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."

Physicists had expected the universe's expansion to be slowing as the galaxies pull on one another with their gravity. But in 1998, two independent teams traced the history of the universe's expansion by studying type 1a supernovae: stellar explosions whose colors tell when they went off and whose brightness reveals how far away they are now. Both teams found that the expansion is speeding up, suggesting that dark energy is blowing up the universe like a balloon.

However, it's possible that dark energy doesn't exist and that the acceleration comes about instead because physicists' understanding of gravity—Albert Einstein's general theory of relativity—isn't quite right. Einstein deduced that gravity arises because mass and energy warp spacetime. In general relativity, given the distribution of mass and energy, spacetime bends to minimize its curvature, denoted R. But in so-called f(R) (pronounced "eff-of-are") theories, spacetime contorts to minimize the curvature plus some extra function of the curvature. That change produces an extra gravitylike force that can either attract or repel under different conditions.

In 2007, theorists Wayne Hu of the University of Chicago in Illinois and Ignacy Sawicki, now at the University of Geneva in Switzerland, showed that, with the right choice of the function f(R), such a theory might explain the accelerating expansion without dark energy. To do that, the extra force has to disappear where gravity is relatively strong, such as within a galaxy or the early universe, and kick in on the largest scales and at later times.

To test such theories, scientists must study the universe on huge scales. Last year, Nichol and colleagues tested f(R) theory by tallying galaxy clusters spanning millions of light-years. If dark energy is stretching space, then it should slow the formation of massive clusters and produce fewer of them than f(R) gravity would. Nichol and colleagues found numbers consistent with dark energy. The analysis is tricky, however. Researchers need to estimate the mass of each cluster, which comes mostly from mysterious, invisible dark matter. So Nichol and colleagues inferred a cluster's mass from x-rays coming from hot gas within it, relying on theoretical modeling of the interplay of ordinary and dark matter.

Now, a team of scientists led by Zuhui Fan, an astronomer at Peking University in Beijing, has taken an approach that measures a cluster's mass directly. Gravity from a massive object can distort the images of things beyond it. A galaxy cluster thus distorts the images of more distant galaxies, so that instead of being oriented randomly in the sky, their elongated shapes align slightly, like fish in a school. The strength of that "weak lensing" directly reveals the mass of the foreground cluster. "You don't rely on the scaling between the cluster's mass and its [ordinary matter] content," says Baojiu Li, a cosmologist at Durham University in the United Kingdom, who worked on the study.

The researchers used data from the 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea in Hawaii, which imaged 5.5 million galaxies to create a weak lensing map covering 154 square degrees of sky. From the "peaks" in the map, they tallied clusters weighing hundreds of times much as our Milky Way galaxy, they report in a paper in press at Physical Review Letters . Those tallies agree with the predictions of dark energy and weaken the case for f(R) theories.

"At the moment, this is the best measurement on the cosmological scale," Nichol says. The new result doesn't quite kill f(R) theory, but if the limit on a key parameter can be lowered by another factor of 10, Nichol says, "I suspect that people will say, 'This theory is not it.'"

However, Hu questions how far the method can be pushed. Testing f(R) gravity further may require accounting for the detailed distribution of dark matter within individual clusters, he says. But that distribution will be modified by the interplay between dark and ordinary matter, Hu says, bringing the issue back into play.

Still, experts say, the new work shows the potential to probe the cosmos with weak lensing. The Large Synoptic Survey Telescope, under construction in Cerro Pachón, Chile, will map weak lensing over 20,000 square degrees—roughly half the sky. The European Space Agency's proposed Euclid spacecraft and NASA's proposed Wide Field Infrared Survey Telescope satellite will employ the technique. "In terms of data quality," Li says, "there's going to be a big improvement from what we have now."