Light from distant galaxies, passing through clusters of matter, gets distorted — much like the bottom of a pool breaks up into ripples of water. Astronomers measure these barely perceptible bends in shape and create a map of gravitational ‘ripples.’ This is weak lensing — a natural detector of invisible matter. However, the accelerated expansion of the cosmos is usually attributed to dark energy, but maybe it’s not there, and gravity itself on huge scales works a little differently. By testing theories where gravity gets a tiny correction, scientists overlaid them onto the distortion map. It turned out that the allowed deviations from Einstein’s law are within a few percent. To spot the difference, they had to average the shapes of millions of galaxies. If gravity were different, the night’s constellations would ‘ripple’ differently, and it would be visible. So lensing became a judge, separating permissible corrections from fantasies.
When a distant quasar, a lensing galaxy, and a telescope line up, an Einstein ring appears, a perfectly round circle of light.