Supermassive black holes rarely travel alone. Most large galaxies hide one at the center, and when galaxies collide, the two central black holes can end up bound together. Astronomers have seen plenty of wide pairs. The tighter ones, the kind that spiral inward and eventually merge, have been much harder to pin down. Researchers at the University of Oxford and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) think the missing systems may be giving themselves away anyway, in brief, repeating flashes of starlight. In a paper published today in Physical Review Letters, they argue that a tight supermassive black hole binary could act like a moving magnifying glass, repeatedly boosting the light from individual stars in the same galaxy. “Supermassive black holes act as natural telescopes,” said Dr Miguel Zumalacárregui from the Max Planck Institute for Gravitational Physics. “Because of their enormous mass and compact size, they strongly bend passing light. Starlight from the same host galaxy can be focused into extraordinarily bright images, a phenomenon known as gravitational lensing.” Artistic impression …