Astronomers have detected a rumbling “cosmic bass note” of gravitational waves thought to be produced by the slow-motion mergers of supermassive black holes across the universe.
The observations are the first detections of low-frequency ripples in the fabric of spacetime and promise to open a new window on the monster black holes lying at the centres of galaxies.
These objects are millions to billions times the mass of the sun and have played a profound role in shaping galaxies, but remain elusive because no light can escape their vicelike grip.
“This is huge news,” said Dr Stephen Taylor, chair of the North American Nanohertz Observatory for Gravitational Waves (Nanograv) consortium, which spearheaded the discovery, and an astrophysicist at Vanderbilt University in Nashville.
Dr Michael Keith, lecturer at Jodrell Bank Centre for Astrophysics, and member of the European group that independently confirmed the signal, stated: “The findings presented today represent the start of a journey to the cosmos in order to unravel some of its mysteries.”
“We are incredibly excited that after decades of work by hundreds of astronomers and physicists around the world, we are finally seeing the signature of gravitational waves from the distant universe.”
Albert Einstein first predicted the existence of gravitational waves a century ago, and a 2016 breakthrough by the US-based Laser Interferometer Gravitational-Wave Observatory (Ligo) delivered proof that space itself can be stretched and squeezed.
Until now, though, scientists have only been able to capture short “chirps” of gravitational waves linked to mergers of black holes or neutron stars only slightly larger than the sun.
The latest observations tune into a far deeper frequency range, with a single complete wave, travelling at the speed of light, taking roughly 30 years to pass by Earth. Scientists think this cosmic rumble is probably produced by the entire population of supermassive black hole binaries over roughly the past 8bn years.
“We think each pair contributes a little wave, which is added to a little wave of another, and all together that is what we may see right now – a sort of murmur of the entire population,” said Prof Alberto Vecchio of the University of Birmingham and a member of the European Pulsar Timing Array (EPTA).
The detection was made by meticulously monitoring more than 100 pulsars – exotic stars that spin hundreds of times each second, creating lighthouse-like beams of radio waves. The pulses produced by these stars are so steady that even small changes in the timing of the expanding and contracting fabric of space, can be detected.
In 2020, with 12 years of data, Nanograv scientists began to see hints of this gravitational hum, and reached out to separate teams in Europe, India, China and Australia, who each agreed to use their own data to provide independent corroboration.
Taylor said the likelihood of the latest results being down to chance is close to one in 10,000, making it compelling evidence, although this falls short of the one-in-a-million gold standard in physics for claiming evidence of a new phenomenon.
There is also an element of uncertainty about the source of the gravitational waves. Supermassive black hole death spirals are viewed as “the most plausible explanation”, Taylor said, but alternative possibilities remain on the table, including a gravitational imprint left on the universe shortly after the big bang.
Prof. Dame Jocelyn Bell Burnell who first discovered pulsars in graduate school, called the new discovery “a significant technical achievement”. “The effect is very small – one part in a thousand million million – so it’s precision work,” she said. “They have been very careful and cautious and not shouted prematurely.”
Prof Andrew Pontzen, a cosmologist at University College London, said: “It’s not often that we get a glimpse of the universe through a totally new lens, but after 15 years of patient work, Nanograv seems to be providing just that. It’s tremendously exciting to see initial evidence for these waves, which will eventually teach us an enormous amount about supermassive black holes, hundreds of millions of times the mass of the sun.”
The findings are outlined in a set of papers published on Thursday in the Astrophysical Journal Letters.
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