Eight years after the first-ever detection of gravitational waves, a feat honoured by Nobel Prize for Physics two years later, scientists have now picked up evidence to suggest that a multitude of gravitational waves are ever-present in any area of the universe, their combined effects constantly deforming and re-shaping spacetime, and altering the motion and behaviour of every heavenly body.
“A common analogy can be this. When a stone is dropped into a lake, it produces short-lived waves in the water. But when raindrops fall on to the lake, every drop creates a wave. These waves interact with each other, and the disturbance on the surface of the lake is the combined effect of all these individual waves. A floating object on the lake, say a paper boat, would experience, and be affected by, the combined effect of all these waves. Also, compared to a single drop of stone, the disturbance produced by rains is longer-lasting. What is happening in the universe is something similar. Large number of gravitational waves, produced by different events, are constantly deforming spacetime. And all heavenly bodies, like earth, move around under the influence of this combined effect,” said Yashwant Gupta, director of the Pune-based National Centre for Radio Astrophysics (NCRA), which is part of the global scientific collaboration behind the latest finding.
The evidence for what is being called ‘gravitational wave background’ was picked up using an entirely different technology compared to the one used for the first detection of gravitational waves in 2015. Six large radio telescopes around the world, including the Pune-based Giant Meterwave Radio Telescope that is operated by NCRA, measured very small delays — in the range of millionths of a second — in the signals coming from faraway rapidly rotating stars called pulsars. Scientists propose that these delays were a result of deformities caused in spacetime by gravitational waves.
Gravitational waves are ripples, or disturbances, produced in the fabric of spacetime by large moving objects, something similar to the ripples produced on the surface of water by a moving boat. The existence of gravitational waves was predicted by Albert Einstein’s general theory of relativity more than a century ago, but its experimental confirmation came only in 2015.
After showing, in 1905, that space and time were not independent entities but had to be woven together as spacetime, Einstein had, in his general theory of relativity in 1915, proposed that spacetime was not a mere transparent, inert, static or fixed background to all the events in the universe. Instead, spacetime was flexible and malleable, interacted with matter, was influenced by it, and in turn, influenced the events that take place there. It was like a soft fabric that responds to, and gets deformed by, a heavy object placed on it.
In 2015, scientists detected gravitational waves for the first time through LIGO (Laser Interferometer Gravitational-wave Observatory) detectors. Those waves were produced by the merger of two black holes that took place about 1.3 billion years ago. But scientists contend that such events, mergers of black holes or explosion of stars, keep happening all the time, regularly producing gravitational waves. Even the simple motion of large bodies can produce detectable gravitational waves.
“Like you have a whole spectrum of electromagnetic waves, from microwaves to radio waves, you can have a wide range of gravitational waves of different wavelengths, frequencies and energies. The gravitational wave that was detected in 2015, and all subsequent detections after that, involved mergers of black holes that were relatively small in size. The gravitational waves produced by them are relatively feeble. Only the waves produced just ahead of the merger, when the energy released was maximum, could be detected. But these are like flashes of gravitational waves, lasting for maybe a few miliseconds,” said Ashoka University vice-chancellor Somak Raychaudhury, a former director of Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA).
“There are much more massive black holes that are constantly merging, black holes that are millions or billions of times larger than our Sun usually at the centre of the galaxies. They can produce detectable gravitational waves from times much before their merger. In fact, the merger process can take millions of years, providing a steady supply of gravitational waves. And there are many such events happening all the time. So, there is a sort of gravitational wave background that exists all the time,” he said.
The ‘noisy’ presence of several such gravitational waves, each with different characteristics, is what is now being referred to as the ‘background hum’. The results were reported simultaneously on Thursday by five international teams, including the one in Pune.
The latest breakthrough is expected to help scientists get a better understanding of the nature and evolution of the universe.
“We are not yet saying that we have been able to establish the presence of gravitational wave background, because the level of confidence for making this assertion is not very high. But we have produced very promising data that points in that direction. Eventually, we should even be able to wean away signals of big individual events producing strong gravitational waves from the current symphony of signals that we are looking at. Because these are markers of large scale interactions in the universe, we can get information about large scale structure of the universe, its evolutionary history and dynamics of events like merger of galaxies,” NCRA’s Gupta said.
In different studies published on Thursday, radio astronomers representing the different teams including Indian Pulsar Timing Array (InPTA) shared that a time aberration, or delay, was observed in the signals emerging from distant rapidly-rotating neutron stars called pulsars that are sometimes spinning more than 1000 times every second. They are so named because they emit pulses of radiation, observed from earth as bright flashes of light, at every rotation. The time period of these pulses of radiation is fixed and predictable, the reason why these neutron stars are called ‘cosmic clocks’.
In order to detect gravitational wave signals, scientists studied several ultra-stable pulsar clocks randomly distributed across our Milky Way galaxy through six of the largest radio telescopes in the world, including GMRT. The arrival of these signals can be calculated accurately, but during experiments it was observed that some of them arrived a little early while a few others were late, the discrepancies ranging in millionths of seconds.
“These irregularities showed consistent effects of the presence of gravitational waves,” said Bhal Chandra Joshi, senior NCRA scientist and the man behind InPTA.
Scientists say that possible sources of these low-frequency gravitational waves could be colliding pair of very large, ‘monster’, black holes, millions of times bigger than our Sun. Such large blackholes are usually found at the centres of galaxies. Gravitational waves originating from the collision or mergers of such blackholes can have very large wavelengths, extending up to light years, and consequently, very low frequencies.
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.
NCRA’s Joshi said Einstein’s theory had predicted that gravitational waves would 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,” he said.
Prof. Michael Kramer, Director, Max-Planck Institute, Germany, also a collaborator, termed the international collaborative effort as scientifically rewarding. “We hope to also serve as a role model for the global International Pulsar Timing Array efforts,” he said.
With inputs from Partha Biswas in Pune
(Anjali Marar works with Raman Research Institute, Bengaluru)
