Einstein hat-trick, with Indian twist on Gravitational waves

Scientists have for the third time in less than two years detected gravitational waves, feeble ripples in space-time predicted by Albert Einstein, but the new observations have also yielded a bonus: a new population of heavier-than-expected black holes.

An international consortium of more than 1,000 scientists today announced its third detection of gravitational waves generated by the merger of two black holes three billion light years away from Earth. The findings were published in the journal Physical Review Letters.

Black holes are the remains of heavy stars that have exhausted their fuel and collapsed into objects so dense that not even light can escape them.

About 40 scientists from 11 institutions across India, including the Indian Institute of Science Education and Research (IISER), Calcutta, participated in the hunt for gravity waves that relies on two laser-based observatories in America.

The consortium made its first-ever direct observation of gravitational waves in September 2015 and a second detection in December 2015, both sets of waves generated by the collision and merger of black holes.

The observatories spotted the third signal on January 4 this year, with detailed analysis indicating that it emerged from the collision of two black holes of 31 and 19 solar masses.

"The first one was big because it was the first, the second one told us the first was not a fluke, the third one now tells us that we should prepare for routine observations of such events," said Sanjit Mitra, associate professor at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, and a member of the consortium.

Scientists say the new finding also "solidifies" the case for a class of black hole pairs, or binaries, with masses larger than what had been seen by the observatories before.

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"We now have confirmation of the existence of stellar-mass black holes larger than 20 solar masses. These are objects we didn't know existed before the (observatories) detected them," David Shoemaker, spokesperson for the consortium at the Massachusetts Institute of Technology, said.

The third merger was one among six candidates identified during the second detection of gravitational waves by the laser-interferometer gravitational wave observatory (LIGO). The findings had been shared with astronomers worldwide to look for electromagnetic signatures from the candidate sites.
"Each gravitational wave signal allows us to test Einstein's general theory of relativity in slightly different ways -- and each time we find that the theory matches very well with the observations," said Rajesh Nayak, the team member at IISER, Calcutta.

Nayak and his student, Anuradha Samajdar, have used the observational data to verify key aspects of Einstein's theory.

In an unusual twist, India's space observatory, Astrosat, helped astronomers identify a new object, spotted in the sky around the same time as the detection of the third gravitational wave signal, as a gamma ray burst and not something related to the black hole merger.

Each time the LIGO consortium detects a signal, it alerts astronomers across the world who use telescopes to look at the sky for signatures of any associated explosions.

After the January 4 signal, a Hawaii-based astronomy team spotted an explosion that was fading fast -- almost exactly in the same part of the sky as the source of the gravitational waves.

A follow-up analysis by Varun Bhalerao at the Indian Institute of Technology, Bombay, and his colleagues, relying on Astrosat observations, revealed that the object was a gamma ray burst that had exploded about 21 hours after the gravitational wave signal.

"This event signalled the birth of a black hole when a massive star imploded in a galaxy several billion light years away," said Dipankar Bhattacharya, a team member at IUCAA.
The IUCAA has described Astrosat's role as "catching an astronomical impostor".

When the LIGO observatory is ready with an upgraded sensitivity in about three years, scientists should, in the light of current theories, be able to detect one gravitational wave signal each week.

Such signals are expected to spawn gravitational wave astronomy and allow scientists to study exotic events such as the mergers of black holes and neutron stars. The two black hole mergers that had provided the previous two signals had occurred 1.3 billion and 1.4 billion years ago.

The third detection also helped confirm Einstein's prediction that gravitational waves should not display a behaviour called "dispersion" -- the phenomenon of waves travelling at different speeds, depending on their wavelengths.

The Indian component of the LIGO consortium played a key role in deriving this result.

Bala Iyer, a professor at the International Centre for Theoretical Sciences, Bangalore, said: "With the third detection of gravitational waves from (colliding) black holes, we discovered a new class of astrophysical sources to test Einstein's theory in extreme conditions."

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