In DepthAstrophysics

Space ripples may untangle black hole tango

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Science  02 Jun 2017:
Vol. 356, Issue 6341, pp. 895
DOI: 10.1126/science.356.6341.895

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Physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) have spotted a third merger of black holes, the ultraintense gravitational fields left behind when massive stars collapse. This time, the subtle tremor of spacetime that signaled the merger also revealed a key feature of the black holes: their spins, which were out of kilter. The observation could help reveal how the black holes paired up in the first place. A pair of black holes could be born from massive stars that collapse while orbiting each other. Or the black holes could form first and pair later—although either scenario is trickier than it sounds. If the black holes started out as paired stars, then they should spin in the same direction as their orbital axis. If the black holes formed before they paired, then they could spin in any direction. On 4 January, LIGO twin detectors in Livingston, Louisiana, and Hanford, Washington, spotted black holes of 31 and 19 solar masses spiraling together 3 billion light-years from Earth. By comparing the second-long ripple picked up by the detectors with previously calculated "waveforms," the LIGO team determined that the black hole spins were not aligned, and that there's an 80% probability that at least one of them spun in generally the opposite sense of the orbital motion. In this case, at least, the dynamical pairing scenario seems more likely.