Gravitational waves detected again, at furthest distance yet

Posted June 02, 2017

The latest gravitational waves were spotted January 4, 2017, by twin lasers in Louisiana and Washington. The merger shares some features with the previous ones: the black holes were bigger than expected, and their merger released a staggering amount of energy. As was the case with the first two detections, the waves-ripples in spacetime-were generated when two black holes collided to form a larger black hole.

Three billion years ago, in a third of a second, two black holes crashed into each other and merged into a single entity, converting two solar masses into energy that shook the fabric of spacetime, sending gravitational ripples across the universe that were detected on Earth last January, researchers announced Thursday.

Most of the remaining mass - almost twice our sun's worth - cascaded out in a powerful burst of invisible energy: gravitational waves that took 3 billion earth years to reach us, and that passed right on by in a fraction of a second. "Indian scientists played a leading role in deriving this result", said Sanjit Mitra from the Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA), researchers of which have participated in the LIGO discoveries.

The European Space Agency, and potentially in collaboration with NASA, is now developing a space-based gravitational wave observatory.

LIGO is funded by the National Science Foundation and operated by the Massachusetts Institute of Technology and Caltech, which conceived and built the project. The second detection was made in December 2015. Located about 1.3 and 1.4 million light-years away, respectively, those resulting black hole masses varied dramatically, between 62 solar masses and 21 solar masses, respectively. In other words, it was turning on its axis in a different direction than it was orbiting its partner black hole. The initial two black holes orbit each other, until they merge and form one larger remnant black hole. Sometimes black holes spin in the same overall orbital direction as the pair is moving-what astronomers refer to as aligned spins-and sometimes they spin in the opposite direction of the orbital motion.

The Cadmium Zinc Telluride Imager (CZTI) on the Indian space observatory AstroSat conducted the most sensitive search for short duration X-ray flashes associated with this event, but did not find anything. "The measured properties of the black holes, particularly their individual spin orientations, provide clues to their correct evolutionary paths". The methods differ in their details, but either is theoretically capable of producing enough black-hole mergers to account for Advanced LIGO's signals. "If we can detect more systems, we can nail down under what circumstances black holes formed and evolved to form binary systems that ultimately merged". The first black hole mergers observed by LIGO produced tell-tale cosmic signatures that meshed perfectly with what Einstein's theory predicted. Ringdown is the gravitational wave that is emitted by the non-spherical merged black hole as it relaxes to become a sphere. This was a huge accomplishment and a bit of a surprise, as prior to that detection scientists didn't know that collapsing stars could form black holes that massive.

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There are now two primary theories about binary black hole formation: The first is that a pair of black holes is formed when a pair of stars explodes. Understanding how binary black holes spin helps scientists determine how they're formed.

In the other model, the black holes come together later in life within crowded stellar clusters. Because LIGO sees some evidence the GW170104 black holes are non-aligned, the data slightly favor this dense stellar cluster theory.

"This paper only reports on a few weeks worth of data, and we plan to run until August", says Chad Hanna, a LIGO scientist from Pennsylvania State University.

Scientists don't know yet how these black hole binary systems form, but there are two general theories.

We can not "see" these things with our own eyes, but now we may have the chance to "hear" them!

Physical Review Letters, DOI not yet available.

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"The new event also provides new opportunities to test Einstein's theory of general relativity".

Each scientist at the teleconference offered what they would like to see from LIGO.

"It looks like Einstein was right-even for this new event, which is about two times farther away than our first detection", said Laura Cadonati, associate professor of physics at Georgia Institute of Technology and deputy spokesperson for the LIGO Scientific Collaboration. After the first run, the machine was shut down and improvements were made to improve the sensitivity. "We expect that by this summer Virgo, the European interferometer, will expand the network of detectors, helping us to better localize the signals".

For much of the past year, the LIGO detectors were offline to allow an upgrade. The next big class of events would be binary neutron star mergers - in part because these events could be seen with both LIGO and traditional telescopes.

LIGO's two L-shaped "receivers" boast vacuum-tube arms that run 2.5 miles in each direction and house powerful laser beams that bounce back and forth between almost flawless floating mirrors.

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