Study offers possible solution to a gravitational wave mystery

Scientists at the University of Colorado Boulder may have solved a pressing mystery about the universe’s gravitational wave background.

That’s the name for the ripples in space and time that move constantly through the cosmos and “jiggle us almost like Jell-O,” according to CU Boulder astrophysicist Julie Comerford.

The study, published recently in “The Astrophysical Journal,” reveals new insights into the evolution of the universe—namely, how smaller galaxies may have coalesced over billions of years to form larger and more complex galaxies like the Milky Way.

Comerford explained that, at any one time in the universe, countless galaxies are in the process of merging.

Each of those galaxies has an aptly named supermassive black hole at its center. As galaxies merge, these black holes spin around each other, whipping in circles until they eventually smack together. The resulting collisions create waves in space and time that are so subtle humans never feel them.

“You can picture lots of people in a swimming pool,” said Comerford, lead author of the new study and professor in the Department of Astrophysical and Planetary Sciences at CU Boulder. “They’re all creating their own waves, and the waves overlap. That’s what the gravitational wave background is like.”

In 2023, several international collaborations, including the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) experiment, reported that they had detected the gravitational wave background for the first time.

There was just one problem: Based on the groups’ measurements, those waves were much larger than scientists had estimated. No one knew why.

In the new study, Comerford and study co-author Joseph Simon, a former postdoctoral researcher at CU Boulder, may have found the explanation.

Using observations of real galaxies and computer simulations the team discovered something that researchers hadn’t accounted for: When a smaller supermassive black hole merges with a larger one, the smaller black hole seems to gain a lot of mass.

That extra mass makes a difference. Just like swimmers doing cannonballs in a pool, larger supermassive black holes produce larger gravitational waves.

“We had a prediction for what the gravitational wave background should be, and what NANOGrav found was larger than expected,” Comerford said. “It was a surprise and a fun new puzzle to figure out.”

Growth spurts

Supermassive black holes, like galaxies themselves, come in all sorts of sizes. Some of these celestial objects are truly humongous, with a mass equal to billions of Earth’s suns. Others are still big, but slightly less so, with a mass millions of times larger than the sun.

For years, many scientists studying the gravitational wave background didn’t believe those smaller black holes mattered, Comerford said. They were too little, the thinking went, to make a meaningful contribution to the gravitational wave background.

Comerford and Simon weren’t so sure.

In part, that’s because galaxy mergers can be messy affairs. When two galaxies come together, gas from those galaxies begins to funnel toward the supermassive black holes at their centers. This gas forms a doughnut-shaped cloud outside the black holes spiraling around each other. Some of that gas falls back into the black holes and makes them larger in the process.

But previous simulations suggested something surprising: The black holes in a merging pair may not grow at the same pace.

“The more massive black hole sits closer to the center of the doughnut where there isn’t much gas,” Comerford said. “The smaller black hole is further out, so it’s closer to where the gas is.”

The beginning

That difference in growth rates, or what the scientists call “preferential accretion,” could matter a lot.

In the current study, Comerford designed a detailed set of equations capturing the physics of how galaxies merge. The group then adjusted those equations to make smaller black holes grow 10% more than larger ones.

That single tweak was enough to make estimates of the gravitational wave background line up with measurements from the NANOGrav experiment.

“They start out little, but because the little ones grow the most, they shouldn’t be discounted,” Comerford said.  

She noted that the study doesn’t completely solve the mystery: Her team has launched a new effort to observe real galaxies in the act of merging to see if their physics line up with what the simulations found.

The effort, she said, is part of a larger push to understand some of the most fundamental questions about the universe. That includes how “primordial” galaxies at the dawn of the universe, which were tiny and made up mostly of gas, may have built the gigantic black holes that exist today.

“I’ve spent my career studying supermassive black holes, and we don’t even know how they form,” Comerford said.