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Scientists reveal second detection of gravitational waves that Einstein predicted

Astronomers say they have heard the echoes of merging black holes for a second time — a discovery that hints that the unseen violence of the universe may be pretty common.

They detected a second gravitational wave. That's the warp in the fabric in the cosmos that Albert Einstein predicted a century ago in his theory of general relativity, something that results only from the most massive collisions in space.

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It's something that can't be seen and that Einstein thought would never be detected, but scientists have found a way to hear it — first one note, in September 2015, and now a second.

With that second, higher-pitch chirp, detected Christmas night in the United States, an international team of scientists that included two from the University of Maryland, College Park has switched from reveling in the "aha" of the initial discovery to a more detailed and telling recording of the universe's invisible chaos.

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"We all just marveled at it for a while," said Peter Shawhan, an associate professor of physics at Maryland and a principal investigator on the research project. "By December we were sure that the first event was genuine and we had a fairly mature draft of that paper, which finally came out in February. But it was very satisfying to know, even then, that we already had a second event on our hands."

Scientists first heard the cosmic crash after turning on a $1.1 billion set of twin instruments known as the Laser Interferometer Gravitational-wave Observatory, or LIGO.

But physicists weren't quite certain if they had lucked into a once-in-a-lifetime chirp from a rare event or if gravitational waves are out there waiting to be heard on a regular basis.

"It's fabulous that our waveform models have pulled out from the noise such a weak but incredibly valuable gravitational wave signal," said Alessandra Buonanno, a physics professor at Maryland and another principal investigator on the LIGO Scientific Collaboration.

More than 1,000 scientists wrote the study published Wednesday in the journal Physical Review Letters.

Such crashes may happen in our own Milky Way once every few hundred millions of years or so, but by looking at so many other galaxies, scientists can hear more, said Barnard College physicist Janna Levin, author of "Black Hole Blues And Other Songs From Outer Space." Levin wasn't part of the study but praised it as sound and significant, as did several other outside experts.

Columbia University astrophysicist Szabolcs Marka, a member of the discovery team, said detecting the first wave was much like going outside and finding a $100 bill. It could be a lucky day like winning the lottery. Finding as second one so soon tells scientists that this is more likely to be a regular windfall.

Marka said he wouldn't be surprised if they will hear a gravitational wave about once a week, once scientists upgrade their equipment.

"I think they're not rare anymore; they were never rare" but were hard to hear until Earth scientists got the right hearing aid, said Louisiana State University physicist Gabriela Gonzalez, the LIGO team's scientific spokeswoman.

The latest sound was from a cosmic crash 1.4 billion light years away. A light-year is 5.9 trillion miles.

The first black hole crash that scientists heard in September was from two massive objects that were unusually huge even for black holes. December's black holes were more normal sized — 14 and eight times the mass of our sun. Those mergers produce faint invisible ripples in the fabric of the cosmos that bunch up like a kink in a net.

But such kinks can't be seen. The wave is only noticed on Earth in incredibly tiny misalignments of split laser beams in detectors both in Hanford, Washington, and Livingston, Louisiana. That mismatch is a vibration that scientists can hear as sound.

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The Christmas Day wave was fainter, but lasted longer, Gonzalez said. Marka called it a crescendo. Levin likened it to a drum beat or whale's song.

"We can now think seriously about gravitational waves as a new way to learn about the universe," said University of Florida physicist Clifford Will, who wasn't part of the team, in an email. "To me, this latest detection says there is a `big band' out there and we are only beginning to dance to the music of cosmic DJ."

The Associated Press and Baltimore Sun reporter Scott Dance contributed to this article.

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