The direct detection of gravitational waves was the first mandate of the laser interferometer gravitational-wave detectors. The excitement is because of its potential to be transformative and to open a new window into a dominantly dark universe. It would then become a powerful tool for astrophysics and cosmology and mature into a sensitive probe for basic physics. All this is possible because gravitational waves propagate unattenuated through large distances and gravitational wave observations are privy to new aspects of underlying physics inaccessible to electromagnetic astronomy.
Gravitational waves must occur in any relativistic theory of gravity, but properties of such waves could be different in alternative theories of gravity. The ideas of black holes and gravitational waves have had a chequered history during the first 50 years of general relativity. The first detection of gravitational waves came from a system of binary black holes and S. Chandrasekhar was doubly right when he said astronomy was the natural home of general relativity.
The Indian participation in LIGO collaboration is under the umbrella of the Indian Initiative in Gravitational-wave Observations (IndIGO), which includes 61 scientists from nine institutions. Indian groups have contributed significantly to understanding the response of the detector to the signals and terrestrial influences, to the method used for detecting the signal, bounding the orbital eccentricity, estimating the mass and spin of the final black hole and the energy and power radiated during merger, confirming that the observed signal agrees with Einstein's general theory of relativity, and to the search for a possible electromagnetic counterpart using optical telescopes.
The current Indian gravitational wave community has grown out of research programmes carried out over three decades by two groups. The group led by me (currently at ICTS-TIFR) at the Raman Research Institute, Bengaluru, in collaboration with scientists in France, had pioneered the mathematical calculations used to model gravitational wave signals from orbiting black holes and neutron stars. The group led by Sanjeev Dhurandhar at Inter-University Centre for Astronomy and Astrophysics, Pune, initiated work on developing data-analysis techniques to detect weak gravitational wave signals buried in the detector noise.
IndIGO was formed in 2009 by a group of researchers, who were keen to establish experimental gravitational wave research in the country with the dream of setting up an advanced detector in India. Over the last decade, the Indian gravitational wave community has spread to a number of institutions and has made significant contributions to the development of novel techniques to identify the weak gravitational wave signals.
The discovery could make it possible to observe our universe in gravitational waves if one can locate the source with additional detectors placed far from the LIGO detectors, forming one or more large triangles. With the direct detection of gravitational waves, we enter a new era of astronomy, of stellar evolution and energetic events involving black holes and neutron stars, some of which are not visible to any other type of telescope. When the LIGO detectors reach their full sensitivity in the coming years and as more detectors are added into the network, such detections will become frequent and varied, opening up a whole new world of astrophysics.
The author is visiting professor, International Centre for Theoretical Physics, Bengaluru, and chair, IndIGO Consortium.