Indian astronomers played key role in detecting cosmic ‘hum’ from giant gravitational waves

A radio telescope near Pune and Indian astrophysicists were part of an international ground-breaking experiment in which scientists collected data for 15 years to "hear" the perpetual chorus of gravitational waves rippling through the universe for the first time - and it's louder than expected.

Cosmic signals picked up by the Giant Metrewave Radio Telescope and critical analysis by Indian scientists including researchers from Raman Research Institute, Bengaluru, played a key role in the discovery of the first evidence of a gravitational wave background that was theorised to exist for a long time but there was no proof.

The breakthrough was led by a team at the North American Nanohertz Observatory for Gravitational Waves, who collaborated with astronomers from Canada, Europe, China, Australia, Japan and India for collecting data painstakingly over the years and analysing them.

Observing a set of a particular class of burnt out stars known as pulsars, they spotted new gravitational waves — ripples in the fabric of space-time — which are by far the most powerful ever measured. The team observed 68 pulsars.

These waves carry roughly a million times as much energy as the one-off bursts of gravitational waves from black hole and neutron star mergers detected for the first time in 2015 by experiments such as LIGO and Virgo.

“It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies,” says NANOGrav scientist Chiara Mingarelli. “This is the first-ever evidence for the gravitational wave background. We’ve opened a new window of observation on the universe.”

The existence and composition of the gravitational wave background — predicted long back but never before heard — presents a treasure trove of new insights into long-standing questions, from the fate of supermassive black hole pairs to the frequency of galaxy mergers.

A key piece of information gathered by the recently upgraded GMRT helped identify the universe’s background hum.

“There was a lot of background noise, which needed to be filtered out to pick up the faint gravitational wave signals. The GMRT did that,” Bal Chandra Joshi, a scientist at the National Centre for Radio Astronomy, Pune and one of the members of the international team, told DH.

Other Indian institutions involved in the project are Indian Institute of Technology, Roorkee; IIT Hyderabad and Indian Institute for Science, Education and Research, Bhopal. All of them are members of the Indian Pulsar Timing Array that collaborated with other such pulsar timing arrays for the research.

“The gravitational wave background is about twice as loud as what I expected,” says Mingarelli, now an Assistant Professor at Yale University. “It’s really at the upper end of what our models can create from just supermassive black holes.”

The deafening volume may result from experimental limitations or heavier and more abundant supermassive black holes. But there’s also a possibility that something else is generating powerful gravitational waves.

Einstein’s theory of general relativity predicts precisely how gravitational waves should affect pulsar signals. By stretching and squeezing the fabric of space, gravitational waves affect the timing of each pulse in a small but predictable way, delaying some while advancing others.

"Such changes, in the order of nano-seconds, were measured during the experiment," said Joshi.

Detecting the background hum is a major scientific accomplishment because of its potential to revolutionise the current scientific understanding of the earliest days of the universe including how the galaxies evolved and the merger and collision between two galaxies.

“Now that we have evidence for gravitational waves, the next step is to use our observations to study the sources producing this hum. One possibility is that the signal is coming from pairs of supermassive black holes, with masses millions or billions of times the mass of our Sun. As these gigantic black holes orbit each other, they produce low-frequency gravitational waves,” said University of Wisconsin-Milwaukee’s Sarah Vigeland who is also a team member.