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Sarah Vigeland.

Last summer, using data from the Green Bank Telescope and other radio telescopes, international research teams announced evidence that a low-frequency gravitational wave background permeates the universe. Where it comes from and what it means are compelling questions without definite answers—at least not yet. 

In 2015, the LIGO and Virgo instruments made the first direct detection of gravitational waves, a ripple in spacetime that radiated outward from the final milliseconds of a massive binary black hole merger. The “chirp” lasted around 0.2 seconds.

The 2023 detection was different. The data came from radio telescopes, not laser interferometers, and the signal wasn’t a chirp. “It’s like a humming, or like a rumbling, of gravitational waves through the whole universe,” says Sarah Vigeland, a physics professor at the University of Wisconsin - Milwaukee who co-led this research for the NANOGrav Collaboration. And the signal gets stronger with time.

Scientists don’t know what’s producing the hum yet, says Vigeland. One theory is, “When you combine the gravitational waves produced by all the supermassive binary black holes in the universe, you get this hum,” she says. There are other possibilities, too. The background may come from more exotic sources, such as cosmic strings, that are theorized to exist but not yet confirmed, she says. “That’s part of why we’re really excited.”

Radio telescopes can’t detect gravitational waves directly. They are sensitive to electromagnetic waves in the radio (and microwave) portion of the spectrum, not to ripples in spacetime. The connection lies in pulsars.

Pulsars are rapidly rotating neutron stars that emit beams of radio waves. The beams swing around like the beacon of a lighthouse with extreme precision, says Vigeland. “The kind that we use, millisecond pulsars, rotate hundreds of times per second, and you can time them down to better than a microsecond over the course of decades.” This stability is key to detecting gravitational waves.

Since gravitational waves distort spacetime, they impact how long it takes radio pulses to reach Earth. The changes are tiny but detectable. Other factors can also impact the timing of individual pulses, so researchers study arrays of pulsars to look for gravitational wave signatures. “If there’s a delay in one [pulse] because of, say, the interstellar medium, it’s not going to affect the other ones, because the pulses are traveling on different paths. But gravitational waves affect them all in a particular fashion,” Vigeland explains.

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In this artist’s interpretation, a pair of supermassive black holes (top left) emit gravitational waves that ripple through the fabric of space-time. Those gravitational waves compress and stretch the paths of radio waves emitted by pulsars (white). By carefully measuring the radio waves, a team of scientists recently made the first detection of the universe’s gravitational wave background. Credit: Aurore Simonnet for the NANOGrav Collaboration.

NANOGrav’s gravitational wave background result relied on data from 68 pulsars, but the number of pulsars they use continues to grow. Finding and characterizing more neutron stars is essential for reducing noise and advancing our understanding of the universe. Vigeland loves this element of the project. “Every stage of the experiment is more astrophysics. In LIGO, for example, they also have to worry about making their detector more sensitive. But when we do it, we're learning about astrophysics because our detector is made up of astrophysical objects,” she says.

Vigeland encourages students interested in gravitational wave astronomy to get good at learning—while skills like programming and statistics are helpful, the field is young and evolving. “It’s a really exciting time,” she says. “We really are opening up a new window on the universe.”