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First black-hole signals
James Hough from the University of Glasgow in the UK agrees with Stuver, pointing out that LIGO’s discovery is also the only direct evidence we have for the existence of any black holes. Astronomers had previously obtained only indirect evidence in the form of X-rays from matter falling into other black holes and the distortion of the orbits of stars at galactic centres that host supermassive black holes.
Hough says the team can be sure that the waves in the GW150914 event came from two merging stellar-mass black holes because the waves are directly related to the size of the system. The radius of the objects is such that they must be black holes for the mass that they have, he explains. “I think that there is no doubt about that at all.”
He adds that the signal from GW150914 was so perfect and clear that it almost needed no sophisticated data analysis to tease it out of LIGO’s data. The signal lasted in the detector for nearly 0.2 s, sweeping from about 30 Hz to 150 Hz, almost exactly as was expected for such a wave.
Matching the templates
According to Stuver, LIGO has a large “template bank” containing detailed simulations and predictions for every possible type of merger – be it binary black holes or neutron stars – with many different permutations and combinations of possible masses. Each template produces a unique gravitational-wave signal and the researchers’ computer system actively looks for a high correlation between it and an incoming signal. If the two are close, a detection is flagged. For GW150914, this correlation was extremely clear and immediately noticeable.