• Simulation showing two black holes before merging. Illustration: T. Dietrich/R. Haas/Max Planck Institute. (Max Planck Institute)Source: Max Planck Institute
A second discovery means the era of gravitational wave astronomy is well and truly here.
Lisa Grossman

New Scientist
16 Jun 2016 - 9:06 AM  UPDATED 16 Jun 2016 - 9:10 AM

They've done it again. On 26 December, 2015, for the second time, the Laser Interferometer Gravitational Wave Observatory (LIGO) caught the ripples in space-time shaken off by the death spiral of a pair of black holes. In other words, we are officially in the era of gravitational wave astronomy.

“This gives us confidence,” says Salvatore Vitale at the Massachusetts Institute of Technology, one of the LIGO team. “It was not just a lucky accident. Seeing a second one tells us clearly that there is a population of black holes there, and we will see a lot of them in the coming science run.”

The discovery was announced at the American Astronomical Society meeting in San Diego on 15 June, and will be published in the journal Physical Review Letters

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The LIGO team made history in February when they announced the first detection of gravitational waves. Albert Einstein predicted they would be produced when massive objects like black holes move around. Dubbed GW150914, the signal arrived at twin detectors in Hanford, Washington, and Livingston, Louisiana, on 14 September last year.

The detectors picked up the minuscule stretching of space-time spurred by the collision of a pair of black holes about 30 and 35 times the mass of the sun, 1.3 billion light years away.

The second signal, called GW151226, also came from a pair of black holes merging. But these were much lighter – about 14.2 and 7.5 times the mass of the sun. They merged to form a black hole of 20.8 solar masses, meaning about 1 solar mass of energy radiated away in gravitational waves during the collision.

“This event radiated the equivalent of the mass of our sun in a couple of seconds,” Vitale says. “Our own sun radiated about a millionth of its mass in 5 billion years. This really gives you the scale of how violent and sudden this release of energy is, as compared to our everyday experience.”

If you could see it, the collision would be 10,000 times brighter than a gamma-ray burst, the brightest explosions we know of in the universe, says Avi Loeb at Harvard University

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Into the mainstream

Those smaller masses are reassuring, because they fit squarely in the 5 to 20 solar mass range of black holes, the kind already observed with X-ray telescopes. Those black holes usually pair up with ordinary stars, and we can see the disc of hot gas that accumulates around the black hole as it steals material from its companion.

“The fact that they detected lower mass black holes brings it closer to mainstream astronomy, where such black holes are often seen,” Loeb says. “It establishes gravitational wave astronomy as a field.”

“When we made the first discovery, it was kind of surprising because they have zero overlap with the known distribution of black holes,” Vitale says. “Now we are back on two black holes with masses that are totally compatible with what we expect. It’s nice to see that we can target a similar population.”

It also meant that LIGO watched more of their deadly waltz. The black holes in the first event were so massive that they swung around each other less than 10 times before merging. In the second collision, the team watched 55 full orbits before the end.

But the signal from the smaller black holes was also more difficult to detect. The first one was so powerful that you could see it in the data with the naked eye. You could even hear it: translating the signal into sound waves gave a “chirp“, a rise in pitch and volume as the black holes circle each other faster and faster.

“If there is any small deviation from general relativity, it will accumulate”

This new one required more targeted algorithms and sophisticated processing, to tease the signal out of the noise.

“The first event was so loud and so screaming in the data, it was found by algorithms that just looked for anything really, not particularly binary black holes,” Vitale says. “For this one it was important to know we were looking for compact binaries.”

“If you tried to make a chirp out of the data all you would hear is ‘ksshhhhhhh’,” says LIGO team member Nergis Mavalvala, also at MIT.

Regardless, seeing more cycles makes this system a better laboratory to test Einstein’s theory of general relativity.

“If there is any small deviation from general relativity, it will accumulate,” Vitale says. “If you have more cycles you have a better hope to see if there is something wrong.” So far, the event matches general relativity perfectly.

The team also measured a new attribute of the black holes: one of the behemoths was spinning slowly. 

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Measuring spin is a way to probe how the black holes formed. If they came from a pair of stars that both exploded and became black holes together, they ought to spin in the same direction. If they were already black holes when they found each other in a dense environment like a globular cluster, they should not.

More detections will help gauge the size of the universe, probe the nature of matter and test general relativity to ever higher precision.

“These are the kinds of things we want to do, and we can hardly do them in any other way,” Vitale says.

But we will need a lot more signals to answer such questions.

“The significance of this paper is it establishes a population, rather than a single example,” says Loeb. “There is a big qualitative difference between having one data point and having two. I look forward to improving the statistics.” 

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