Again! LIGO Detects second merger of a binary black hole

Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence

Jun 16
Bram Slagmolen

Scientists have detected gravitational waves for a second time, caused by the collision of two black holes 14 and eight times the size of the sun.

The team, including scientists from Australian Consortium for Interferometric Gravitational Wave Astronomy (ACIGA), glimpsed the black holes orbiting each other 27 times in their last second before coalescing. The signal was 10 times longer than that of the first gravitational wave, which was announced in February this year.

The signal was detected by the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States, said Professor Susan Scott, from the Australian National University.

“This has cemented the age of gravitational wave astronomy,” Scott said.

“This shows data is going to flow, that will enable us to map a lot more of the Universe than we’ve seen before.”

The violent collision happened approximately 1.4 billion years ago in a distant galaxy. During the journey to Earth, the gravitational waves died down so much that they stretched the LIGO detectors only a tiny fraction of the width of a proton.

"Each new detection of colliding black holes reveals something new about these mysterious and awe-inspiring objects," said Professor Andrew Melatos from the University of Melbourne.

"For example, by accumulating statistical data on black hole masses, and combining these data with computer modelling, we learn about the birth histories of black holes. The more events we capture, the more we learn!"

Gravitational waves are caused by violent cosmic events such as collisions between stars or black holes, or explosions such as supernovae. They were predicted by Albert Einstein in 1916, but he thought they would be too small for humans to ever detect.

“The fact that we have so quickly detected gravitational waves from a second pair of colliding black holes is very exciting as it suggests these events are more numerous than many researchers previously believed. We also anticipate that other sources of gravitational waves may be detected in the future as the detectors are made even more sensitive, ” said Dr Eric Thrane from Monash University.

Until gravitational waves were detected, nearly all astronomy had relied on electromagnetic observations – visible light, radio waves, X-rays and so on – said Dr Robert Ward, a LIGO researcher from the ANU.

“I'd always imagined there would be electromagnetic counterparts in our first discoveries, but instead we found these invisible collisions of black holes purely through the gravitational waves they emitted with no counterparts at all,” Dr Ward said.

The chair of the Australian Consortium for Interferometric Gravitational Astronomy, Dr Bram Slagmolen, said he was proud of the contribution Australian scientists had made to the detection.

“There’s massive enthusiasm among Australian scientists, we’ve come up with lots of innovative technology and ideas,” said Dr Slagmolen.

“Advanced LIGO is such a massive machine and it’s fantastic to see it operate in the way we intended.”

Professor David McClelland, from ANU and Leader of Australia’s Partnership in Advanced LIGO, said that Australian scientists were already working on projects which would enhance the sensitivity of the LIGO detectors.

“Our world-leading quantum optical devices will triple the searchable volume of our universe, we’ll see many more discoveries announced over the next few years.”

The discovery has been published in Physical Review Letters, PRL 116, 241103 (2016).

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