LONDON: In a first, scientists have found three closely orbiting super-massive black holes in a galaxy more than four billion light years away, a discovery that may aid the search for gravitational waves predicted by Einstein An international team, including Oxford University scientists, examined six systems thought to contain two super-massive black holes.The team found that one of these contained three super-massive black holes - the tightest trio of black holes detected at such a large distance - with two of them orbiting each other rather like binary stars.The finding suggests that these closely-packed super-massive black holes are far more common than previously thought."What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein's Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth," said Dr Roger Deane from the University of Cape Town , who led the study."Not only that, but using the combined signals from radio telescopes on four continents we are able to observe this exotic system one third of the way across the Universe," said Deane."General Relativity predicts that merging black holes are sources of gravitational waves and in this work we have managed to spot three black holes packed about as tightly together as they could be before spiralling into each other and merging," said Professor Matt Jarvis of Oxford's Department of Physics, an author of the paper."The idea that we might be able to find more of these potential sources of gravitational waves is very encouraging as knowing where such signals should originate will help us try to detect these 'ripples' in space-time as they warp the Universe," said Jarvis.The team used a technique called Very Long Baseline Interferometry (VLBI) to discover the inner two black holes of the triple system.The discovery was made with the European VLBI Network, an array of European, Chinese, Russian and South African antennas, as well as the 305 metre Arecibo Observatory in Puerto Rico At this point, very little is actually known about black hole systems that are so close to one another that they emit detectable gravitational waves.The research was published in the journal Nature.