Scientists at the University of Colorado Boulder think they may have cracked a mystery that has puzzled astronomers since the first detection of the universe’s gravitational wave background—a faint but constant hum of ripples moving through space and time.
Gravitational waves are tiny distortions in space-time caused by massive cosmic events.
According to astrophysicist Julie Comerford, they move through the universe all the time, gently “jiggling” everything they pass through, even us—though the effect is far too subtle for humans to feel.
In a new study published in The Astrophysical Journal, Comerford and her colleague Joseph Simon explored why the gravitational wave background detected in 2023 was much stronger than scientists had predicted.
That detection, made by several international teams including North American Nanohertz Observatory for Gravitational Waves, was a major milestone—but it came with a surprise.
To understand the mystery, the researchers looked closely at how galaxies grow and merge over billions of years.
Across the universe, galaxies are constantly colliding and combining. At the center of nearly every large galaxy sits a supermassive black hole, with a mass millions or even billions of times greater than the Sun.
When two galaxies merge, their central black holes begin circling each other like cosmic dancers.
Over time, they spiral inward and eventually collide. Each of these enormous mergers sends out gravitational waves. When countless mergers happen across the universe, their waves overlap, creating a background signal—similar to how waves from many swimmers combine in a busy pool.
For a long time, scientists assumed that only the biggest black holes mattered for this background signal. Smaller black holes were thought to be too weak to make a real difference. But Comerford and Simon suspected otherwise.
Their work focused on what happens to black holes during galaxy mergers.
As galaxies collide, gas flows toward the black holes, forming a thick, doughnut-shaped ring around them. Surprisingly, the researchers found that smaller black holes may gain mass faster than larger ones during this process.
The smaller black hole tends to orbit closer to the gas-rich region, allowing it to “feed” more efficiently.
Using computer simulations and real galaxy data, the team adjusted their models so that smaller black holes grew just 10 percent more than their larger partners. That small change made a big difference. Suddenly, the predicted strength of the gravitational wave background matched what NANOGrav observed.
This result suggests that smaller black holes play a much larger role in shaping the universe than previously believed. While the study doesn’t answer every question, it offers a promising explanation and points scientists toward what to look for next.
Comerford and her team are now observing real galaxies in the process of merging to test whether nature behaves the same way as the simulations.
Ultimately, this research could help answer one of astronomy’s biggest questions: how the earliest, tiny galaxies gave rise to the massive black holes that dominate the universe today.
Source: KSR.
