Pune’s GMRT becomes first Indian facility to detect gravitational waves

 In a major breakthrough, an international team of astronomers Thursday announced scientific evidence confirming the presence of gravitational waves using pulsar observations.

Operated by the National Centre for Radio Astrophysics (NCRA), India’s Giant Metrewave Radio Telescope (GMRT) located in Pune was among the six of the world’s largest radio telescopes that paved the way for this discovery of nano-hertz gravitational waves.

This major announcement has come eight years after the first detection of gravitational waves which was proposed by Physicist Albert Einstein a century ago.

In two different studies published on Thursday, radio astronomers representing the Indian Pulsar Timing Array (InPTA) and European Pulsar Timing Array (EPTA) shared that a time aberration was observed in the signals emerging from these pulsars.

Nicknamed as cosmic clocks, pulsars are rapidly spinning neutron stars that send out radio signals at regular intervals which are seen as bright flashes from the Earth. As these signals are accurately timed, there is a great interest in studying these pulsars and to unravel the mysteries of the Universe. In order to detect gravitational wave signals, scientists explore several ultra-stable pulsar clocks randomly distributed across our Milky Way galaxy and create an ‘imaginary’ galactic-scale gravitational wave detector.

There are several signals travelling through spacetime of the Universe. But, the presence of gravitational waves influences the arrival of these signals when detected from Earth. It was noticed that some signals arrive early while others, with a slight delay (less than a millionth of a second).

These nano-hertz signals were heard as humming from the Universe. These were caused due to the presence of gravitational waves and due to signal irregularities emerging from pulsars, said the scientists.

“These irregularities showcased consistent effects on the resultant emanating gravitational waves at ultra-low frequency,” said Bhal Chandra Joshi, senior NCRA scientist and the man behind forming InPTA.

It is expected that ultra-low frequency gravitational waves, also known as nano-hertz gravitational waves, emerge from a colliding pair of very large ‘monster’ black holes, many crores of times heavier than our Sun. Such ‘monster’ black holes are believed to be located in the centre of colliding galaxies. The signals or ripples that emerge from within these blackholes are known as nano-hertz gravitational waves. Their wavelengths can be many lakhs of crores of kilometres and oscillate with a periodicity anywhere between a 1 year to 10 years. When there is continuous arrival of these nano-hertz gravitational waves, it creates a consistent humming in our Universe, which gets detected using powerful radio telescopes from the Earth.

In all, six of the world’s most powerful and large radio telescopes – uGMRT, Westerbork Synthesis Radio Telescope, Effelsberg Radio Telescope, Lovell Telescope, Nançay Radio Telescope and Sardinia Radio Telescope — were deployed to study 25 pulsars over a period of 15 years. In addition to data from these facilities, highly sensitive uGMRT data of more than three years were analysed too. It has been concluded that radio flashes from these pulsars were affected by the nano-hertz gravitational waves believed to emerge from ‘monster’ black holes.

Along with scientists from NCRA, the InPTA comprises experts from Indian Institute of Science Education and Research, Bhopal, Raman Research Institute (RRI), Bengaluru, IIT-Roorkee, IIT-Hyderabad, Institute of Mathematical Sciences, Chennai.

Even though the Laser Interferometer Gravitational Observatory (LIGO) captured these waves lasting over a few seconds, PTAs observed these signals in a different frequency range.

“But our galaxy-sized PTA could sense a permanent vibration of the gravitational wave background in nano-hertz frequencies,” said Prof. A Gopakumar from Tata Institute of Fundamental Research (TIFR), Mumbai.

According to Joshi, Albert Einstein had stated that gravitational waves change the arrival times of these radio flashes, and thereby affect the measured ticks of our cosmic clocks.

“As these changes are tiny, astronomers need sensitive telescopes like uGMRT and a collection of radio pulsars to separate these changes from other disturbances. Such slow variations of the signal have meant that it takes decades to look for these elusive nano-hertz gravitational signals,” Joshi said.

Yashwant Gupta, centre director, NCRA, said, “The whiteband receiver systems designed and built for the uGMRT played a crucial role in obtaining high quality data from low frequency radio bandwidth.”

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