Decoding the hum of the universe

A consortium of astronomers from across the world, including from India, have detected the signature for the background hum of the universe. Called the gravitational wave background (GWB), these are ripples in the fabric of spacetime pervading the space around us. The global collaboration of radio astronomers called the International Pulsar Timing Array (IPTA) announced the detection on June 29, 2023.

In September 2015, astronomers observed the first direct evidence of a gravitational wave using the Laser Interferometer Gravitational-Wave Observatory (LIGO). The wave was generated when two stellar-mass blackholes (blackholes tens of times heavier than our sun) merged.

But gravitational waves, much like their electromagnetic counterparts, have a spectrum ranging from very high frequencies to very low frequencies. "Imagine two massive objects, like black holes or neutron stars, that orbit each other and eventually merge. They emit low-frequency, long-wavelength gravitational waves when they are orbiting far apart. As they get closer, the frequency of the waves also gets higher, while the wavelength gets smaller," says Prof Manjari Bagchi, a professor at the Institute of Mathematical Sciences, Chennai, and part of the Indian Pulsar Timing Array (InPTA).

LIGO and other gravitational wave detectors based on Earth can only detect the highest frequency waves. To detect the really low-frequency waves, we need much larger detectors—the size of galaxies. 

Different sources around the universe generate many low-frequency gravitational waves, mainly theorised to result from the merger of supermassive black holes (millions or even a billion times heavier than our sun) like the one at the centre of our Milky Way. Much like how raindrops falling in a puddle create many tiny waves that interfere, creating a chaotic-looking puddle, this stream of gravitational waves from sources around the universe would then interfere with each other and create a continuous noise of gravitational waves that can be detected all around us.

This background noise of low-frequency gravitational waves called the gravitational wave background (GWB), is what astronomers have now seen signatures of. Its detection, however, required us to turn the entire Milky Way galaxy into a giant gravitational wave detector.

Astronomers observed pulsars—the neutron stars with enormous beams of high-energy particles shooting out from their magnetic poles to detect the GWB. The pulses of a pulsar are known to be extremely precise in their period, making them useful as accurate cosmic clocks. This precision of the pulsars is precisely what helped the astronomers detect the GWB.

"The time between the pulses from pulsars is constant. If the pulses reach us at different time intervals, then there must be some change in the distance between the earth and the pulsar. Once we model for all other changes, any slight changes in the timing between pulses have to be because of the change in the curvature of spacetime, which is a gravitational wave," explains Prof Bagchi. 

Pulsar Timing Array

To ensure it's not an anomaly causing the change in timing, astronomers observe an array of pulsars - 87 to be exact. Hence the name Pulsar Timing Array. "If there was a change in just one of the pulsar’s timings, then it can be attributed to a change in the pulsar itself, like a starquake (similar to an earthquake but happening on a star) or something. But when all the pulsars are showing a predictable change in their timing, it has to be a change in the curvature of space itself, and hence has to be a gravitational wave," she adds.

The low-frequency gravitational waves also have extremely long wavelengths. This means the distance between the crest and the trough of a single wave can be light years long. To observe the effect of such a long wave on a pulsar, the detectors must be placed lightyears apart. The solution was to use pulsars, which are far apart from each other and the Earth, thus making a galactic-sized detector.

Radio telescopes had to be placed around the earth to observe the pulsars spread across the skies. The International Pulsar Timing Array (IPTA), a multi-institutional, multi-telescope collaboration, was set up to achieve this. It comprises the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the Parkes Pulsar Timing Array (PPTA) in Australia, and the Indian Pulsar Timing Array (InPTA).

The InPTA is a collaboration between Indian and Japanese astronomers and uses the upgraded Giant Meter Wave (uGMRT) radio telescope based in Pune to observe the pulsars. Data from the InPTA were combined with those of the EPTA to observe the signature for the GWB. The result agrees with the results of the NANOGrav data and the PPTA data.

The detection of the GWB opens up a whole new set of tools to observe the universe around us. Although we have been able to detect them, our instruments are not yet sensitive enough to gather information about the source of each individual wave from the noise of the GWB. New additions to the IPTA, like the Chinese and African pulsar timing arrays, are in the pipeline to help increase the detection sensitivity. 

Such detection wouldn’t have been possible or would have taken much longer without the global collaboration and data sharing that the IPTA enabled. "The future lies in combining global efforts. Even within India, although the uGMRT is being operated by NCRA, Pune, the scientists working on it come from institutes all over the country and Japan. So, humanity should work together," concludes Prof Bagchi, underlining the importance of collaborations and open data sharing.

(The author is with Research Matters)