In 2011 a passionate debate flared up about who deserves the credit for the discovery that our universe is expanding. Here are some of the background facts. By February 1922, US astronomer Vesto Slipher had already measured the redshifts for 41 galaxies. British astrophysicist Arthur Eddington, who listed them in his 1923 book The Mathematical Theory of Relativity (Cambridge University Press), noted that “the great preponderance of positive (receding) velocities is very striking.” He did add, however, that the lack of observations from the Southern Hemisphere precluded any definitive conclusions.

In 1924 Swedish astronomer Knut Lundmark provided tentative, qualitative evidence for the expansion. However, his results did not carry much weight, since he relied on the implausible assumption that all galaxies have the same diameter and his correlation between velocity and distance was not readily apparent.

A stronger case for an expanding universe came from Belgian priest and cosmologist Georges...

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