the Hubble constant was around 50 to 55. Despite its name, the Hubble constant wasn’t a constant—a value unchanging over time. It told you only how fast the universe was expanding now—its current rate of expansion—and for this reason astronomers sometimes referred to it as the Hubble parameter. It told you nothing, however, about how much the expansion rate was changing over time. That value—Sandage’s second number—astronomers called the deceleration parameter because it would tell you to what extent the universe was slowing down. From the Hubble parameter you could extrapolate backward into
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