For decades, scientists have disagreed about a fundamental question: how quickly is the Universe expanding' But this year, astronomer Wendy Freedman announced results that could help to put the controversy to rest. The long-standing puzzle has been that two methods to measure the cosmic expansion rate, known as the Hubble constant, give stubbornly different answers. Studies using fluctuations in the cosmic microwave background ' the afterglow of the Big Bang ' suggest that for every megaparsec (Mpc; or 3.2 million light years) farther out one looks, galaxies rush away 67 kilometres per second faster. But when scientists, including Freedman, measured the recession rate of far galaxies and estimated their distance, they got a larger Hubble constant: variously 72'74 km s'1 Mpc'1. The method for estimating the distance of galaxies is crucial. It relies on observing the brightness of supernovae (exploding stars) in those galaxies. To calibrate how a supernova's apparent brightness...
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