Hubble knows

By Charles Seife in Washington DC HOW fast is the Universe expanding? NASA announced last week that scientists have finally reached a consensus, drawing the main work of the Hubble Space Telescope (HST) to a close. But the consensus is less than unanimous. “We all agree,” says Jeremy Mould, an astronomer at the Australian National University in Canberra and co-leader of the NASA Hubble team. “We just don’t agree that we agree.” Not everyone takes that line, however. “That’s a bunch of hooey,” says Allan Sandage, an astronomer at the Carnegie Observatories in Pasadena. “The impression NASA tried to give to the media is that it’s all solved, but that is not true.” In an expanding Universe, the farther away a particular galaxy is, the faster it is receding. The primary goal of the HST has been to measure the figure that describes this—known as the “Hubble constant”—in part by examining a class of pulsating stars called Cepheids. The NASA Hubble team now concludes that galaxies are speeding away at 70 kilometres per second per 3.2 million light years, plus or minus 10 per cent. “We used to disagree by a factor of two. Now we are just as passionate about 10 per cent,” says Robert Kirschner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. “A factor of two is like being unsure if you have one foot or two. Ten per cent is like arguing about one toe.” But Sandage is less comfortable with the discrepancy. His group uses the HST to measure the Hubble constant by looking at supernovae. Their data, from four separate experiments, suggest the expansion is slower, with values of between 55 and 60. “Our best value from the HST is 58 plus or minus 2,” he says. Other astronomers think that despite the fuss, astronomers are still moving towards consensus. “The high values are slowly coming down,
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