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The problem with string theory, according to some physicists, is that it makes too many universes. It predicts not one but some 10500 versions of spacetime, each with their own laws of physics. But with so many universes on the table, how can the theory explain why ours has the features it does? Now some theorists suggest most—if not all—of those universes are actually forbidden, at least if we want them to have stable dark energy, the supposed force accelerating the expansion of the cosmos. To some, eliminating so many possible universes is not a drawback but a major step...

Do We Live In A "Stop And Go" Universe? Anyone who drives is familiar with the frustration of being caught in "stop and go" traffic, a phenomenon found in urban areas all over the world. Astronomers have found that stop-and-go traffic is even more widespread than that, affecting galaxies throughout the universe. Today at the 202nd meeting of the American Astronomical Society, Robert Kirshner (Harvard-Smithsonian Center for Astrophysics), on behalf of the international High-z Supernova Search Team led by Brian Schmidt (Mount Stromlo Observatory), presented evidence that the expanding universe slowed for billions of years before galaxies began accelerating,...