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

"Previous work has identified potential examples of black holes on their way to merging, but the case presented by Boroson and Lauer is special because the pairing is tighter and the evidence much stronger, " said Jon Miller, an astronomer at the University of Michigan.

BBC: Dancing black hole twins spotted