Strong gravitational lenses are hard to find. Since the late 1970s, when the first one was observed, astronomers have discovered only a few hundred. But that is about to change. In the next decade, a new generation of astronomical sky surveys will probe the cosmos with unprecedented sensitivity. Scientists predict that the data from those surveys will contain more than 100 000 lenses. The first data release, coming from the space telescope Euclid, launched in July 2023, is slated to occur this month.
Gravitational lensing is a consequence of general relativity: Massive objects curve the space around them and bend the trajectories of passing photons. Sometimes the effect is minor; in what is termed weak gravitational lensing, the paths of photons are only slightly warped. Strong lensing occurs when the light from a background object is so severely deflected by a massive foreground object that astronomers observe it as...
