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Simulation of light emitted by a pair of supermassive black holes spiraling inward, viewed from above the plane of the disk. Credit: NASA's Goddard Space Flight Center

When scientists first detected gravitational waves, from the violent collision of two black holes 1.3 billion years in the past, the ripples in space-time made a distinctive chirp, followed by a signal like a ringing bell. (The signals actually had to be converted into frequencies we can hear.) Since that first detection in 2015, every black hole collision has sounded pretty much the same. But according to a new study based on computer simulations, black holes actually sing a more elaborate swan song.

The team, which included Pablo Laguna, former chair of the School of Physics at Georgia Tech and now professor at The University of Texas at Austin, found that when two black holes collide and merge, the remnant black hole chirps not once, but multiple times and in a way that reveals information about its shape. Their study has been published in Communications Physics.

"While a relation between the gravitational waves and the behavior of the final black hole has been long conjectured, our study provides the first explicit example of this kind of relation," Laguna said in a press release.

The team discovered that the final black hole acts like a kind of gravitational-wave lighthouse. When the two original parent black holes are of different sizes, the final black hole initially looks like a teardrop, with a pointy tip on one side and a wider, smoother back on the other. The black hole emits more intense gravitational waves from the tip. As the remnant black hole spins, its point and backside repeatedly point towards observers watching along its equator.

So why haven't we observed these more complex signals?

"We need either more sensitivity in the detectors or we need to get lucky with a very strong signal," Laguna said, "and we also need to get a bit lucky having a more optimal orientation of the binary."

In other words, our observations so far have been from the top of the remnant black hole, like looking down on a spinning top. The effect they modeled, however, is only apparent when viewed from the side. Fortunately, the LIGO and VIRGO gravitational wave detectors are currently being upgraded. With any luck, they may be sensitive enough to detect the more intricate swan songs of colliding black holes.

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