In a brilliant piece of detective work, Mario Livio clarifies the mysterious differences between Georges Lemaître's original 1927 paper, which announced the expansion of the Universe, and its 1931 translation from French into English (Nature 479, 171–173; 2011). He suggests that “Lemaître was not at all obsessed with establishing priority for his original discovery”. However, Lemaître's important later paper, 'L'expansion de l'Univers' (Ann. d'Ap. 13, 344–345; 1950), argues otherwise.

In that paper, Lemaître writes (translated from the French):“The title of my note leaves no doubt about my intentions: a Universe of constant mass and growing radius accounts for the radial velocities of the extra-galactic nebulae.”