Controversy persists over who first found that the Universe is expanding. Last year, Mario Livio quashed suggestions that Georges Lemaître's 1927 theoretical prediction of expansion was deliberately suppressed (Nature 479, 171–173; 2011). Since then, another contender has emerged.
The joint NASA and Infrared Processing and Analysis Center Extragalactic Database of Galaxy Distances, in Pasadena, California, which I co-lead, has tabulated and made public the historical distance estimates published by Edwin Hubble and his contemporaries to prove expansion (see I. Steer J. R. Astron. Soc. Can. 105, 18–20; 2011). These reveal that measurements by a Swedish astronomer, Knut Lundmark, were much more advanced than formerly appreciated.
Lundmark was the first person to find observational evidence for expansion, in 1924 — three years before Lemaître and five years before Hubble. Lundmark's extragalactic distance estimates were far more accurate than Hubble's, consistent with an expansion rate (Hubble constant) that was within 1% of the best measurements today.
However, Lundmark's research was not adopted because it relied on one unproven method (galaxy diameters), cross-checked with one unproven distance to the Andromeda galaxy, which was derived from a type Ia supernova observed in 1885 and mistaken for a normal nova (W. Huggins and W. F. Denning Nature 32, 465–466; 1885).
Hubble's research in 1929 yielded a value for the Hubble constant that was inaccurate by almost an order of magnitude. It was adopted because it was derived from multiple methods — including one still in use (brightest stars) — and was cross-checked with multiple galaxies with distances based on proven Cepheid star variables.
Lundmark established observational evidence that the Universe is expanding. Lemaître established theoretical evidence. Hubble established observational proof.
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Steer, I. Who discovered Universe expansion?. Nature 490, 176 (2012). https://doi.org/10.1038/490176c