Bengaluru: For the first time, physicists have detected strong evidence of the long-hypothesised background “hum” of nanohertz gravitational waves — omnipresent ultra-low frequency ripples in spacetime spreading throughout the universe, from multiple sources all around us, continuously. The findings were made through a global collaboration, with unique contribution from Indo-Japanese Pulsar Timing Array (InPTA) teams through the Pune-based GMRT telescope.
A number of research institutes from India are a part of the InPTA: National Centre for Radio Astrophysics-Tata Institute of Fundamental Research (NCRA-TIFR) which leads the project, TIFR-Mumbai, IMS Chennai, Indian Institute of Technology (IIT) Hyderabad, Indian Institute of Science Education and Research (IISER) Bhopal, Raman Research Institute (RRI) — participating with active work. Students from IISER-Mohali, Indian Institute of Science (IISc) Bengaluru, IISER- Kolkata, and IISER-Thiruvananthapuram have contributed to the work.
IIT-Roorkee and IIT-Hyderabad were actively involved in the computing, with their supercomputers PARAM Ganga and PARAM Seva, respectively. Japanese team members come from Kumamoto University and Osaka University.
The scientists released the dataset through multiple papers published or uploaded simultaneously in The Astrophysical Journal Letters and the Astronomy & Astrophysics Journal Thursday, as well as on the e-print archive arXiv. There are more papers expected to come out in the next few weeks as a part of the data pipeline.
Future work into these findings is expected to confirm at least one source of this constant background signals, which will in turn help understand the evolution of early galaxies and our own Milky Way.
These background gravitational waves are of frequencies in the order of nanohertz, with high wavelengths stretching up to lakhs and crores of kilometers, and a period of oscillation spanning years to decades.
Such waves are predicted by Einstein’s theory of relativity to be generated by two extreme supermassive black holes orbiting each other and moving in large in-spiralling arcs through spacetime, eventually colliding. But source objects for this detected background signal have not been detected yet.
The findings were made after an observation period of nearly fifteen years, starting 2007-2008, using “cosmic clocks” called pulsars.
Since ground-based interferometers, such as the US-based Laser Interferometer Gravitational-Wave Observatory (LIGO), are not sensitive enough to detect these ultra-low frequency waves, astronomers used pulsars — rapidly spinning neutron stars that emit bursts of energy with a precise period. These were spread out across the cosmos, thus turning the entire Milky Way galaxy into a giant interferometer for the study.
At the end of 15 years, the scientists were able to confirm the arrival of a wave at 75 pulsars, by confirming its impact on them; the arrival of a pulse from each pulsar changed by a mere 1 microsecond. The scientists describe the precision levels as comparable to measuring the distance to the Moon to within a thousandth of a millimeter
“This is one of the most precise measurement ever made in astrophysics,” said Bhal Chandra Joshi, senior radio astronomer and the founding director of the Indian Pulsar Timing Array, which was a part of the effort. “We can get precisions to the level of a few centimetres for the position of a black hole.”
The work is a massive international collaboration by the International Pulsar Timing Array (IPTA) consortium, made up of the Parkes PTA (PPTA) from Australia, the European PTA (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Indo-Japanese PTA (InPTA), and the Chinese PTA (CPTA), and involving hundreds of scientific institutes.
“This effort is a fantastic combination of hard work of several graduate, postgraduate, and PhD students, as well as postdoctoral researchers, as well as researchers from Japan,” said Joshi.
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What are nanohertz gravitational waves?
While stars and supernovae can be observed through the electromagnetic spectrum, the effects of intensely massive bodies like black holes and neutron stars, that disrupt the very fabric of spacetime, can be detected as gravitational waves traveling at the speed of light. These waves bend and flex spacetime, altering both space and time by a fraction of a second.
Interferometers such as LIGO are devices that detect these waves by observing periodic signals with high precision and catching a minute variation in the time they take to reach their target — which would have been caused by the flexing of spacetime by a passing gravitational wave. Multiple interferometers help improve accuracy of the signal and also help triangulate the source.
Black holes are the most massive — meaning heaviest, or carrying the most mass — objects in the universe. Their mass is measured in terms of the mass of the sun for comparison, and they range anywhere from three solar masses (smallest black hole known) to 100000000000 (10 thousand crores) or 1011 solar masses.
The larger extremes are called supermassive black holes (SMBHs), and they are present at the centre of all galaxies. Our own Milky Way orbits an SMBH called Sgr A* (pronounced Sagittarius A star).
To understand how these massive bodies affect the fabric of spacetime, scientists use a rubber sheet example where each astronomical body is a proportionally massive ball on a rubber sheet. More massive objects cause a larger depression in the fabric, pulling objects farther away towards themselves, denoting gravity.
When the most extreme of these SMBHs orbit each other and collide, they continuously emit gravitational waves at extremely low frequencies, measured in nanohertz. Unlike smaller black holes and neutron star combinational pairs detected by LIGO at much higher frequencies and eventually end at collision, these are a persistent and continuous signals with no end in sight.
A. Gopakumar, the project lead from InPTA, who started the project along with Joshi, calls these SMBHs “hyper massive black holes”.
“The fabric of spacetime has four dimensions, the fourth being time,” he explained. “Distortions in these are perceived as gravitational waves. What we have found here is a persistent ringing of such waves all around us in the universe, and which is not going to end in the foreseeable future in human timescales.”
On top of that, he added, is the fact that the source for such waves is not just one or two SMBHs, but millions of hyper-massive blackhole pairs emitting at this frequency.
“There could be alternative sources, but it’s quite a demanding process and the simplest explanation is hyper-massive binary black holes. And that’s what we expect to find,” Gopakumar said.
When NANOGrav released their 12.5-year dataset in 2020, there were early hints of this background signal without actual evidence, explained Nihan Pol, nano gravitational wave researcher at Vanderbilt University, USA, and an author on the papers. “In this 15-year dataset, we now have more data, and a lot more pulsars, and we see a subsequent increase in the measured evidence for the presence of the nanohertz background.”
How do pulsars detect GWs?
Pulsars (pulsating radio sources) are rapidly spinning neutron stars that emit pulses of energy with high precision.
“They spin at the rate of hundreds or thousands of times per second, with pulses having time periods in milliseconds,” explained Gopakumar. They are so precisely consistent that they are used as a reference for other measurements involving periods, and were used so for this experiment as well.
In 1967, astronomer Jocelyn Bell Burnell made the first detection of a radio pulsar while studying data from a new radio telescope she had helped build. The continuous periodic signal led her and her colleagues to nickname the signal Little Green Men or LGM, as a play on extraterrestrial signal, although they were confident they would find the source.
And they did, as the team discovered their second pulsar, and more. Later in 1974, her supervisor Antony Hewish and physicist Martin Ryle developed revolutionary radio telescopes and won the joint Nobel Prize in Physics for their “decisive role in the discovery of pulsars”.
For Bell Burnell, the past few days have been “manic”.
“This is not so much a pulsar discovery itself as using pulsars to make another discovery. Because of the precision of the pulses from a pulsar you can tell if the pulsar moves (or is moved). What has been seen is that the pulsars are ‘bobbing’ around in the gravitational waves (as a duck would bob around in water waves). So the pulsar enables us to ‘see’ the gravitational wave,” she told ThePrint.
A change caused to the time period of a pulsar by a passing gravitational wave is detectable only on a multi-year scale because it results in a such a minute change noticeable only at multiple digits after the decimal point.
“Pulsar timing arrays are multi-decade projects, and gain better sensitivity to lower frequencies as they collect more pulsar timing data. The nanohertz gravitational wave background reported in this work also grows louder at lower frequencies,” said Pol. “Put together, this means that the evidence for this nanohertz GWB should grow with increasing timespans of pulsar timing datasets, which is exactly what we are seeing in NANOGrav data.”
As these pulses travel through the interstellar medium, which is made up of gas, dust, and plasma, noise is introduced into the signal.
Here is where the crucial contribution by the InPTA comes in.
The InPTA utilises the unique properties of the Pune-based Giant Metrewave Radio Telescope (GMRT), which is operated by National Centre for Radio Astrophysics (NCRA-TIFR) near Pune. GMRT has been operational since 2016, collecting data, but joined IPTA only in 2020.
“Our niche is that we are the only ones who can observe at two ranges of low radio frequencies that no one else is operating. This helps distinguish the noise from the interstellar medium from the actual pulsar signal,” explained Mayuresh Surnis, nanohertz GW researcher, professor at IISER-Bhopal and an author on the Indo-European papers. “With GMRT data, other sources that cause changes to a pulsar’s emission are also eliminated.”
The EPTA sets, when combined with the InPTA sets, improved in precision, providing cleaner data about pulses, which in turn helped calculate characteristics of the Nano GW that would have passed through the source pulsar.
While InPTA’s Indian and Japanese researchers use the upgraded GMRT, PPTA uses the Parkes Radio Telescope, EPTA uses the five largest European telescopes — Lovell Telescope, Westerbork Synthesis Radio Telescope, Effelsberg Telescope, Nancay Radio Telescope, Sardinia Radio Telescope, NANOGrav uses data collected by the now defunct Arecibo telescope and Green Bank radio telescope, and CPTA uses the Five-hundred-meter Aperture Spherical radio Telescope(FAST).
Significance and future
The released datasets contain individual datasets from PPTA, NANOGrav, and combined datasets from EPTA and InPTA. The publication of the papers were timed with each other. Now, the IPTA team has already begun to combine results from all PTAs, which would result in more accurate findings that would help trace the sources for some of these nanohertz GWs.
“Imagine someone drops multiple stones in a pond. You come by after a few minutes, and you can see now see the ripples all over but you have no idea where the rocks were dropped. That’s the signal we’re trying to detect,” said Surnis.
The combined dataset will offer information into all 75 pulsars used in the study over the last 15 years and their data, and help triangulate sources in the sky.
Everyone involved is confident that the search will lead to a black hole-black hole pair, with results expected to be announced in one of the next two dataset releases, within the next two years.
“The gold standard in physics for detecting a new phenomenon is at five sigma. We are not there yet, but are confident we will be when we get results from the combined dataset,” said Gopakumar.
The scientists are not entirely discounting alternatives.
“One possible explanation is a population of inspiraling supermassive BH binary systems. But other explanations, some of which invoke exotic particles like axions, can also explain the signal we see in this dataset. Right now it is too early to say for sure if the waves are being produced by one source or the other, and future, longer, more sensitive datasets should let us make a more conclusive statement,” said Pol.
Detecting the source would make it easy to calculate the mass of these likely SMBHs through Einstein’s equations, which would in turn make their evolution understandable — and whether they can solve a big mystery.
“There are large galaxies today around us and we don’t exactly know how they formed,” explained Surnis. “The current theory is that smaller galaxies with smaller black holes merged to form larger spiral galaxies such as ours. We can understand how today’s galaxies evolved by understanding these early SMBHs that at one point emitted these nanohertz GWs.”
Joshi remembered the early days of difficulty in requesting to operate telescopes such as GMRT because request proposals need to include a reasonable demonstration of expected results. Since Nanohertz GW observations occur over a period of 12 to 15 years, he had started experimenting at the Ooty radio telescope before 2016.
Now knowing the advantage GMRT offers to pulsar data, Joshi believes InPTA will play a key role in the future of nanohertz gravitational wave astronomy.
“Findings such as these are a testament to relentless inquiry and curiosity, which is what has led to all of human progress. Fundamental research has spurred the development of many technologies that are in daily use today,” said Joshi. “For nanohertz gravitational wave astronomy, this is just the beginning. In the decades to come, there will be many more such discoveries.”
(Edited by Poulomi Banerjee)