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Hint for relentless humming of universe by low frequency gravitational
waves
An International team of astronomers from India, Japan and Europe has
published the results from monitoring pulsars, nature’s best clocks,
using six of the World's most sensitive radio telescopes, including India’slargest telescope uGMRT. These results provide a hint of evidence forthe relentless vibrations of the fabric of the universe, caused byultra-low frequency gravitational waves. Such waves are expected tooriginate 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 thegravitational wave spectrum.
Such dancing monster Black Hole pairs, expected to lurk in the centers ofcolliding galaxies, create ripples in the fabric of our cosmos, andastronomers call them nano-hertz gravitational waves as their wavelengths
can be many lakhs of crores of kilometers. The relentless cacophony ofgravitational waves from a large number of supermassive black hole pairs
create a persistent humming of our universe. The team, consisting ofmembers of European Pulsar Timing Array (EPTA) and Indian Pulsar
Timing Array (InPTA) consortia, published their results in two papers in
the Astronomy and Astrophysics journal and their results hint at thepresence of such gravitational waves in their data set.
These light-year-scale ripples can only be detected by synthesizing agalactic-scale gravitational-wave detector using pulsars-the only accessiblecelestial clocks for the humans. Pulsars are a type of rapidly rotating
neutron stars that are essentially embers of dead stars, present in ourgalaxy. 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 pulsars using the best radio telescopes of theworld including India’s premiere radio telescope, the uGMRT, situatednear Pune. In recent years, uGMRT has made significant contributions inprecisely recording the little flashes of pulsar’s radio beams so that we canuse pulsars as celestial clocks.
“According to Einstein, gravitational waves change the arrival times ofthese radio flashes and thereby affect the measured ticks of our cosmicclocks. 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-TIFR,
Pune who founded the InPTA collaboration during the last decade.
Scientists of the EPTA in collaboration with the Indo-Japanese colleaguesof the InPTA have reported detailed results of analysing pulsar datacollected 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 range and the flexibility of India’s largest
radio telescope - the uGMRT. The analysis of this unique data set hasrevealed that the measured rate of ticking of these cosmic clocks has
characteristic irregularities, common across the twenty-five pulsars thathave been monitored. This is consistent with the effect produced bygravitational waves at ultra-low frequency (waves that oscillate withperiods between one and ten years).
Prof Yashwant Gupta, Centre Director at NCRA-TIFR which runs theuGMRT, commented “It is fantastic to see our unique uGMRT data being
used for the ongoing international efforts on gravitational wave
astronomy. Carrying out high precision timing of pulsars for such apurpose was one of the crucial science targets of the major upgrade of theGMRT that we carried out during 2013-2019, and I am extremely pleased
to see it producing fruit within the first few years. The wideband receiversystems that we designed and built for the upgraded GMRT are the ones

that have enabled the high quality data from the lower radio frequencies
that the GMRT is contributing to the international collaboration”.
Not surprisingly, nano-hertz frequency gravitational waves carryinformation about some of the best-kept secrets of the Universe. The
cosmic population of black hole pairs with masses that are ten-to-hundredcrores 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 otherphenomena that may have taken place when the Universe was in its
infancy, just a few seconds old, also produce these waves at theseastronomically long wavelengths. According to Prof. A. Gopakumar,TIFR, Mumbai, and Chair of the InPTA consortium, “The resultspresented today mark the beginning of a new journey into the Universe tounveil some of these mysteries. More importantly, this is the first time thatan 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 ofthe pulsars, which have been going on for decades, are being comparedwith 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-size GW detector synthesised by incorporating 25
meticulously chosen pulsars in our Milky Way Galaxy makes it possible toaccess 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 theUnited States of America. “Interestingly, kilometer-sized LIGO sees
flashing gravitational wave signals that last for seconds. In contrast, ourgalaxy-sized PTA is beginning to sense a permanent vibration of the fabricof our universe or in other words a gravitational wave background at
nano-hertz frequencies. The resulting new window to the universe isexpected to get wider with new telescopes like the Square Kilometre Array(SKA) in the near future where India is expected to play a decisive role”,
says Prof. A. Gopakumar.

The current results are based on a coordinated observing campaign using
the five largest radio telescopes in Europe: the 100-m Effelsberg radio
telescope in Germany, the Lovell Telescope of the Jodrell Bank
Observatory in the United Kingdom, the Nancay Radio Telescope inFrance, the Sardinia Radio Telescope in Italy and the Westerbork
Synthesis Radio Telescope in the Netherlands. To complement this dataset, observations with the upgraded Giant Metrewave Radio Telescope inIndia were included in the analysis. Once a month, the European
telescopes are additionally added together to give an extra boost to thesensitivity.
“Our collaboration between colleagues across Europe, India and Japan not
only shows that an international effort is successful and very rewarding
scientifically, but we hope to also serve as a role model for the global IPTAefforts”, says Prof. Michael Kramer, Director - Max-Planck-Institut fürRadioastronomie, Bonn, Germany who along with Prof. Bhal Chandra
Joshi is instrumental in creating close collaborations between theEuropean and Indian PTAs.

The analysis of the European and Indian Pulsar Timing Array(EPTA+InPTA) data which is presented today has revealed the presenceof a common signal across the pulsars in the array which is broadly inagreement with being due to gravitational waves. “The signal is persistentthroughout the many years of monitoring of these pulsars, as if these
cosmic clocks are pitching and rolling in the waves of space-time. Thisemerging evidence is in line with what astrophysicists expect”, says Prof.

Keitaro Takahashi, Kumamoto University, Japan who leads Japaneseefforts with his Indian and European colleagues.
The EPTA+InPTA results are complemented by the coordinatedpublications 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 isseen by NANOGrav and consistent with results reported by the CPTA andPPTA.
“The results reported by the EPTA+InPTA collaboration aretantalizingly close to the discovery of nano-hertz gravitational waves andare the culmination of many years of efforts by many scientists including
early career researchers and undergraduate students. We are grateful to
the NSM computing facilities at IIT Hyderabad and IIT Roorkee, alongwith the computing infrastructure of NCRA, Pune and TIFR, Mumbai
which helped to unveil these results’’, states Prof. Shantanu Desai of IIT,Hyderabad.
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 theconstraints 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.
“This is really exciting!”, says Dr. Pratik Tarafdar of The Institute ofMathematical Sciences, Chennai, who feels that “we are within a whiskerof achieving such dynamic range where one can finally listen to the bass
sections in this cosmic gravitational-wave-symphony."In the subsequent years, the IPTA consortium expects to find gravitational
waves from individual pairs of monster black holes like the one suspected
to lurk in a very active galaxy called OJ 287. Such discoveries will
enhance astrophysical information that can be extracted from PTA
observations, similar to the iconic neutron star merger GW170817,
observed by LIGO and many telescopes in the electromagnetic spectrum
during 2017. “Along with the upcoming Square Kilometre Array, thispancontinental collaboration is expected to fully complement thediscoveries from future space-based and ground-based gravitational wave
observatories including LIGO-India”, believes Prof. Bhal Chandra Joshi.
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 fromKumamoto University, Japan.

Technical Press Release
Tuning in for the relentless humming of our universe

Indian and Japanese astronomers, which form the Indian
Pulsar Timing Array (InPTA) collaboration (https://inpta.iitr.ac.in/),have been using the premiere low frequency Indian radio telescope,
the upgraded Giant Metrewave Radio Telescope (uGMRT), located 80 kmfrom Pune, to make precise measurements of tiny effects on signals
from about 20 milli-second pulsars,nature’s best clocks, for the last 5years in the quest of these gravitational waves. Recently, they combined
their data with similar high-precision higher frequency data obtained using
5 large radio telescopes in Germany, France, United Kingdom and Italy
over the last 24 years to carry out one of the most sensitive searches forthese waves.

The results of this search by the InPTA and European Pulsar Timing
Array collaboration (EPTA), published on June 29, 2023, reveal evidenceof an emerging signal similar to nano-Hertz Gravitational Waves.
Results consistent with this search have also been publishedsimultaneously by three other international pulsar timing array (PTA)
experiments.
It is highly likely that the combination of the data from allthese experiments, including unique low frequency data contributedby the InPTA, under the auspices of International Pulsar Timing
Array (IPTA) later next year may lead to an unambiguous discoveryof these elusive gravitational waves with light-year wavelengths.
Gravitational waves, predicted by Einstein almost a century back,shrink and stretch the space and time around us. This ripple
in space-time is imperceptible to us, but can alter the tick-tockof a clock by a very small amount in Einstein's theory. PTA experiments
like the InPTA attempt to measure such changes in celestial clocks
called radio pulsars. These are faint radio sources, which emit clock
like radio pulses with a regularity comparable to the best terrestrial atomicclocks.
GWs passing near the earth can shorten or lengthen by a very tiny amountthe observed period of these millisecond period pulsars. A sensitive
telescope like the uGMRT can be used to count the number of pulses
received every second making it possible to measure these periods to a
fraction of a billionth of a second. The InPTA along with its international
partners in Australia, Europe and North America has been precisely doing
this for the last decade or more.
Now all clocks have imperfections. So, how do astronomers separate
the noise of these clocks from GWs? These GWs are emitted by massive
black hole binaries or pairs with a period of about decade or frequencyof nano-Hertz, which corresponds to the time each black hole of the pairtakes to go around its partner. While the GWs from a single pair is tooweak, GWs from many such pairs combine to form a much stronger "hum"or background. This "music" of black holes is not only very characteristic
in its "sound", it leaves different imprints on pairs of pulsar clocks
being used to "listen" to it. While the sound is similarly loud on two
pulsars separated by a small angle in the sky, its similarity or "correlation"is different as the angle between two pulsars changes. Astronomers use
these peculiarities of the GWs signal to differentiate the "black hole hum"from other types of "noise" which a single pulsar clock may show.
Amongst these noises are imperfections caused by propagation of radio
waves in the medium between us and the pulsar clock. It turns out thatthese are best measured at radio frequencies near 400 MHz, where theuGMRT is currently the world’s most sensitive radio telescope.
The InPTA astronomers have used this unique feature of the uGMRT,
apart from its sub-array and seamless wideband frequency coveragecapabilities to provide precise characterization of this noise to our
European colleague in the InPTA-EPTA data combination.
The InPTA-EPTA collaboration has used this combined data to
distinguish the propagation and other noise from the GWs signal.
In the recently published papers, these experiments have hinted
at a gradual emergence of GWs signal with the expected two peculiar
characteristics mentioned above - the spectrum and the spatial
correlation. While the "smoking gun" evidence is stillemerging, the results impact our understanding of how thegalaxies evolved in our universe and of the cosmic strings anddark matter. With the ongoing effort to combine the InPTA-EPTA
data with the North American Nano-Hertz Gravitational Wave
(NANOGrav), the Parkes Pulsar Timing Array (PPTA) and MeerTime
Pulsar Timing Array (MPTA) data, the IPTA hopes to open thisnew window to the Universe in the next year or so.
Contacts:
Keitaro Takahashi: Kumamoto University, Japan(keitaro@kumamoto-u.ac.jp)

P Arumugam: 8979890366, arumugam@ph.iitr.ac.in., IIT Roorkee
Bhal Chandra Joshi: 9422315542, bcj@ncra.tifr.res.in, DAE,
NCRA-TIFR, Pune/IIT Roorkee
Yashwant Gupta: ygupta@ncra.tifr.res.in NCRA-TIFR, Pune
A. Gopakumar: TIFR, Mumbai, 09869039269, gopu.tifr@gmail.com
Manjari Bagchi: IMSc, Chennai; 96771 46926
Shantanu Desai: IIT Hyderabad 9619127162 shantanud@phy.iith.ac.in
T. Prabu: 944 857 3994, prabu@rri.res.in, RRI, Bengaluru,
Mayuresh Surnis: IISER Bhopal, msurnis@iiserb.ac.in 7666183077
Yogesh Maan: ymaan@ncra.tifr.res.in NCRA–TIFR, Pune