The universe is full of merging black holes, but in the vast majority of cases, the product is too weak for us to detect. However, two physicists think they have found a way to enhance the power of existing gravitational wave detectors to pick up faraway mergers we currently miss, unleashing a new wave of discoveries about the nature and origins of black holes.
In 2015 we detected gravitational waves from merging black holes for the first time. The collision caused an eddy in the fabric of spacetime that spread out through the universe until it gently shook the Laser Interferometer Gravitational-Wave Observatory (LIGO) on Earth, almost a billion light-years from the source.
It was one of the discoveries of the decade, until it was overtaken by last year's merging neutron stars. In the 30 months since we have detected a total of six. All of these were quite close by, astronomically speaking, and in terms of the size of the universe, it is estimated that mergers occur once every 200 seconds.
When black holes merge they produce a set of waves with distinctive frequency patterns, which, Dr Eric Thrane of Monash University told IFLScience, could be converted to sound waves to produce a characteristic whoop. However, unless the merger happens quite close by, cosmologically speaking, the waves are so subtle they get lost in the gravitational noise of the universe, like trying to pick up a signal on a radio dominated by static.
Extending the analogy, Thrane asked us to imagine listening to a radio when beyond a station's normal range. “You might not be able to work out what song was playing, or even the genre, but if you listen long enough you'll pick some things up and be able to work out the style of music even if you never hear a complete song,” he said. “If your brain can do this there must be an algorithm, so we came up with a statistically rigorous method for listening to the radio of the universe.”
After feeding the algorithm with enough test data, Thrane and colleague Dr Rory Smith were able to detect simulated mergers, and now they are keen to apply this to real observations from the network used to pick up those we have already found.
In Physical Review X, the pair predicts their new method should be a thousand times more sensitive to distant mergers than existing ones.