The apocalypse is still on, apparently — at least in a galaxy about 3.5 billion light-years from here. Last winter, a team of Caltech astronomers reported that two supermassive black holes appeared to be spiraling together toward a cataclysmic collision that could bring down the curtains in that galaxy. The evidence was a rhythmic flickering from the galaxy’s nucleus, a quasar known as PG 1302-102, which Matthew Graham and his colleagues interpreted as the fatal mating dance of a pair of black holes with a total mass of more than a billion suns. Their merger, the astronomers calculated, could release...