Astronomers have spotted a pair of supermassive black holes on a collision course in a galaxy 2.5 billion light-years away. Coincidentally, the pair will begin producing gravitational waves in roughly 2.5 billion years, the researchers estimate. The duo can be used to estimate how many supermassive black hole pairs are detectable in the nearby, present-day universe and when the historic first detection of the background 'hum' of their gravitational waves will be made.