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LIGO black holes
An artist’s conception shows gravitational waves emanating like ripples in space time as two black holes approach each other in their orbits. (Credit: T. Pyle / LIGO)

It looks as if gravitational-wave watchers are in for a bumpy, beautiful ride. Scientists using the Laser Interferometer Gravitational-Wave Observatory, or LIGO, have confirmed the detection of another merger involving two faraway black holes.

The observations, which were made last Christmas and reported today in a paper published by Physical Review Letters, support the idea that LIGO could open up a whole new branch of astronomy focusing on gravitational disturbances and black holes.

“It is a promising start to mapping the populations of black holes in our universe,” Gabriela Gonzalez, a Louisiana State University astrophysicist who serves as the spokesperson for the LIGO Scientific Collaboration, said in a news release.

She and her colleagues say this smash-up was smaller than the first black-hole merger, which was observed in September and reported by the LIGO team in February. That clash involved black holes that were 29 and 36 times as massive as the sun. This one brought together black holes that were eight and 14 times the sun’s mass.

During the smash-up, an amount of mass roughly equivalent to the sun’s was converted into gravitational-wave energy in accordance with Albert Einstein’s E=mc2 formula. The merger resulted in a single spinning black hole, 21 times as massive as the sun.

Gravitational-wave ripples from the crash spread out through spacetime for 1.4 billion years. Late on Christmas night, the signal reached LIGO’s two detectors, located on the Hanford Reservation in Washington state and in Livingston, La. The signal swept through the Louisiana detector first. It hit Hanford about 1.1 milliseconds later.

The detectors recorded the signal’s rising frequency pattern as a “chirp” that ended with the black hole merger.

Fred Raab, the head of LIGO Hanford, said it was fortunate that scientists extended their observations over the holidays. “The dedication of our staff in supporting this decision paid off,” Raab said in a news release.

LIGO’s analysts worked for months to verify the first detection as well as the December event. The September event turned out to be the focus of February’s announcement. Meanwhile,the team continued to analyze December’s data.

Michael Landry, detection lead scientist at LIGO Hanford, said the December signals were more suited to analysis. “Unlike our first detection, which was louder than we expected, this second, fainter event was exactly the kind of signal that our computer algorithms were supposed to dig out of the noise,” he said.

Gonzalez said the December signals reverberated for more time – for about one second – in the detector’s most sensitive band.

Localization comparison
The approximate locations of the gravitational-wave events detected in September and December are shown on this sky map of the southern hemisphere. Because there are only two detectors, the probabilities for the locations are defined by long, narrow ellipses. The later event occurred on Dec. 26 GMT, but Dec. 25 local time. (Credit: Axel Mellinger / LIGO)

LIGO’s twin detectors are placed 3,000 miles apart so that any signals can be verified and triangulated. The designs are virtually identical. Two sets of L-shaped vacuum tubes, each measuring 2.5 miles on a side, house ultra-sensitive laser interferometers that can detect distortions in spatial dimensions to an accuracy of one-ten-thousandth of the width of a proton.

The first-generation LIGO observatories began looking for evidence of gravitational waves in 2002 but turned up nothing during their initial runs. Starting in 2008, scientists gave the detectors a major upgrade to increase their sensitivity. The first gravitational-wave signals showed up during the testing phase for the “Advanced LIGO” equipment.

“With the advent of Advanced LIGO, we anticipated researchers would eventually succeed at detecting unexpected phenomena, but these two detections thus far have surpassed our expectations,” France Cordova, director of the National Science Foundation, said in a statement. Over the past quarter-century, NSF and its international partners have put about $1.1 billion into the project.

LIGO’s readings serve as the strongest confirmation to date for Einstein’s century-old general theory of relativity. Looking ahead, LIGO could provide the best way for scientists to understand how black holes behave.

“We can begin to make predictions about how often we might be hearing gravitational waves in the future,” said Caltech’s Albert Lazzarini, deputy director of the LIGO Laboratory. “LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe.”

The more events LIGO records, the more quickly scientists can pry loose the dark secrets surrounding black holes. The paper published in Physical Review Letters actually refers to a third event, which was recorded last Oct. 12. Scientists didn’t count that event as a definitive detection.

“The third event, LVT151012, is more likely a black-hole binary coalescence than noise, with Russian-roulette-type odds,” LIGO Hanford’s Raab explained in an email to GeekWire. “But just like we don’t advise playing even a single round of Russian roulette, although you would likely survive it, at this early stage of gravitational-wave astronomy, we don’t accept Russian-roulette odds for claiming detections.”

Still more gravitational-wave readings should pop up when LIGO resumes collecting data this fall – and when European scientists ramp up their Virgo interferometer in Italy. Meanwhile, a European space mission called LISA Pathfinder is blazing the trail for a space-based gravitational-wave observatory.

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