Artist’s impression of a pair of supermassive black holes orbiting each other
A stronomers have long assumed that two black holes that circle close to each other are always destined to become one in a cataclysmic merger that spans aeons. But that needn’t always be the case, recent research finds. Physicists found that it is theoretically possible for two black holes to remain at a fixed distance from each other thanks to their mutual gravitational pull being perfectly counterbalanced by the speed at which the universe is expanding. “Viewed from a distance, a pair of black holes whose attraction is offset by cosmic expansion would look like a single black hole,” said Óscar Dias, a physicist at the University of Southampton. “It might be hard to detect whether it is a single black hole or a pair of them.”
They demonstrate that two black holes could be delicately balanced, despite conventional theories predicting otherwise, by pointing out “a logical inconsistency in the proof of one theorem and a limiting assumption in another,” says Toby Wiseman, a professor of theoretical physics at Imperial College London. One of the key assumptions in those theorems is that the region around black hole pairs is empty. However, according to the standard model of cosmology, our current best description of the universe, dark energy causes the universe to expand at an accelerated rate. This dark energy is sometimes considered equivalent to the puzzling cosmological constant in the theory of general relativity.