One of the greatest discoveries of the 20th Century was the discovery of expanding Universe. Of course, what this means is that see the light from galaxies redshifted, with more distant galaxies having a larger redshift. Importantly, this is precisely what you expect from a Universe described by general relativity, in terms of the expanding space-time metric given by the Friedmann-Robertson-Walker metric, and governed by the Friedmann equations.
Converting a galaxies redshift into a velocity (by treating it as a Doppler shift), Hubble's law is v = Ho d, where v is the velocity, d is the distance, and Ho is the (in)famous Hubble constant. It is Ho that tells us how fast the Universe is expanding at the moment.
Measuring Ho was a big preoccupation of 20th Century astronomy, with us finally finding it is around 72 km/s/Mpc. But who was the first to measure Ho, essentially by plotting the distance against redshift for galaxies and measuring the slope? Credit is typically given to Hubble. But the situation is not so clear.
As mentioned in Letters to Nature, a recent paper on astro-ph suggests that Lematire beat Hubble to the "linear velocity–distance relationship" (i.e. Hubble's law) by two years.
Today's astro-ph paper by Sidney van den Bergh muddies the water even more. Directly quoting his abstract;
The 1927 discovery of the expansion of the Universe by Lemaitre was published in French in a low-impact journal. In the 1931 high-impact English translation of this article a critical equation was changed by omitting reference to what is now known as the Hubble constant. That the section of the text of this paper dealing with the expansion of the Universe was also deleted from that English translation suggests a deliberate omission by the unknown translator.
However, a recent paper by Jean-Pierre Luminet directly names the translator as the famous astronomer Arthur Eddington, leader of the expeditions to verify Einstein's general theory of relativity by examining the deflection of starlight by the Sun. As explained by Luminet;
Next, Eddington carried out an English translation of the 1927 Lemaitre article for publication in the Monthly Notices of the Royal Astronomical Society. Here took place a curious episode: for an unexplained reason, Eddington replaced the important paragraph quoted above (where Lemaitre gave the relation of proportionality between the recession velocity and the distance) by a single sentence: "From a discussion of available data, we adopt R'/R = 0,68x10-27cm-1 (Eq. 24)". Thus, due to Eddington's (deliberate?) blunder, Lemaitre will never be recognized on the same footing as Edwin Hubble for being the discoverer of the expansion of the universe.
So, we are left with the fact that at some level, Hubble's law should probably be known as Lemaitre's law. History is never as simple as the textbooks make out! At some point I'll write something about the strange case of Ollin Eggan and the vanishing Greenwich Observatory documents.