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For nearly 20 years, physicists have known that the expansion of the universe has begun to speed up. This bizarre acceleration could arise because some form of mysterious dark energy is stretching space. Or, it could signal that physicists' understanding of gravity isn't quite right. But a new study puts the screws on a broad class of alternative theories of gravity, making it that much harder to explain away dark energy.

The study is also path setting because it exploits an effect called weak lensing in which the gravity from closer galaxies distorts the images of more distant ones. "That's the future," says Bob Nichol, an observational cosmologist at the University of Portsmouth in the United Kingdom who was not involved in the study. "If you look to the next decade, there's going to be an explosion of this data."

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