Gravitational Waves Create a Constant ‘Hum’ Across the Universe

Breakthrough research suggests the continuous ripples in spacetime could be caused by pairs of supermassive black holes, spiraling toward collisions

The wide grey dish of a radio telescope, with trees and blue sky in the background
The Arecibo Observatory in Puerto Rico, one of the radio telescopes used to detect the pulses from pulsars in the new research. The telescope started to fall apart in 2020 and was decommisioned. Stocktrek via Getty Images

The universe is filled with a constant hum of gravitational waves, according to new observations from separate teams of scientists around the world.

Gravitational waves are ripples in the fabric of the universe, caused by the motion of giant, accelerating celestial bodies. They travel through space-time like waves in an ocean, but at the speed of light. In 2015, scientists at the Laser Interferometer Gravitational-Wave Observatory detected these waves for the first time, in a massive breakthrough that rocked the world of astrophysics. That finding provided crucial support for Albert Einstein’s theory of relativity, which predicted the existence of these waves.

But the new evidence points to a different kind of gravitational wave—a constant, ambient hum that’s known as the gravitational wave background. These waves could be coming from pairs of supermassive black holes, the researchers say, and they suggest the fabric of the universe is continuously in motion.

“These observations reveal a rolling, noisy universe alive with the cosmic symphony of gravitational waves,” Sean Jones, assistant director for the Directorate of Mathematical and Physical Sciences at the National Science Foundation, said at a public news briefing Thursday. It “shatters the perception of a static universe.”

Scientists unveiled the findings Wednesday evening in a collection of papers from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), published in the Astrophysical Journal Letters. Separate teams from Europe, India, China and Australia also released papers describing the same hum.

The breakthrough evidence came from 15 years of documenting radio waves that emanate from the remains of exploded stars, called pulsars.

In 1967, astrophysicist Jocelyn Bell discovered pulsars, and other scientists later realized they could be used to hunt for gravitational waves, according to Quanta Magazine’s Jonathan O’Callaghan. Pulsars spin hundreds to thousands of times per second while emitting a powerful beam of radio waves. To a radio telescope on Earth, one of these dead stars is perceived as a pulse each time its beam swings by our planet. The timing of these pulses is extremely precise and predictable—so any slight perturbations in these pulses would be evidence of gravitational waves rippling through spacetime, changing the distance between Earth and the pulsar.

“We can create these models that basically let us know the time of [a pulse’s] arrival to precisions that rival atomic clocks,” Thankful Cromartie, an astrophysicist at Cornell University and member of NANOGrav, tells Scientific American’s Meghan Bartels. “So, we know when there’s something happening, something at play that’s causing the pulsars to tick a little bit off-time.”

From measurements of 67 pulsars taken by the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico, the scientists say they have picked up on these telltale perturbations, just as the theory of relativity predicted.

“We’re very happy to announce that our hard work has paid off,” Stephen Taylor, the NANOGrav chair, said at a press briefing this week, per Wired’s Ramin Skibba.

Still, the finding has a slight caveat: A statistical measure of the possibility the results are due to random chance is slightly larger than what physicists require to definitively call a result a discovery, notes the New York Times’ Katrina Miller. But the scientists expect to find more evidence of the gravitational wave background in the future.

“We’ll get there eventually,” Chiara Mingarelli, an astrophysicist at Yale University and a member of NANOGrav, tells the publication. The fact that teams from around the world released similar findings from observations with different telescopes adds to the strength of the evidence, writes Wired.

The researchers hope to further study these pulsar signals to learn more about the evolution of the universe, colliding galaxies and merging black holes, according to a statement from NANOGrav.

“It’s incredibly exciting, because we think we’re starting to open up this new window on the gravitational-wave universe,” Sarah Vigeland, an astrophysicist at the University of Wisconsin-Milwaukee and a member of NANOGrav, tells Scientific American.

Now, the NANOGrav team wants to focus on finding the source of this ever-present hum. They think these waves could come from pairs of supermassive black holes—each with the mass of millions to billions of suns—spiraling around each other as their host galaxies collide. Perhaps, scientists say, the overlapping gravitational waves from many of these pairs form the universe’s background hum.

“Our earlier data told us that we were hearing something, but we didn’t know what. Now we know that it’s music coming from the gravitational universe,” Scott Ransom, an astronomer at the National Radio Astronomy Observatory, says in the statement. “As we keep listening, we’ll likely be able to pick out notes from the instruments playing in this cosmic orchestra.”

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