Todd Boroson and Tod Lauer of the US National Optical Astronomy Observatory analysed some 17, 500 spectra from the Sloan Digital Sky Survey, and have now found just such a pair of emissions coming from a distant quasar.

BBC: Dancing black hole twins spotted

On 17 March 2005, Andrzej Udalski, professor of astronomy at Warsaw University, Poland, and leader of the Optical Gravitational Lensing Experiment (Ogle), noticed that just such an event was about to occur near the centre of our galaxy.

BBC: Artist's concept of a distant world (BBC)