Georges Lemaître

Georges Henri Joseph Édouard Lemaître was born in Charleroi, Belgium, in 1894. He began studying civil engineering, served as an artillery officer in the Belgian army through the First World War, and afterward turned to mathematics and physics while simultaneously preparing for the priesthood. He was ordained a Catholic priest in 1923 and spent the rest of his life as both an ordained clergyman and a working scientist, holding throughout that the two domains answered different questions and must not be confused.

In the mid-1920s Lemaître studied with Eddington at Cambridge and with Shapley at Harvard, and earned a doctorate at MIT. In 1927, working independently of Friedmann — whose papers he had not seen — he rederived the equations of an expanding universe from general relativity. But he went further than Friedmann: he connected the expansion directly to observation, deriving a linear relation between a galaxy's recession velocity and its distance and estimating its slope from the available redshift data. This is the relation that, two years later and with better data, became known as Hubble's law; in 2018 the International Astronomical Union recommended it be called the Hubble–Lemaître law in recognition of his priority.

Lemaître's boldest step came in 1931. Reasoning that an expanding universe must have been smaller and denser in the past, he proposed that it began from a single quantum of enormous density that he called the 'primeval atom' — a concrete physical origin for the cosmos in a finite past. This was the conceptual seed of what Fred Hoyle would later dismissively name the 'Big Bang'. Einstein, who had earlier resisted the expanding universe, is reported to have told Lemaître after a 1933 seminar that his was the most beautiful and satisfactory explanation of creation he had ever heard.

Lemaître also recognised, decades before it became fashionable, that Einstein's cosmological constant need not be an embarrassment: he interpreted Λ as a vacuum energy density with its own physical meaning, anticipating the modern view of dark energy. He spent his career at the University of Louvain and served as president of the Pontifical Academy of Sciences. He died in 1966, shortly after learning of the discovery of the cosmic microwave background — the relic radiation that confirmed the hot, dense beginning his primeval atom had foretold.