Light from a distant quasar, the bright core of a galaxy where gas swirls around a supermassive black hole, passes by another galaxy. The gravity of its stars bends the light's path like imperfect glass. As a result, the quasar appears multiple times in images, and its brightness flickers slightly. From these flickers, astronomers can infer the size of the glowing disk, even though the disk itself is invisible even in powerful telescopes.

The researchers took images of seven such quasars from the Hubble Space Telescope and, for the first time, measured the disk out to a distance of 14 light-days from the black hole. For comparison, they used spectral lines that originate far from the disk and are unaffected by the gravitational lens. It turned out that the disk is three times larger than the standard theory predicts. Moreover, the temperature at its outer edges drops more slowly than expected.

This means our understanding of how matter falls onto a black hole is incomplete. Perhaps the disk is thicker, or magnetic fields are stronger within it. Astronomers use disk size to estimate black hole masses, so these new data could change our picture of the most massive objects in the universe.