Ars Technica: Pairs of supermassive black holes are expected to be relatively common because every galaxy has a central black hole and galaxies often merge. One such binary may have been spotted in 2014. Data collected over the last 20 years by NASA's Galaxy Evolution Explorer (GALEX) and the Hubble Space Telescope have confirmed the identification. The black holes have an orbital period of about five years. According to calculations accounting for their mass, the black holes are each moving at about 7% of the speed of light and are separated by about 0.007–0.017 pc, roughly the diameter of the solar system. The speed was confirmed thanks to a relativistic effect known as beaming, which was detectable in the GALEX and Hubble images. Similar to the Doppler effect, beaming refers to the change in brightness of a light-emitting object as it moves toward or away from an observer at relativistic speeds.
