When black holes collide, the ensuing cosmic drama was assumed to play out under the cloak of darkness, given that both objects are invisible. But now astronomers believe they have made the first optical observations of such a merger, marked by a blaze of light a trillion times brighter than the sun. The flare was linked to a known black hole merger detected last year by the gravitational wave observatory, Ligo, which picked up ripples sent out through the fabric of space. The latest observations suggest that when these cataclysmic events occur within the accretion disk of an even more gigantic black hole, they are brilliantly illuminated by the surrounding dust and gas, making them also visible to optical telescopes. “This supermassive black hole was burbling along for years before this more abrupt flare,” said Matthew Graham, a research professor of astronomy at the California Institute of Technology and lead author of the work. “We conclude that the flare is likely the result of a black hole merger.” The authors have not entirely ruled out other sources, but Saavik Ford, a co-author based at the City University of New York, said the window of doubt was narrow. “We are 99.9% sure,” she said. Prof Alberto Vecchio, the director of the Institute of Gravitational Wave Astronomy at the University of Birmingham, said experts would now be watching closely to see how the latest observations align with a detailed analysis of the same event due to be published in the coming months by Ligo scientists. “If the two independent observations line up … this would really be something rather spectacular,” he said. The observations came after Ford and her colleague, Barry McKernan, made theoretical predictions that black hole mergers would be visible, contrary to expectations, if they occurred against the backdrop of the accretion disk of a third supermassive black hole. Ford and McKernan teamed up with Graham, a project scientist for the Zwicky Transient Facility (ZTF), an all-sky survey telescope designed to spot bright events. “It turns out to be perfect for something like this,” said Ford. The scientists trawled through the Zwicky data looking for any flares that coincided in place and time with known collisions that had been detected by Ligo, which releases public alerts each time a detection is made. One event stood out: a merger referred to as S190521g that Ligo detected in May last year. “It’s certainly not one of the things you would have predicted three years ago when we started the survey,” said Graham. Closer analysis suggested the merger had taken place in the vicinity of a distant supermassive black hole called J1249+3449, with a diameter equivalent to Earth’s orbit around the sun. The pair of smaller black holes sat at the outer reaches of the accretion disk, a halo of stars, dust and gas swirling around the vast central sinkhole. “These objects swarm like angry bees around the monstrous queen bee at the centre,” said Ford. As the pair of black holes, each around the size of the Isle of Wight and with a combined mass of 150 suns, spiral inwards and coalesce, gravitational waves are sent out across space and the new, merged object experiences a kick in the opposite direction, sending it ploughing through the dust and gas of the disk and out into surrounding space. “It’s the reaction of the gas to this speeding bullet that creates a bright flare, visible with telescopes,” said McKernan. If confirmed, the observations could help to resolve a central problem in black hole astronomy: that there are far more heavyweight black holes than there ought to be. Black holes form from collapsed ancient stars. Bigger black holes form when these merge, but some black holes are so big that in theory it ought to have taken longer than the age of the universe for them to snowball to their observed size. A potential explanation is that if black holes cluster together in accretion disks then multiple rounds of mergers become far more likely. “If you have a place where you can retain these black holes in one place, then you can pair them up efficiently,” said Vecchio. “The gas is the glue that keeps them all together.” Ligo observations are not easily able to resolve this question, because gravitational wave astronomy is not able to pinpoint exactly where in the sky a merger has taken place, but if the same events could be seen using conventional telescopes, an answer could be imminent. The findings are published in the journal Physical Review Letters