In a few months I'll be teaching my high-school students about cosmology and, in particular, how Edwin Hubble discovered that the universe is expanding. That's how most of us learned it, anyway, but it's not the whole story. In fact, it's not even correct.
Hubble came to his game-changing revelation when he compared his distance measurements of a few dozen galaxies (or "spiral nebulae," as they were then called) with prior observations by astronomer Vesto Slipher, who'd analyzed their spectra and found they were racing away from us at various speeds. In 1929 Hubble published "A Relation between Distance and Radial Velocity Among Extra-Galactic Nebulae" — not mentioning Slipher at all, by the way — and forever transformed how we view our universe.
The problem is that Hubble wasn't the first to come to this conclusion. Two years earlier, a Belgian priest and cosmologist named Georges Lemaître had used Slipher's redshifts along with earlier, less precise distance estimates to reach the same astounding conclusion. But few took notice at the time, because Lemaître published his findings, in French, in the little-known journal Annals of the Scientific Society of Brussels. Nor was he a self-promoter.
A Jesuit priest who trained at the University of Cambridge, Harvard, and MIT, Georges Henri Joseph Édouard Lemaître was more theorist than observer. At the time he was preoccupied with showing that Einstein's concept of a static, eternal universe was untenable. A static universe was contradicted by Einstein's own general theory of relativity unless an artificial-seeming adjustment was applied (Einstein's "cosmological constant"). Lemaître used the sparse data then available to derive expansion rates of 625 and 575 km per second per megaparsec (or Mpc, about 3¼ million light-years), with the difference depending on certain assumptions. Hubble's value, put forward two years later, was 530 km/s/Mpc.
Lemaître's analysis wasn't available in English until 1931 when, at the invitation of British astronomers, it appeared in the Monthly Notices of the Royal Astronomical Society. But the English translation omitted several key paragraphs, a critical equation, and a footnote about the expansion rate he'd derived. So for all these years credit for discovering the expanding universe has gone to Hubble, and the all-important expansion coefficient is known as the Hubble Constant — even though Lemaître was the first to publish a number for it.
The saga might have ended there. However, earlier this year astronomers reopened the Case of the Missing Paragraphs. First came an assertion by amateur historians Michael Way and Harry Nussbaumer, posted online in April and later published in Physics Today, that Lemaître should get the discovery credit.
The sleuthing took a darker turn when Canadian astrophysicist Sidney van den Bergh suggested that the translator of Lemaître's manuscript had intentionally omitted the critical equation and text, so as to preserve Hubble's discovery claim. In June, South African mathematician David Block went so far as to suggest that Hubble himself might have been involved in the censorship. (Hubble was notoriously territorial, as evidenced by his snub of Slipher.)
Now we know the real story, thanks to detective work by Mario Livio (Space Telescope Science Institute). In the November 10th issue of Nature, Livio describes how he dug into the Royal Astronomical Society's archives and unearthed a previously unseen letter from the Belgian priest to William Marshall Smart, editor of the Monthly Notices at that time. Having translated the treatise to English himself, Lemaître notes, "I did not find advisable to reprint the provisional discussion of radial velocities which is clearly of no actual interest . . . ."
"Lemaître was not at all obsessed with establishing priority for his original discovery," Livio explains. "Given that Hubble’s results had been published in 1929, Lemaître saw no point in repeating his own more tentative earlier findings in 1931."
Still, I hope science historians and astronomy teachers everywhere will now give more credit where credit is due. And perhaps someday, as Block exhorts, NASA or the European Space Agency will see fit to launch a Lemaître Space Telescope to honor this visionary yet modest cosmologist — who, by the way, went on to propose what we now know as the Big Bang Theory.