Physicist and astronomer Georges Lemaître was born #OTD in 1894. In 1927 he proposed a model of an expanding Universe. Then, in 1931, he ran the clock backwards and hypothesized its origin in a “unique atom” or “unique quantum,” paving the way for modern ideas about the Big Bang.

Lemaître and the expanding universe were the subject of a Google doodle a few years ago.
Image: the Google doodle team

(Lemaître was not the first to propose an expanding universe. Alexander Friedman’s work appeared five years earlier.)
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Lemaître, a Jesuit, entered the priesthood after serving in WWI. Later, he studied with Eddington at Cambridge. He worked on cosmology and stellar astronomy, applying insights from the still-new field of relativity. His studies took him to Harvard and MIT.

In 1927 he published a paper in Annales de la Société Scientifique de Bruxelles, claiming that relativity implies an expanding Universe. His proposal assumed a once-static configuration (the idea preferred by Einstein at the time) that suddenly begins expanding.

Lemaître pointed out that this would imply an expansion velocity for galaxies — what we now refer to as Hubble's Law. (The idea that other galaxies existed outside our Milky Way was only two years old!)

Einstein was skeptical. After all, he'd added a cosmological constant term to the equations of general relativity in 1917 to avoid exactly this sort of behavior!

But the work of Edwin Hubble, Vesto Slipher, Annie Jump Cannon, Milton Humason, and others eventually established a clear relationship between galactic distance and redshift. Between that and scientists like Eddington promoting Lemaître's idea, Einstein came around.

In 1931, Lemaître pursued a radical line of thought. He ran the clock of his model backwards and concluded that the Universe must have a beginning, an extremely dense and energetic phase "very different from the present order of Nature."