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In 1929 Edwin Hubble proved that our universe is expanding by showing that the farther a galaxy is from us, the faster it is speeding away into space. This velocity-distance relation came to be called Hubble's law, and the value that describes the rate of expansion is known as the Hubble constant, or H0. Like the speed of light, H0 is a fundamental constant, and it is a key parameter needed to estimate both the age and size of the universe. Since the late 1950s astronomers have been arguing for an H0 value between 50 to 100 kilometers per second per megaparsec, a lack of precision that produced an unacceptably wide range of ages for the universe—anywhere from 10 to 20 billion years. Using the Hubble Space Telescope, Freedman and her colleagues measured H0 to an unprecedented level of accuracy, deriving a value of 72, with an uncertainty of 10 percent—a milestone achievement in cosmology. The new result suggests that our universe is about 13 billion years old, give or take a billion years, and it's a value that sits comfortably alongside the 12 billion years estimated for the age of the oldest stars.
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