding universe compelling, especially after Eddington highlighted the instability of the static model Einstein had proposed.

Although the paper was gaining acceptance, the scientific community wasn’t ready for what Lemaître proposed next. In his May 1931 note in Nature, Lemaître suggested that an expanding universe implies a single origin. “If we go back in the course of time we must find fewer and fewer quanta, until we find all the energy of the universe packed in a few or even in a unique quantum,” he wrote.

The note, titled “The beginning of the world from the point of view of quantum theory,” suggested that this quantum, an unstable atom he later called the “primeval atom,” could have undergone some kind of super-radioactive process, expanding and decaying into smaller atoms and launching the expanding universe.

Lemaître seemed to know this might be a hard sell, and his opening line got right to the point: “Sir Arthur Eddington states that, philosophically, the notion of a beginning of the present order of Nature is repugnant to him.”

Eddington, Einstein, and other scientists at the time felt that even discussing a cosmic beginning was edging outside the realm of science. That Lemaître was first a mathematician and a priest may have reinforced this idea. “When I spoke to [Einstein] about the primeval atom, he stopped me: ‘No, that suggests creation too much,’” Lemaître recalled.

Lemaître considered science and religion as distinct domains and grounded the cosmic origin discussion in thermodynamics and quantum theory. “If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning,” he wrote in Nature. “If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time. I think that such a beginning of the world is far enough from the present order of Nature to be not at all repugnant.”

Although his hypothesis was met with skepticism, Lemaître continued developing his idea and it gained some traction, even in the press. A 1932 Popular Mechanics article by Donald H. Menzel introduced Lemaître’s “sensational theory” this way: “Out of a single, bursting atom came all the suns and planets of our universe!”

Cosmologist George Gamow was key to transitioning from Lemaître’s primeval atom hypothesis to the modern Big Bang theory. In 1948, Gamow and his student Ralph Alpher detailed in Physical Review how the extreme conditions of a radiation-dominated early universe could produce some chemical abundances, such as helium. They, in collaboration with Robert Herman, also predicted a cosmic microwave background to the universe, a relic of this extremely hot, dense period. In 1965, Arno Penzias and Robert Wilson announced their discovery of the cosmic microwave background, reinforcing the growing scientific consensus around the Big Bang theory.

Lemaître died in 1966, reportedly just a few days after learning of the discovery by Penzias and Wilson. During his prolific career, he published insightful and often forward-thinking contributions not just on the birth of the universe, but also the cosmological constant, cosmic rays, and the formation of galaxies.

The radical innovation introduced by Lemaître in his 1931 note, Luminet wrote, “consisted in linking the structure of the universe at large scales with the intimate nature of the atoms.”