According to the Big Bang theory, the expansion of the observable universe began with the explosion of a single particle at a definite point in time.

Georges Lemaître, (1894-1966), Belgian cosmologist, Catholic priest, and father of the Big Bang theory. Photo courtesy of AIP Emilio Segré Visual Archives, Dorothy Davis Locanthi Collection.

This startling idea first appeared in scientific form in 1931, in a paper by Georges Lemaître, a Belgian cosmologist and Catholic priest. The theory, accepted by nearly all astronomers today, was a radical departure from scientific orthodoxy in the 1930s. Many astronomers at the time were still uncomfortable with the idea that the universe is expanding. That the entire observable universe of galaxies began with a bang seemed preposterous.

Lemaître was born in 1894 in Charleroi, Belgium. As a young man he was attracted to both science and theology, but World War I interrupted his studies (he served as an artillery officer and witnessed the first poison gas attack in history). After the war, Lemaître studied theoretical physics, and in 1923 was ordained as an abbé. The following year, he pursued his scientific studies with the distinguished English astronomer Arthur Eddington, who regarded him as “a very brilliant student, wonderfully quick and clear-sighted, and of great mathematical ability.” Lemaître then went on to America, where he visited most of the major centers of astronomical research. Later, he received his Ph.D. in physics from the Massachusetts Institute of Technology.

In 1925, at age 31, Lemaître accepted a professorship at the Catholic University of Louvain, near Brussels, a position he retained through World War II (when he was injured in the accidental bombing of his home by U.S. forces). He was a devoted teacher who enjoyed the company of students, but he preferred to work alone. Lemaître’s religious interests remained as important to him as science throughout his life, and he served as President of the Pontifical Academy of Sciences from 1960 until his death in 1966.

In 1927, Lemaître published in Belgium a virtually unnoticed paper that provided a compelling solution to the equations of General Relativity for the case of an expanding universe. His solution had, in fact, already been derived without his knowledge by the Russian Alexander Friedmann in 1922. But Friedmann was principally interested in the mathematics of a range of idealized solutions (including expanding and contracting universes) and did not pursue the possibility that one of them might actually describe the physical universe. In contrast, Lemaître attacked the problem of cosmology from a thoroughly physical point of view, and realized that his solution predicted the expansion of the real universe of galaxies that observations were only then beginning to suggest.

By 1930, other cosmologists, including Eddington, Willem de Sitter, and Einstein, had concluded that the static (non-evolving) models of the universe they had worked on for many years were unsatisfactory. Furthermore, Edwin Hubble, using the world’s largest telescope at Mt. Wilson in California, had shown that the distant galaxies all appeared to be receding from us at speeds proportional to their distances. It was at this point that Lemaître drew Eddington’s attention to his earlier work, in which he had derived and explained the relation between the distance and the recession velocity of galaxies. Eddington at once called the attention of other cosmologists to Lemaître’s 1927 paper and arranged for the publication of an English translation. Together with Hubble’s observations, Lemaître’s paper convinced the majority of astronomers that the universe was indeed expanding, and this revolutionized the study of cosmology.

A year later, Lemaître explored the logical consequences of an expanding universe and boldly proposed that it must have originated at a finite point in time. If the universe is expanding, he reasoned, it was smaller in the past, and extrapolation back in time should lead to an epoch when all the matter in the universe was packed together in an extremely dense state. Appealing to the new quantum theory of matter, Lemaître argued that the physical universe was initially a single particle—the “primeval atom” as he called it—which disintegrated in an explosion, giving rise to space and time and the expansion of the universe that continues to this day. This idea marked the birth of what we now know as Big Bang cosmology.

It is tempting to think that Lemaître’s deeply-held religious beliefs might have led him to the notion of a beginning of time. After all, the Judeo-Christian tradition had propagated a similar idea for millennia. Yet Lemaître clearly insisted that there was neither a connection nor a conflict between his religion and his science. Rather he kept them entirely separate, treating them as different, parallel interpretations of the world, both of which he believed with personal conviction. Indeed, when Pope Pius XII referred to the new theory of the origin of the universe as a scientific validation of the Catholic faith, Lemaître was rather alarmed. Delicately, for that was his way, he tried to separate the two:

“As far as I can see, such a theory remains entirely outside any metaphysical or religious question. It leaves the materialist free to deny any transcendental Being… For the believer, it removes any attempt at familiarity with God… It is consonant with Isaiah speaking of the hidden God, hidden even in the beginning of the universe.”

In the latter part of his life, Lemaître turned his attention to other areas of astronomical research, including pioneering work in electronic computation for astrophysical problems. His idea that the universe had an explosive birth was developed much further by other cosmologists, including George Gamow, to become the modern Big Bang theory. While contemporary views of the early universe differ in many respects from Lemaître’s “primordial atom,” his work had nevertheless opened the way. Shortly before his death, Lemaître learned that Arno Penzias and Robert Wilson had discovered the cosmic microwave background radiation, the first and still most important observational evidence in support of the Big Bang.