Haridwar : An international team of astronomers from India, Japan and Europe has recently published the results from monitoring nature's best clocks, pulsars using six of the world's most sensitive radio telescopes, including India's largest telescope, uGMRT. These results provide scintillating evidence for the relentless vibrations of the fabric of our universe caused by ultra-low frequency gravitational waves.

Such waves are expected to originate from a large number of dancing monster black hole pairs, crores of times heavier than our Sun. The team's results are a crucial milestone in opening a new, astrophysically-rich window in the gravitational wave spectrum.

Such dancing monster Black Hole pairs, expected to lurk in the centers of colliding galaxies, create ripples in the fabric of space-time, which the astronomers call nano-hertz gravitational waves. The relentless cacophony of gravitational waves from a large number of supermassive black hole pairs create a persistent humming of our universe.

The team, consisting of members of the European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array (InPTA) consortia, published their results in two seminal papers in the Astronomy and Astrophysics journal, and their results hint at the presence of such gravitational waves in their data set. Prof. P. Arumugam and his senior PhD student, Jaikhomba Singha, are part of these ground-breaking results.

"These results have culminated due to years of efforts of many scientists, including early career researchers and undergraduate students. I am very grateful that IIT Roorkee has been able to constantly contribute in various ways in achieving these results. The NSM facility, PARAM Ganga, installed at IIT Roorkee, among various other facilities, has played a crucial role in this global effort. I hope IIT Roorkee will continue to support the various efforts of this stellar collaboration," states Prof. Arumugam, Department of Physics, IIT Roorkee.

These light-year-scale ripples can only be detected by synthesizing a galactic-scale gravitational-wave detector using pulsars-the only accessible celestial clocks for humans. Pulsars are a type of rapidly rotating neutron stars that are essentially embers of dead stars, present in our galaxy. Fortunately, a pulsar is a cosmic lighthouse as it emits radio beams that flashes by the Earth regularly, just like a lighthouse near a harbor. Astronomers monitor these objects using the best radio telescopes of the world, including India's premiere radio telescope, the uGMRT, situated near Pune.

"According to Einstein, gravitational waves change the arrival times of these radio flashes and thereby affect the measured ticks of our cosmic clocks. These changes are so tiny that astronomers need sensitive telescopes like the uGMRT and a collection of radio pulsars to separate these changes from other disturbances. The slow variation of this signal has meant that it takes decades to look for these elusive nano-hertz gravitational waves," explains Prof. Bhal Chandra Joshi of NCRA Pune and Adjunct Faculty, IIT Roorkee, who founded the InPTA collaboration during the last decade.

Scientists of the EPTA in collaboration with the Indo-Japanese colleagues of the InPTA, have reported detailed results of analysing pulsar data collected over 25 years with six of the world's largest radio telescopes. This includes more than three years of very sensitive data collected using the unique low radio frequency radio telescope, the uGMRT. The analysis of this unique data set has revealed that the measured rate of ticking of these cosmic clocks has characteristic irregularities common across the twenty-five pulsars that have been monitored. This is consistent with the effect produced by gravitational waves at ultra-low frequency (waves that oscillate with periods between one and ten years).

Not surprisingly, nano-hertz frequency gravitational waves will unravel some of the best-kept secrets of the Universe. The cosmic population of black hole pairs with masses that are ten-to-hundred crores times more than the mass of our Sun are expected to be formed when their parent galaxies merge and such a population emits gravitational waves at these frequencies. Further, various other phenomena that may have taken place when the Universe was in its infancy, just a few seconds old, also produce these waves at these astronomically long wavelengths.

According to Prof. A. Gopakumar, TIFR, Mumbai, and Chair of the InPTA consortium, "The results presented today mark the beginning of a new journey into the Universe to unveil some of these mysteries. More importantly, this is the first time that an Indian telescope's data is used for hunting gravitational waves".

To detect these gravitational-wave signals, astronomers in a "Pulsar Timing Array" (PTA) collaboration exploit many ultra-stable pulsar clocks distributed across our Milky Way galaxy to create a "galactic-scale gravitational-wave detector". Measurements of the exact arrival times of the pulsars, which have been going on for decades, are being compared with each other to study the influence of gravitational waves.

As radio signals travel through space and time, the presence of gravitational waves affects their path in a characteristic way: some pulses will arrive a little (less than a millionth of a second) later, some a little earlier. This gigantic galactic-scale GW detector synthesised by incorporating 25 meticulously chosen pulsars in our Milky Way Galaxy makes it possible to access the variations in the pulse arrival times created by gravitational waves with a frequency of oscillation 10 billion times slower than those first observed in 2015 by the two ground-based LIGO detectors in the United States of America.

The current results are based on a coordinated observing campaign using the five largest radio telescopes in Europe, complemented by the observations with the upgraded Giant Metrewave Radio Telescope in India. The analysis of the European and Indian Pulsar Timing Array (EPTA+InPTA) data which is presented today has revealed the presence of a common signal across the pulsars in the array which is broadly in agreement with being due to gravitational waves. The EPTA+InPTA results are complemented by the coordinated publications made by other PTAs across the world, namely the Australian (PPTA), Chinese (CPTA) and North-American (NANOGrav) pulsar timing array collaborations. This same evidence for gravitational waves is seen by NANOGrav and consistent with results reported by the CPTA and PPTA.

Singha, a senior PhD scholar from IIT Roorkee, says, "This is an extremely exciting time for early career researchers. We are in an era where an international team of researchers across the globe are all collaborating and trying to listen to the humming of our universe. The present results will open a plethora of exhilarating science for us in future."

Importantly, work is already in progress where scientists from the four collaborations - EPTA, InPTA, PPTA and NANOGrav - are combining their data sets under the auspices of the International Pulsar Timing Array (IPTA) to create an array consisting of over 100 pulsars that may allow them to reach this goal in the near future. This combined IPTA data set is expected to be more sensitive, and scientists are excited about the constraints they can place on the GWB along with understanding various other phenomena that may have taken place when the Universe was in its infancy, just a few seconds old, which can also produce gravitational waves at these astronomically long wavelengths.

The InPTA experiment involves researchers from NCRA (Pune), TIFR (Mumbai), IIT (Roorkee), IISER (Bhopal), IIT (Hyderabad), IMSc (Chennai) and RRI (Bengaluru) along with their colleagues from Kumamoto University, Japan.

Prof. K K Pant, the Director of IIT Roorkee, said, "Congratulations to the InPTA team and our esteemed researchers from IIT Roorkee for their remarkable findings and impactful research. I am delighted to learn about the utilization of IIT Roorkee's cutting-edge facilities, such as PARAM Ganga, in this endeavor. This achievement exemplifies the power of international collaborations in attaining greater scientific goals and significantly contributing to our understanding of the universe."

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