Caption

This illustration, using Hubble Space Telescope images of Supernova Refsdal, shows how the gravity of massive galaxy cluster MACS J1149.6+2223 bends and focuses the light from a supernova behind it, resulting in multiple images of the exploding star. This phenomenon is called gravitational lensing. Gravitationally lensed supernovae offer astronomers a unique way to calculate the Hubble constant—the rate at which the universe is accelerating. One research team is now readying astronomers to find, and study, these rare objects using NASA’s upcoming Nancy Grace Roman Space Telescope, set to launch by May 2027.

The upper graphic shows that when the star explodes, its light travels through space and encounters the foreground galaxy cluster. If the cluster were not present, astronomers would detect only the supernova light that is directed straight at Earth and would see only a single image of the supernova. In the case of the multiply imaged supernova, however, the light paths are bent by the cluster’s gravity and redirected onto new paths, several of which are pointed at Earth. Astronomers, therefore, see multiple images of the exploding star, each one corresponding to one of those altered light paths. Each image takes a different route through the cluster and arrives at a different time, due, in part, to differences in the length of the pathways the light follows to reach Earth. Precisely measuring this difference in arrival times between the multiple images leads to a combination of distances that constrain the Hubble constant.

In the lower graphic, the redirected light passes through a giant elliptical galaxy within the cluster. This galaxy adds another layer of lensing, once again redirecting several light paths that would otherwise have missed us, and focusing them so that they do reach Earth.