 "How did scientists hear humming from the Universe and why is it significant")
An international team of astronomers, comprising scientists from India, Japan, and Europe, have made an exciting discovery by monitoring pulsars using six of the world's most sensitive radio telescopes, including India's largest telescope, the upgraded Giant Metrewave Radio Telescope (uGMRT).
What are the findings?
By using these telescopes, scientists were able to hear a "humming" or vibrations caused by ultra-low frequency gravitational waves.
These waves originate from a number of "dancing monster black hole pairs" found in colliding galaxies and create vibrations in the fabric of space-time. Astronomers are calling these vibrations nano-hertz gravitational waves.
Why is this discovery significant?
The discovery is a significant breakthrough in understanding the gravitational wave spectrum, and it has opened a new window of exploration in astrophysics. It has also deepened the understanding of the Universe and is a great example of the power of international collaboration.
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"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," said A Gopakumar, the chair of the Indian Pulsar Timing Array consortium & professor at TIFR, Mumbai.
Pulsars and how they are monitored
Pulsars are rapidly rotating neutron stars, the remnants of dead stars, that emit regular radio beams as they rotate.
These pulsars are monitored using radio telescopes, such as India's uGMRT.
The detection of gravitational-wave signals relies on a collaboration called the Pulsar Timing Array (PTA). The experiment is called PTA, as some of these millisecond pulsars can rotate 100 times a second, scientists.
Scientists used pulsars as cosmic beacons for this experiment to detect these light-year-scale ripples.
Where do the gravitational waves come from?
These gravitational waves can occur when black holes merge as a result of galaxies merging, emitting gravitational waves at astronomically long wavelengths.
By monitoring these pulsars using radio telescopes, such as India's uGMRT, researchers have collected data over 25 years, including three years of highly sensitive observations. The analysis of this unique dataset revealed consistent irregularities in the ticking rates of the monitored pulsars, indicating the influence of ultra-low frequency gravitational waves with oscillation periods ranging from one to ten years.
"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 over the last decade.
Precise measurements of the arrival times of pulsar signals, spanning decades, are compared to study the effects of gravitational waves. These waves slightly alter the arrival times of radio pulses, enabling the detection of variations at a frequency ten billion times slower than the waves initially observed in 2015 by the LIGO detectors in the United States.
The team: An EPTA and InPTA collaboration
The international team consisted of members of the European Pulsar Timing Array (EPTA) and Indian Pulsar Timing Array (InPTA) consortium. Both are members of the International Pulsar Timing collaboration (IPTA).
InPTA is a pulsar timing experiment committed to searching for low-frequency nanoHz gravitational waves since 2016. It is an Indo-Japanese collaboration that uses the uGMRT to monitor a sample of nearby millisecond pulsars.
"These results have culminated due to years of efforts of many scientists, including early career researchers and undergraduate students," stated P Arumugam, professor at the Department of Physics, IIT Roorkee, who was also part of the team of scientists.
The InPTA experiment involves researchers from seven Indian institutes, including NCRA (Pune), TIFR (Mumbai), IIT (Roorkee), IISER (Bhopal), IIT (Hyderabad), IMSc (Chennai) and RRI (Bengaluru) along with colleagues from Kumamoto University, Japan.
The research, published in two papers in the Astronomy and Astrophysics journal, suggests the presence of these gravitational waves in the data collected by the EPTA and InPTA consortia.
Future prospects of research
The EPTA and InPTA data analysis revealed a common signal across the pulsars, indicating the presence of gravitational waves. Similar findings have been reported by other PTA collaborations worldwide, including Australia (PPTA), China (CPTA), and North America (NANOGrav).
Researchers are already collaborating under the International Pulsar Timing Array (IPTA) to combine their data sets. This collaboration aims to incorporate over 100 pulsars into the array, enabling even more sensitive observations and potentially unlocking further insights into the early Universe and its associated phenomena.
Jaikhomba Singha, a senior PhD scholar from IIT Roorkee who was a part of the research team, said, "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."