The night sky looks smooth, but it's actually clumpy — vast communities of galaxies that bend light from distant objects like lenses. When a short, powerful radio burst goes off somewhere in the universe, its path to an Earth telescope can pass near such a clump.
Over several years, the Canadian radio telescope CHIME caught hundreds of fast radio bursts — flashes lasting milliseconds. The scientists took the catalog of these bursts and overlaid it on a map of galaxy clusters built from optical images. It turned out that 26 bursts came from regions where a cluster lies along the line of sight. One of them repeated, and two crossed the famous Coma cluster.
Comparison with computer models showed that this number of coincidences is not random: bursts really do occur more often near clusters. About half of the effect is explained by clusters having more burst sources in their galaxies, and half by clusters brightening distant bursts that we otherwise wouldn't notice.
This result provides a new tool: by studying how often bursts coincide with clusters of different masses and at different distances, we can test hypotheses about what objects produce them. One burst, FRB 20211113A, passed near a very strong gravitational lens — the cluster Abell 2218. This may be the first candidate for a lensed fast radio burst.
The Coma cluster contains thousands of galaxies, and its mass is so great that light from more distant objects passing nearby is noticeably bent, as if we were looking through wavy glass.
In Carl Sagan's novel "Contact," an alien signal comes from the direction of a galaxy cluster, where gravitational lensing helps detect a faint source.