LIGO detects third black-hole collision — and it’s not quite like the others

Scientists with the Laser Interferometer Gravitational-wave Observatory, or LIGO, have detected the signal from a cataclysmic collision between two black holes that lie 3 billion light-years away – much farther than the previous two discoveries.

The findings, described in a paper accepted to Physical Review Letters, cement the idea that gravitational-wave astronomy – a whole new way to observe some of the most powerful events in the universe – is here to stay.

“We’re really moving from novelty to new observational science — a new astronomy of gravitational waves,” said MIT’s David Shoemaker, spokesman for the LIGO scientific collaboration.

Astronomers document the universe in different wavelengths of light, from visible and infrared all the way to X-rays and gamma rays. But black holes do not emit light as far as we know — making them very difficult to study. By picking up deformations in space-time, LIGO allows scientists to “hear” these mysterious phenomena, even if they can’t see them with telescopes.

“We’re entering a whole new kind of astronomy,” said Clifford Johnson, a theoretical physicist at USC who was not involved in the work. “Every time we find a new way of looking in the sky … we understand our universe in a whole new way, at a whole new level.”

The new signal, called GW170104, was picked up in the early morning hours of Jan. 4 by the twin L-shaped detectors in Hanford, Wash., and Livingston, La. The ripple was triggered as two black holes, spinning around slowly toward one another, finally succumbed to each other’s gravitational tug — and merged. The collision resulted in the creation of a new, single black hole.

Gravitational waves are ripples in the fabric of space-time, caused by objects accelerating or decelerating through space. Their existence was predicted over a century ago by Albert Einstein as part of his general theory of relativity, but they were thought to be so faint as to be virtually undetectable.

LIGO changed that. Last year the collaboration announced that its twin detectors had picked up a passing distortion in late 2015 caused by two black holes crashing into one another. A second soon followed. With the third confirmed find announced Thursday, scientists are finally moving LIGO’s work from the examination of singular curiosities to demographic studies of the sky’s invisible denizens. And already, this third discovery is revealing that there may be some diversity in this mysterious cosmic population.

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In the meantime, LIGO is set to wrap up its current observing run in late summer – right around the time that the European Virgo detector is set to go online. With a little bit of overlap between the two runs, and a little bit of luck, the two detectors just might be able to see the same events – which would allow scientists to get even better measurements of these violent cosmic phenomena.

Eventually, scientists might expect to catch a gravitational event once or twice a week, or perhaps even on a daily basis. But for now, the discovery of these gravitational wave events continue to thrill scientists, Kamionkowski said.

“Five or 10 years from now, we’re gonna have another event discovered, and then I’ll be like, ‘Oh yeah, another gravitational wave event,’” Kamionkowski said. “But I’m still amazed every time they discover every one of these things. The glow from last year is still there.”

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