The detection of the expansion of the universe is one of the 20th century’s most important scientific discoveries. It is still widely held that in 1929 Edwin Hubble discovered the expanding universe1 and that his discovery was based on his extended observations of redshifts in spiral nebulae. Both statements are incorrect. The circumstances of the discovery were well documented in two books: The Day We Found the Universe, by Marcia Bartusiak, and Discovering the Expanding Universe, by one of us (Nussbaumer) and Lydia Bieri. Both books were positively reviewed in the December 2009 issue of PHYSICS TODAY (page 51). Other writers have stated the facts correctly as well.2
Alexander Friedmann in 1922 was the first to publish nonstatic solutions3 to Albert Einstein’s field equations. However, he did not extend that work into a cosmological model built on astronomical observations. Some five years later, Georges Lemaître also discovered dynamical solutions.4 In the same publication in which he reported his discovery, he extracted (on theoretical grounds) the linear relationship between velocity v and distance r: v = Hr. Combining redshifts published by Gustaf Strömberg (who relied mostly on Vesto Slipher’s work) and Hubble’s determination of distances from magnitudes, he calculated two values for the Hubble constant H, 575 and 670 km s−1 Mpc−1, depending on how the data are grouped. Lemaître concluded from those results that the universe was expanding. Two years later Hubble found the same velocity–distance relationship on observational grounds1 from practically the same observations that Lemaître had used. However, Hubble did not credit anyone for the redshifts, most of which again came from Slipher.
Several of today’s professional astronomers and popular authors believe that the entirety of Lemaître’s 1927 French-language paper was republished in English5 in 1931 with the help of Arthur Eddington. That is also incorrect; the two pages from the 1927 paper that contain Lemaître’s estimates of the Hubble constant are not in the 1931 paper, for reasons that have never been properly explained.
Unfortunately, several prominent people writing in the popular press continue to promote Hubble’s discovery of the expansion of the universe. See, for example, Brian Greene’s Op-Ed piece in the New York Times on 15 January 2011.
There is a great irony in these falsehoods still being promoted today. Hubble himself never came out in favor of an expanding universe; on the contrary, he doubted it to the end of his days. It was Lemaître who was the first to combine theoretical and observational arguments to show that we live in an expanding universe.