On Wednesday evening, an international consortium of research collaborations revealed compelling evidence for the existence of a low-pitch hum of gravitational waves reverberating across the universe.
The scientists strongly suspect that these gravitational waves are the collective echo of pairs of supermassive black holes — thousands of them, some as massive as a billion suns, sitting at the hearts of ancient galaxies up to 10 billion light-years away — as they slowly merge and generate ripples in space-time.
“I like to think of it as a choir, or an orchestra,” said Xavier Siemens, a physicist at Oregon State University who is part of the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, collaboration, which led the effort. Each pair of supermassive black holes is generating a different note, Siemens said, “and what we’re receiving is the sum of all those signals at once.”
The findings were highly anticipated, coming more than 15 years after NANOGrav began taking data. Scientists said that, so far, the results were consistent with Albert Einstein’s theory of general relativity, which describes how matter and energy warp space-time to create what we call gravity. As more data is gathered, this cosmic hum could help researchers understand how the universe achieved its current structure and perhaps reveal exotic types of matter that may have existed shortly after the Big Bang 13.7 billion years ago.
“The gravitational-wave background was always going to be the loudest, most obvious thing to find,” said Chiara Mingarelli, an astrophysicist at Yale University and a member of NANOGrav. “This is really just the beginning of a whole new way to observe the universe.”
Gravitational waves are created by any object that spins, such as the rotating remnants of stellar corpses, orbiting black holes, or even two people “doing a do-si-do,” Mingarelli said. But unlike other types of waves, these ripples stretch and squeeze the very fabric of space-time, warping the distances between any celestial objects they pass by.
“It sounds very sci-fi,” Mingarelli said. “But it’s for real.”
Gravitational waves were first detected in 2016 as audible chirps by the Laser Interferometer Gravitational-Wave Observatory, or LIGO, collaboration; the breakthrough solidified Einstein’s theory of general relativity as an accurate model of the universe and earned the project’s founders the Nobel Prize in physics in 2017. But LIGO’s signals were mostly in the frequency range of a few hundred hertz, and were created by individual pairs of black holes or neutron stars that were 10 to 100 times as massive as our sun.
In contrast, the researchers involved in this work were looking for a collective hum at much lower frequencies — one-billionth of one hertz, far below the audible range — emanating from everywhere all at once.
At the lowest frequencies, that hum is so loud “that it could be coming from hundreds of thousands, or possibly a million, overlapping signals from the cosmic merger history of supermassive black hole binaries,” Mingarelli said.
The signal was discovered by studying the behavior of rapidly spinning stars called pulsars, using a method that in 1993 earned two scientists the Nobel Prize in physics for indirectly measuring the effects of gravitational waves.
The NANOGrav team simultaneously published four studies in The Astrophysical Journal Letters, as well as two papers on the preprint server arXiv.org, detailing the collection and analysis of the data and the different interpretations of the result.
If the signal does arise from orbiting pairs of supermassive black holes, studying the gravitational-wave background will shed light on the evolutionary history of these systems and the galaxies surrounding them. But the gravitational-wave background could also be coming from something else, like hypothetical cracks in space-time known as cosmic strings.
Or it could be a relic of the Big Bang, akin to the cosmic microwave background, which led to fundamental discoveries about the structure of the universe to within 400,000 years of its beginning. The gravitational-wave background would be an even better primordial probe, Mingarelli said, because it would have been emitted almost instantaneously.
In 2020, after more than 12 years of gathering data, the NANOGrav team released results from monitoring the timing of 45 pulsars. Even then, Siemens said, the researchers saw tantalizing hints of a gravitational-wave background, but they needed to track more pulsars for longer amounts of time to confirm that they were indeed correlated, and to claim a discovery. So the NANOGrav team approached colleagues through the International Pulsar Timing Array — an umbrella organization that includes collaborations based in India, Europe, China, and Australia — and coordinated an effort to uncover the gravitational-wave background together.
Fast-forward to Wednesday: Each collaboration is now publishing results from independently collected data, all of which support the existence of a gravitational-wave background. The NANOGrav team has the largest data set, with 15 years of measurements from 67 pulsars, each monitored for at least three years.
The findings carry a confidence level in the range of 3.5- to 4-sigma, just shy of the 5-sigma standard generally expected by physicists to claim a smoking-gun discovery. That means the odds of seeing a result like this randomly are about 1 in 1,000 years, Mingarelli said. “That’s good enough for me, but other people want once in a million years,” she said. “We’ll get there eventually.”