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LIGO black hole merger
An artist’s conception shows two merging black holes similar to those detected by LIGO. (LIGO / Caltech / MIT / Sonoma State Illustration / Aurore Simonnet)

The Laser Interferometer Gravitational-wave Observatory has detected its third confirmed black hole merger, and this one’s a doozy: LIGO’s latest discovery is about 3 billion light-years away, which is more than twice as far away as the first two finds.

The gravitational wave signature of the newly reported smash-up, known as GW170104, also confirms that there’s a heavyweight class for stellar-mass black holes.

“It clearly establishes a new population of black holes that were not known before LIGO,” said Bangalore Sathyaprakash, a physicist at Penn State and Cardiff University.

What’s more, the way the black holes spun around each other before they merged suggests that they may not have been born together, but wandered into each other’s way. That’s a first for gravitational wave observations.

Maybe that’s not as unusual as it sounds. After all, the era of gravitational wave observations is less than two years old, dating back to LIGO’s first-ever direct detection in September 2015. That history-making discovery was followed by the second confirmed detection three months later.

The third detection was made on Jan. 4 of this year, and is being reported today in a paper accepted for publication in Physical Review Letters.

“The key thing to take away from this third high-confidence event is that we’re really moving from novelty to new observational science,” said MIT’s David Shoemaker, the main spokesperson for the LIGO Scientific Collaboration.

Gravitational waves are fleeting distortions in the fabric of spacetime, caused by dramatic events such as supernovae or the collisions of black holes and neutron stars. Physicist Albert Einstein said that they should be given off as a consequence of his general theory of relativity, but it took decades to nail down the evidence conclusively with LIGO.

LIGO’s team uses two detectors, built near Livingston in Louisiana and on the Hanford site in Washington state. Each detector has two L-shaped arms, extending out 2.5 miles in each direction. Laser light is reflected by mirrors inside the arms, in an arrangement that can detect distortions in spatial dimensions to an accuracy of less than a thousandth of the width of a proton.

A whopper of a black hole

Scientists compare readings from the two detectors to make sure that what they’re seeing are true gravitational waves rather than one-off anomalies. Close analysis of the distortions over the course of a fraction of a second can tell them how big the black holes are, as well as their alignment and spin.

The first two confirmed detections resulted in black holes that weigh 62 times as much as the sun, and 21 times as much. LIGO’s team says the third case involved black holes that weighed 31.2 and 19.4 solar masses. They smashed together to create a bigger black hole, about 48.7 times as massive as the sun.

The collision was so violent that some of the black holes’ mass, equivalent to twice the weight of the sun, was converted directly into energy in accordance with Einstein’s E=mc2 formula. That’s what fueled the gravitational wave outburst.

Before LIGO, physicists weren’t sure whether stars could collapse into black holes weighing more than 20 times the sun’s mass. But thanks to LIGO’s observations, they now know that heavy stellar-mass black holes can exist – probably in cases where the stars have low metallicity.

Black hole masses
LIGO has discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (shown in purple). The three confirmed detections by LIGO (GW150914, GW151226, GW170104), and one lower-confidence detection (LVT151012), point to stellar-mass binary black holes that, once merged, are larger than 20 solar masses. (LIGO / Caltech / Sonoma State / Aurore Simonnet)

Out of alignment?

The newly reported detection also sheds light on a question about binary black holes that’s been puzzling physicists. Do they always have to form in pairs, or can they form separately and link up later in life?

The readings for GW170104 indicate that at least one of the black holes might have been spinning in a direction that was different from the two objects’ orbital motion around each other. That would imply that the black holes were not born together as stars.

“This is the first time that we have evidence that the black holes may not be aligned, giving us just a tiny hint that binary black holes may form in dense stellar clusters,” Sathyaprakash said.

Georgia Tech’s Laura Cadonati, deputy spokesperson for the LIGO Scientific Collaboration, told GeekWire that the finding represents “a new tile to put in the puzzle” of black hole formation.

Einstein was right … again

The first two black hole mergers that LIGO detected took place no more than 1.4 billion light-years away. The fact that this third merger happened at a distance of about 3 billion light-years gave scientists a chance to test yet another of Einstein’s hypotheses.

Some theories of gravity suggest that gravitational waves should disperse as they travel through spacetime, just as light waves from the sun spread out into the colors of the rainbow when they pass through a prism or a curtain of raindrops.

Einstein’s version of relativity theory, however, rules out that kind of dispersion. Sure enough, when LIGO’s scientists looked closely at the data, they saw no sign that the waves were spreading.

“It looks like Einstein was right — even for this new event, which is about two times farther away than our first detection,” Cadonati said in a news release. “We can see no deviation from the predictions of general relativity, and this greater distance helps us to make that statement with more confidence.”

Black hole location
This 3-D projection of the Milky Way galaxy onto a transparent globe shows the probable locations of three black-hole merger events, plus a fourth possible detection at lower significance (LVT151012, green). The outer contour for each represents the 90 percent confidence region. The innermost contour signifies 10 percent confidence. (LIGO / Caltech / MIT / Leo Singer / Milky Way image by Axel Mellinger)

The next wave is on the way

GW170104 was detected during the second run for the Advanced LIGO system, which is still in progress. This run started last November and is expected to last until the end of August. Then the detectors will undergo yet another round of upgrades.

“The upgrades will improve our reach into space, and allow for our third observation run ultimately to commence,” said Caltech’s Mike Landry, who directs LIGO’s Hanford Observatory. “We’re looking forward to making additional binary black hole discoveries in that run, but also the possibility of detections that include matter.”

For example, LIGO has a good chance of spotting its first neutron star merger during the third run, Landry said.

Sometime this year, the European VIRGO gravitational-wave detector is expected to join the hunt in Italy. Detectors are being planned in India and Japan as well. Those facilities will not only boost the capability to detect gravitational waves, but also help scientists pinpoint the source of outbursts more precisely.

Eventually, physicists expect to pick up gravitational-wave signals once a week on average, or even once a day.

“This is why NSF started providing support for LIGO more than 40 years ago,” France Cordova, director of the National Science Foundation, said in a statement. “We know this is just the beginning. This ‘window on the universe’ will continue to expand, and NSF looks forward to being a part of future upgrades that promise to increase the frequency of detections to even a daily basis.”

More than 1,000 researchers contributed to the paper to be published in Physical Review Letters, titled “GW170104: Observation of a 50-Solar-Mass Binary Black Hole Coalescence at Redshift 0.2” (B.P. Abbott et al.).

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