Some astronomers doubt that the technique can outperform supernova surveys and measurements of baryonic acoustic oscillations — ripples in cosmic structure dating from several hundred thousand years after the Big Bang. “I think the jury’s still out,” says David Schlegel, principal investigator of the Baryon Oscillation Spectroscopic Survey at the Lawrence Berkeley National Laboratory in Berkeley, California. He points out, for instance, that a key assumption in weak lensing — that galaxies are oriented randomly — is turning out not to be true, forcing astronomers to correct for a systematic effect.
But others say that it is important to take diverse approaches to the dark-energy mystery. Chris Stubbs, an astrophysicist at Harvard University in Cambridge, Massachusetts, says that, a decade ago, astronomers were intent on simply proving that the effect was real. Now there is little doubt, and they need techniques such as weak lensing to explore more-detailed questions — for instance, whether the effect varies with distance, direction or the density of matter, and whether the effects of dark energy can be distinguished from those of variable gravity. “We are only now entering an era where we’re attacking the dark-energy problem with instruments and experiments that were explicitly optimized for that purpose,” says Stubbs.