Published: Fri, June 02, 2017
Research | By Francis Brooks

Third Gravitational Wave Event Detected

Third Gravitational Wave Event Detected

It was the third confirmed detection of coalescing black holes detected so far by the US -led Laser Interferometer Gravitational-Wave Observatory, or LIGO, a project made up of two observing stations, one near Hanford, Washington, and the other 1,800 miles away near Livingston, Louisiana, and hundreds of scientists around the world.

The newfound black hole, formed by the merger, has a mass about 49 times that of our sun. And new LIGO lead Dave Shoemaker said that he's still dreaming of seeing something new: "I would like to see in all three detectors, the two LIGO and the VIRGO detectors, signals which arrive within the time window that says they came from the same source".

The lasers picked up the vibrations of two black holes that were 20 and 30 times bigger than the sun before they hurtled toward one another and merged into a massive black hole.

"It is remarkable that humans can put together a story, and test it, for such odd and extreme events that took place a billion years ago and a billion light-years distant from us", he said. LIGO is just starting to piece together this puzzle.

After undergoing tune-ups to improve its sensitivity, LIGO began its second observing run on November 30, 2016.

"The University of MS is so pleased to be a member of this global collaboration of talented scientists and engineers, which is producing such astounding breakthroughs", Chancellor Jeffrey Vitter said. The previous two sightings of gravitational waves were also produced by black-hole mergers, but LIGO researchers think this is the first event in which the spin of one of the merging black holes could have been pointing in the opposite direction to the orbital rotation of the black holes. The second set was detected in December 2015, and announced the following July.

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

"It clearly establishes a new population of black holes that were not known before LIGO discovered them", said LIGO scientific collaboration member Bangalore Sathyaprakash of Penn State and Cardiff University. The collisions produce more power than is radiated by all of the stars in all of the galaxies in the entire observable universe.

That signal took almost 3 billion light years to reach Earth - twice as far off as the other detections.

LIGO made the first-ever direct observation of gravitational waves in September 2015. As well as orbiting each other, each black hole can be spinning on its own axis - much like the Earth does. One is that they are remnants of stars that were already paired before they collapsed into black holes. "We thought we were walking through the world knowing how black holes happened, and now we're seeing that there's some real discovery space here". But in which direction do they spin?

The new data also helps astronomers determine how binary black holes are created.

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An illustration of two merging black holes, similar to those detected by LIGO.

The modeling of the waveforms detected in the recent LIGO observation, said Cadonati, "disfavor" such an effect, and indicate that the spins of the two black holes were more likely tilted with respect to their orbits around one another. The instrument compresses or expands the mirrors at the ends of the tunnels when a gravitational wave strikes the observatory. But some physicists have speculated that so-called "primordial black holes"-black holes created in the first second of the universe's existence-might make up a significant fraction of the mysterious dark matter".

There are two leading models for how these massive black holes are brought together. "Also, the data suggests that at least one of the black holes in this binary system might have been spinning in a direction that is not completely aligned with the orbital rotation of the binary, providing potential clues on how these binaries might have formed". It's possible these black holes formed far apart from each other inside a globular cluster. The black holes would, as a result, maintain the spin of their former stars, which would have been aligned.

Christopher Berry, of the University of Birmingham and a researcher on the project, said: "We're not just in this business to detect gravitational waves".

According to relativity, all frequencies of gravitational waves should travel at the same speed.

The gravitational wave detections confirm a major prediction of Albert Einstein's 1915 general theory of relativity and mark the beginning of the new field of gravitational-wave astronomy.

Gravitational waves provide a kind of "fingerprint" for this spin, explained physicist and LIGO scientist Laura Cadonati of Georgia Tech. The greater the object, the more ripples it would send out through spacetime. The Italy-based VIRGO detector is nearly in place and will join in to collect data later in 2017, a spokesperson for VIRGO said, at a tele-conference organised by LIGO collaboration.

LIGO detects waveforms, which are readouts of the ripples in the fabric of the universe caused by masses moving through it. They are also working on technical upgrades for LIGO's next run, scheduled to begin in late 2018, during which the detectors' sensitivity will be improved.

Today's announcement was made by scientists from the LIGO Scientific Collaboration and the Virgo Collaboration, an global team working to bring a third gravitational wave detector, Virgo, online this summer in Italy.

Although LIGO appears to be "uniquely suited" to observing these events, David Reitze, executive director of the LIGO Laboratory, said he hopes to see other types of astrophysical events soon.

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