India is going to have a major role in the future detection of gravitational waves. The first direct observation of the phenomenon announced on Thursday in Washington DC is the biggest breakthrough astrophysicists have been waiting for to take a giant leap into understanding the universe. But apart from the excitement of finding a gravitational wave, the discovery of the presence of binary black holes in the universe, and a whole new toolbox with which to look into space, Thursday's announcement held great cheer for Indian astrophysicists.
“There is a proposal to build a LIGO observatory in India in collaboration with USA and other international partners,” said Parameswaran Ajith of the International Centre for Theoretical Sciences in Bangalore and member of the international scientific collaboration on gravitational waves. "Scientists are hoping that the current discovery will accelerate the decision-making process on this."
The Laser Interferometer Gravitational-wave Observatory, known as LIGO, founded by Caltech and MIT scientists in 1992 is a collaboration of more than 900 scientists who have all been looking for gravitational waves, which have been elusive till very recently. But the project has two detectors, both located within the United States – one in Livingstone, Louisiana, and the other at Hanford, Washington. The relative proximity of the detectors that are looking for signals from across the vast expanse of space limits the observatory’s range.
Signal, yes. But from where?
There are only three other gravitational wave detectors in the world at Italy, Germany and Japan. But the continuous detection of gravitational waves will need more separated and independent detectors across the world to ensure that a signal is indeed coming from deep space and not from a local source. Having a detector in India will help narrow down the source of a gravitational wave to a specific part of the sky.
Take, for example, the wave that was detected by LIGO’s Livingstone and Hanford detectors in September 14, 2015. It was generated 1.3 billion years ago when two large black holes, each about 150 kilometers in diameter but packed with 30 times the mass of the sun, spun around each other, then accelerated to half the speed of light and finally collided to become became one massive black hole. The cataclysmic event sent out a burst of energy, a gravitational wave that passed through all matter in the universe to finally reach the earth and the interferometer set up to detect it.
In picking up the signal the two detectors in the US that had only just been upgraded to Advanced LIGO, or aLIGO, could guess at the swathe of space where this event had occurred.
An aLIGO detector in India would help triangulate a gravitational wave signal and shrink the area in which to look for its origins.
There are other advantages to triangulating a signal from a detector in India.
“The gravitational wave that we saw was from two black holes and we knew that there would be no light from it because there is no real gas around black holes,” explained Karan Jani, doctoral candidate at the Center for Relativistic Astrophysics at the Georgia Institute of Technology who has been studying gravitational waves for eight years and joined LIGO in 2014. “But if a regular star and a neutron star collide, then you will see jets of light that telescopes on earth can point to. LIGO will see it first as gravitational waves come faster because it is the purest form of signal. So then you can point the telescopes to exactly that part of the sky.”
LIGO-India has been conceived as a collaboration between LIGO and a consortium called IndIGO formed by three Indian institutions – the Institute of Plasma Research in Gandhinagar, the Inter-University Centre for Astronomy and Astrophysics in Pune and the Raja Ramanna Centre for Advanced Technology in Indore. The detector in India will be identical to the aLIGO detectors in the US. It will consist of a Michelson Interferometer that has two perpendicular arms, each four kilometres long.
To check for gravitational waves the detector will split a laser beam and project the twin beams along these arms till they hit mirrors at the end. A gravitational wave, which expands and contracts all matter as it passes through, will cause a phase shift in the light that will be seen as the returning beams merge.
The LIGO network is set get wider and stronger in coming years. Advanced LIGO is at one-third of its ultimate design sensitivity. LIGO co-founder Kip Thorne said that further upgradations will make the detectors three times better and enable scientists to see three times farther into the universe.
India’s physicists have got a positive signal that LIGO-India, which is currently awaiting government approval, will now be put on the fast track.
“The fact that the Prime Minister tweeted it is just amazing,” said Jani. “This shows that he has an eye on mega science projects, that he – or the people around him – are aware of what’s going on in fundamental science. If they know that this is the greatest discovery in a hundred years, I think they will leave no stone unturned to make sure India is a part of it.”
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