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LIGO confirms gravitational wave for third time

LIGO confirms gravitational wave for third time

An global team of researchers has made a third detection of gravitational waves, ripples in space and time, in a discovery that provides new insights into the mysterious nature of black holes and, potentially, dark matter.

Referred to as GW170104 for its detection date, the newest gravitational wave signal originated when two heavy black holes violently collided, one about 20 times the mass of our sun and the other about 30 times the mass of the sun.

This time, the two black holes were unequal in size, one significantly lighter than the other.

Just like the previous times, the observation was triggered by one of the most violent events in the universe: the merger of two black holes. The first one was 62 solar masses, the second 21.

No wonder we can detect the shock waves of such a distant event. "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 new detection occurred during LIGO's current observing run, which began November 30, 2016, and will continue through the summer. Scientists think it's likely that one scenario is dominant in the universe and accounts for nearly all observed events, since it would be unusual for multiple scenarios to produce equal numbers of events in a fine-tuned balance.

The first direct observation of gravitational waves was made in September 2015 during the first observing run.

The second came shortly after, in December 2015, and was a distance of 1.4 billion light-years. In a paper published in the journal Physical Review Letters, researchers describe the collision of two black holes that merged to form a larger black hole located about 3 billion light-years away.

"Most of the gold we see in the solar system might have come from a binary neutron star collision that produced something like a Jupiter mass of gold and dispersed it in all directions", Creighton says.

The collision of a pair of colossal, stellar-mass black holes has made itself heard, almost 3 billion light years away, through a cosmic microphone on Earth. Like planets, black holes spin about their axis.

Astronomers have since been struggling mightily to understand how black holes (which, for the most part, are remnants of collapsed stars) can wind up so close to each other, without having been close enough to have merged during their stellar lifetimes. It makes a lot of sense for the black holes to spin individually in the same direction they are spinning against each other, but as the new data shows, sometimes they spin in the opposite direction.

Intriguingly, the observations are consistent with one of the more exotic explanations for the nature of dark matter - that dark matter haloes observed around galaxies are made up of a smattering of small black holes. The black holes in the first and second detections were located 1.3 billion and 1.4 billion light-years away, respectively.

Both theories may explain a slice of the black hole binary population, said LIGO Executive Director David Reitze of Caltech - but the question is how big each slice is.

Researchers have now confirmed that a third black hole merger was detected on 4 January 2017, earning it the memorable name GW170104. "Einstein created this theory 100 years ago and with no ability to observe gravitational waves, so for him to be 100 percent correct would be quite a feat". The sizes of the black holes are doubled to improve visibility. Scientists think the aligned black hole pairs are born together when two circling stars become two black holes - since the stars would have been spinning in alignment, the black holes should do the same.

"Although our measurement can not precisely determine if the black holes were tilted, we have indication that at least one of the two black holes was misaligned, which favors the first theory", said Cadonati. The black holes pair up after they sink to the center of a star cluster.

The black holes can spin in any direction relative to their orbital motion.

LIGO sees some evidence that the GW170104 black holes are anti-aligned, mildly favoring the dense stellar cluster theory. "We'll need more events to be able to statistically disentangle what is happening", said Daniel Holz, an astrophysicist at the University of Chicago and a LIGO member who has worked on the common-envelope scenario. And because the gravitational wave arrived undiminished, it provides yet another proof of one of Einstein's theories, showing that gravity travels at light speed.

A gravitational wave signal has been detected and confirmed for the third time.

The Indian team in LIGO includes scientists from CMI Chennai, ICTS-TIFR Bengaluru, IISER-Kolkata, IISER-Trivandrum, IIT-Bombay, IIT-Madras, IIT-Gandhinagar, IIT-Hyderabad, IPR Gandhinagar, IUCAA Pune, RRCAT Indore, TIFR Mumbai and UAIR Gandhinagar.

"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, an associate professor at the Georgia Institute of Technology.


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