The Great Cosmic Wiggle: Quasars Reveal Gravity’s Hidden Rhythm

Gravitational waves are constantly washing over Earth, but an astrophysicist aims to capture them in an entirely new way—by watching distant quasars appear to wiggle due to spacetime distortions.

Using data from the Gaia satellite, he’s searching for three-dimensional effects that previous techniques might have missed.

Exploring a New Method to Detect Gravitational Waves

Astrophysicist Jeremy Darling from the University of Colorado Boulder is exploring a bold new method to measure one of the most mysterious forces in the cosmos: the gravitational wave background. These invisible waves ripple constantly through space, subtly stretching and squeezing the very fabric of the universe.

Published in The Astrophysical Journal Letters, Darling’s research could help unlock deep cosmic secrets, including how gravity behaves at its most basic level. By capturing more precise measurements of gravitational waves, scientists hope to uncover the many “flavors” of gravity that may exist across the universe.

“There is a lot we can learn from getting these precise measurements of gravitational waves,” said Darling, professor in the Department of Astrophysical and Planetary Sciences. “Different flavors of gravity could lead to lots of different kinds of gravitational waves.”

Black Hole Collisions and Cosmic Ripples

To understand these waves, imagine Earth as a tiny buoy floating in a vast, stormy ocean. That ocean is space itself, and gravitational waves are the swells passing beneath.

According to Darling, the waves come from some of the most dramatic events in the universe. Throughout cosmic history, gigantic black holes have spiraled toward each other, eventually crashing together in colossal collisions. These epic mergers are thought to unleash powerful gravitational waves that ripple across the cosmos.

These waves continuously wash over Earth, but they’re incredibly subtle. The ones Darling aims to measure move slowly, taking years or even decades to pass by. Although we can’t feel them, these gentle undulations may hold the key to understanding the universe in a whole new way.

Pulsars: Clocks of the Cosmos

In 2023, a team of scientists belonging to the NANOGrav collaboration achieved a coup by measuring that cosmic wave pool. The group recorded how the universe’s gravitational wave background stretched and squeezed spacetime, affecting the light coming to Earth from celestial objects known as pulsars, which act somewhat like cosmic clocks.

But those detailed measurements only captured how gravitational waves move in a single direction—akin to waves flowing directly toward and away from a shoreline. Darling, in contrast, wants to see how gravitational waves also move from side to side and up and down compared to Earth.

Quasars and a New Technique

In his latest study, the astrophysicist got help from another class of celestial objects: quasars, or unusually bright, supermassive black holes sitting at the centers of galaxies. Darling searches for signals from gravitational waves by precisely measuring how quasars move compared to each other in the sky. He hasn’t spotted those signals yet, but that could change as more data becomes available.

“Gravitational waves operate in three dimensions,” Darling said. “They stretch and squeeze spacetime along our line of sight, but they also cause objects to appear to move back and forth in the sky.”

Galaxies in Motion

The research drills down on the notoriously tricky task of studying how celestial objects move, a field known as astrometry.

Darling explained that quasars rest millions of light-years or more from Earth. As the glow from these objects speeds toward Earth, it doesn’t necessarily proceed in a straight line. Instead, passing gravitational waves will deflect that light, almost like a baseball pitcher throwing a curve ball.

Those quasars aren’t actually moving in space, but from Earth, they might look like they are—a sort of cosmic wiggling happening all around us.

Wiggling Lights and Massive Challenges

“If you lived for millions of years, and you could actually observe these incredibly tiny motions, you’d see these quasars wiggling back and forth,” Darling said.

Or that’s the theory. In practice, scientists have struggled to observe those wiggles. In part, that’s because these motions are hard to observe, requiring a precision 10 times greater than it would take to watch a human fingernail growing on the moon from Earth. But our planet is also moving through space. Our planet orbits the sun at a speed of roughly 67,000 miles per hour, and the sun itself is hurtling through space at a blistering 850,000 miles per hour.

Earth’s Wild Ride Through Space

Detecting the signal from gravitational waves, in other words, requires disentangling Earth’s own motion from the apparent motion of quasars. To begin that process, Darling drew on data from the European Space Agency’s Gaia satellite. Since Gaia’s launch in 2013, its science team has released observations of more than a million quasars over about three years.

Darling took those observations, split the quasars into pairs, and then carefully measured how those pairs moved relative to each other.

Awaiting Future Revelations

His findings aren’t detailed enough yet to prove that gravitational waves are making quasars wiggle. But, Darling said, it’s an important search—unraveling the physics of gravitational waves, for example, could help scientists understand how galaxies evolve in our universe and help them test fundamental assumptions about gravity.

The astrophysicist could get some help in that pursuit soon. In 2026, the Gaia team plans to release five-and-a-half more years of quasar observations, providing a new trove of data that might just reveal the secrets of the universe’s gravitational wave background.

“If we can see millions of quasars, then maybe we can find these signals buried in that very large dataset,” he said.

Reference: “A New Approach to the Low-frequency Stochastic Gravitational-wave Background: Constraints from Quasars and the Astrometric Hellings–Downs Curve” by Jeremy Darling, 27 March 2025, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/adbf0d

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