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Science just reported two gravitationally bound massive black holes that are slowly receding from each other. Under normal circumstances two such bound black holes would be approaching each other as they gain mass by capturing matter near their event horizons. For two black holes to be slowly receding they would need to be losing mass (unless they are part of a three body system that might account for the recession). Could these two black holes be located in a region of low matter density and their recession be a signature of mass loss due to Hawking Radiation?

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    $\begingroup$ The paper is behind a paywall, but from the abstract I guess the relative motion means the apparent relative motion as viewed from Earth and not the change in the distance between the black holes. For example if the orbit is seen side on the black holes would appear to move towards then away from each other twice in every orbit. Their spacing would appear constant only if we were looking down on the plane of the orbit in an exactly perpendicular direction. $\endgroup$ – John Rennie Jun 28 '17 at 15:45
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    $\begingroup$ The temperature of supermassive black holes is far, far lower then the temperature of the CMB, so the black holes will be gaining mass from the CMB faster than they can lose it by Hawking radiation. $\endgroup$ – John Rennie Jun 28 '17 at 15:46

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