Georges Lemaître (1927): The First Case for an Expanding Universe

Georges Lemaître, a Belgian priest and physicist, was the first to show that the universe is expanding. In a 1927 paper he found an expanding solution of Einstein’s equations, tied it to the redshifts of galaxies and estimated the expansion rate at about 625 km/s per megaparsec, two years before Edwin Hubble’s famous paper.

The question before the paper

Einstein had applied general relativity to the whole universe assuming it was static, but Alexander Friedmann had shown in 1922 that the equations also allow universes that change size. Meanwhile astronomers had measured that most spiral galaxies are receding. Nobody had connected the two: was the recession of galaxies the visible sign of a universe that is getting bigger?

What the paper found

• An expanding universe of constant mass. Lemaître found a solution of Einstein’s equations for a homogeneous universe whose radius grows with time.
• Redshifts explained. He interpreted the redshifts of galaxies as a direct consequence of that expansion, not as ordinary motion through a fixed space.
• A velocity-distance law with a number. From published galaxy velocities and distances he derived an expansion rate of about 625 km/s per megaparsec, close to what Hubble found in 1929.

How he did it

The paper is mostly mathematics: Lemaître worked through the equations of general relativity for a uniform universe and asked what happens to light crossing a space that stretches while the light travels. The answer is a redshift that grows with distance. He then checked the idea against real data, combining galaxy velocities already in the literature with the distances available at the time, and extracted the expansion rate from their ratio.

Why it mattered

This is the first paper to put general relativity, an expanding universe and the observed recession of galaxies into a single picture. Lemaître later pushed the idea backward in time to a “primeval atom”, the seed of what became Big Bang cosmology. That hot-beginning picture led to the 1948 theory of element formation and, eventually, to the 1965 detection of the cosmic microwave background.

Yet the paper barely registered at first. It appeared in French in a little-read Belgian journal and stayed largely unnoticed until Arthur Eddington promoted it. By then Hubble’s 1929 velocity-distance relation, based on Mount Wilson observations at what is now Carnegie Science, had become the reference result.

What we know now

An English translation appeared in Monthly Notices of the Royal Astronomical Society in 1931, but it left out the paragraphs with the numerical estimate of the expansion rate. For decades this fed suspicion that someone had suppressed Lemaître’s priority. In 2011 the astrophysicist Mario Livio showed from archival letters that Lemaître made the cut himself, judging the passage “of no actual interest” by then.

In 2018 the International Astronomical Union recommended renaming Hubble’s law the Hubble-Lemaître law, with 78% of votes in favor. The expansion rate he first estimated is still contested today, as the Hubble tension.

Who wrote it

Georges Lemaître wrote the paper alone. He was both a Catholic priest and a trained physicist, and his work linked the abstract mathematics of relativity to astronomical data at a time when few theorists looked at telescopes and few observers read relativity.

Read the original

Lemaître, G. (1927). “Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques.” Annales de la Société Scientifique de Bruxelles A47: 49-59. Free to read as a scanned PDF at the NASA Astrophysics Data System. The 1931 English translation is at doi:10.1093/mnras/91.5.483.

Questions people ask

Who first discovered that the universe was expanding?

Georges Lemaître, in 1927. He derived an expanding universe from general relativity, linked it to galaxy redshifts and estimated the expansion rate. Edwin Hubble’s 1929 observations made the case that convinced astronomers, which is why the law now carries both names.

Did Lemaître propose the Big Bang theory?

Yes, in its first form. After showing in 1927 that the universe is expanding, Lemaître proposed that it began from a dense “primeval atom”. The physics of a hot early universe was worked out later by others, but the core idea of a beginning traces back to him.

Why is Hubble more famous than Lemaître for cosmic expansion?

Lemaître’s paper appeared in French in a small Belgian journal, and his 1931 English translation dropped the numerical estimate. Hubble’s 1929 paper, in a widely read American journal, presented the data directly, so his name stuck until historians and the IAU corrected the record.