Colliding Black Holes May Sing Different Gravitational Songs

What is the sound of two black holes colliding? Some of them chirp. But a truly massive, fast-spinning black hole — such as the one featured in the movie "Interstellar" — might create a more dynamic song.

Colliding black holes don't actually create sound waves, but they do create gravitational waves — distortions to space-time, the fabric of reality itself. In February, scientists with the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration announced the first-ever direct detection of gravitational waves.

To help the general public understand the signal that LIGO detected, the researchers transformed the data into sound waves. As the black holes circle each other faster and faster, the sound climbs in pitch, like a slide whistle. The final collision produces a high-pitched chirp (listen to it here), and then the sound is abruptly cut off — the song stops because the two black holes have become one. [The Search for Gravitational Waves in Pictures]

This simple cosmic song may not be the only music these gravitational-wave emitters are capable of producing. At the American Physical Society April Meeting, held April 16 to 19 in Salt Lake City, Niels Warburton, a postdoctoral fellow at the MIT Kavli Institute, discussed simulations showing what kind of gravitational-wave "song" should be produced by collisions involving black holes that spin faster and are significantly larger than those that have been detected by LIGO.  

Extreme collisions

To illustrate the new research, Warburton used the black hole Gargantua from the movie "Interstellar" as an example. In the film, a planet orbiting closer to this monster experiences extreme time dilation, so that one hour on the surface of the planet is equal to seven years on a spaceship nearby.

Astrophysicist Kip Thorne (who is also a founding member of LIGO) was deeply involved with the film and the science therein. He wrote in his book "The Science of Interstellar" that in order to cause the level of time dilation portrayed in the movie, the black hole would have to spin at nearly the fastest possible speed that scientists believe is possible for a black hole. More specifically, "1 part in 100 trillion less than maximum rate allowable," Warburton said.

(While it has not been demonstrably proven, it is thought that if a black hole were to spin faster than this maximum, its event horizon would shrink so far back as to leave a naked singularity, Warburton said — a result that has defied physical models until now.)

For their study, Warburton and his colleagues looked at very massive black holes spinning a little slower than Gargantua — only about 99.99 percent of the maximum theoretical speed. 'Interstellar' Science: The Movie's Black Hole Explained (Video )

Before black holes collide, they spiral around one another, getting closer and closer together. One black hole will circle the other until it reaches a point known as the lowest stable orbit, after which it "falls in" to its companion, Warburton explained.

But the faster a black hole spins, the closer that lowest stable orbit gets to its event horizon, or the point beyond which nothing (not even light) can escape, he said. And what their research shows is that when the companion black hole can get extremely close to its companion, the gravitational waves emitted by the pair are very different from what had been expected.

The two black holes that LIGO observed merged together and produced a "chirp" — that is, the frequency of the signal rose steadily, then was cut off abruptly when the two objects combined. But Warburton and his colleagues showed that fast-spinning black holes create a signal that reaches a peak frequency, and then starts to lower in frequency, before fading out.

"Instead of chirping, you get this kind of singing sound from the black hole," Warburton said. "It'll rise, it won't get cut off, it'll sing, and then it's quiet at the end."

"[It's] a completely different gravitational-wave signature … than what was detected [by LIGO]," he said. If a gravitational-wave detector picked up a signal that looked like the one the researchers' model describes, "you would know you were looking at a gargantuan system, something that is rotating extremely close to the maximum," he said.

This runs contrary to what scientists expected from a merger involving a very fast-spinning black hole, according to Jolyon Bloomfield, a lecturer at MIT, who presented research at the same press conference.

"It was certainly very unexpected to see something that didn't chirp," Bloomfield said, when asked during the press conference what he thought of the results. "Every template that we've seen so far … has had this beautiful, chirping feature, and we just assumed that [if we] make [the spin of the black hole] bigger … it chirps bigger. But this is quite interesting work that says no, the chirp actually goes away. Something else is happening here."

Hunting for gravitational songs

The work Warburton presented focuses mainly on a scenario involving a black hole millions of times more massive than the sun, spinning very fast, and colliding with a much smaller companion black hole — something on the order of tens of times the mass of the sun. To detect these signals would require a very large gravitational wave detector like the European Space Agency's eLISA mission, which is scheduled for launch in the 2030s. However, Warburton said that some of these strange gravitational-wave songs could also be created by two midsize black holes, and those signals could potentially be detected by LIGO.

Will gravitational-wave detectors pick up signals created by these superfast-spinning black holes? Warburton said that such a scenario depends on how common these objects are in the universe.

"There are theoretical arguments that suggest that 99.8 percent is the most maximal speed you will find," Warburton said. "But until the detection of gravitational waves recently, people thought that the biggest black holes you would see would only be 15 solar masses. And the [black holes that LIGO] saw were double that: 30 solar masses."

"So these things might not be that common in the universe," he said. "But when you're doing gravitational-wave data analysis, you need to kind of know what you're looking for in advance … And so we've shown what to look for in the data stream in order to detect these particularly exotic objects."

The new work could also help explain how very massive black holes form, Warburton said, because an object's spin can indicate how it acquired its mass. If a massive black hole formed from smaller black holes merging together, it shouldn't have an extremely high spin rate, he said.

A paper describing this research is available on the open-access website arXive.org, and the paper has been submitted for publication, according to Warburton.

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