Both the retrospective and the prospective interpretations of the Hubble Constant have stoked ongoing controversy in the 90 years since Edwin Powell Hubble published the first definitive evidence of an expanding universe in 1929. Recently, the controversy has taken on yet another guise, as increasingly precise techniques for measuring the expansion rate have begun to yield distinctly different predictions. The discrepancy has cosmologists wondering whether they are missing important elements in their models of how the Universe evolved from the Big Bang to today. – Aeon