Bold claim: a faint cosmic hum could be the key to finally unraveling the mystery of how fast our universe is expanding. For decades, scientists have known that the cosmos grows larger over time, and they quantify this rate with the Hubble constant. Different methods should agree because they’re rooted in the same physics, but instead they don’t. The so-called Hubble tension arises because early-universe measurements disagree with those from the more recent universe, leaving cosmologists with a stubborn puzzle at the forefront of modern science.
A collaborative team from The Grainger College of Engineering at the University of Illinois Urbana-Champaign and the University of Chicago has proposed a fresh, gravitational-wave–driven approach to pin down the Hubble constant. Gravitational waves are minuscule ripples in spacetime produced by cataclysmic events like merging black holes. This new method refines earlier gravitational-wave techniques, and as detectors grow more sensitive, it could yield sharper measurements that help shrink the gap created by the Hubble tension.
Nicolás Yunes, a physics professor at Illinois and founding director of ICASU, called the result highly significant: an independent way to measure the Hubble constant could be crucial for resolving the tension. He emphasized that their method boosts the accuracy of Hubble constant inferences drawn from gravitational waves.
Daniel Holz, a professor at UChicago, described the work as introducing a truly new cosmological tool. By leveraging the background gravitational-wave hum from countless distant black-hole mergers, the team shows we can learn about the universe’s age and composition. This marks an exciting, novel direction, and they plan to apply their approach to future datasets to further constrain the Hubble constant and other key cosmological quantities.
The full team includes Illinois graduate student Bryce Cousins (NSF Graduate Research Fellow and lead author), Illinois graduate student Kristen Schumacher (NSF Fellow), Illinois postdoctoral researcher Ka-wai Adrian Chung, and UChicago postdoctoral researchers Colm Talbot and Thomas Callister (Kavli Institute Postdoctoral Fellows). The findings have been accepted for publication in Physical Review Letters and will appear in the March 11 issue; the complete paper is already available on arXiv.
How scientists gauge the universe’s expansion
Since the early 1900s, researchers have pursued two main routes to measure cosmic expansion. One relies on electromagnetic observations, the other on gravitational waves. A well-known electromagnetic approach uses standard candles—stellar explosions such as supernovae—whose intrinsic brightness is understood. By comparing their true luminosity with how bright they appear from Earth, scientists estimate their distance and visualize how quickly they’re receding, which informs the expansion rate.
In recent years, gravitational waves have opened a complementary path. These waves arise when extremely dense objects like black holes collide. The waves propagate at light speed, spreading through space much like ripples on a pond after a stone is dropped. The LVK Collaboration—consisting of the LIGO, Virgo, and KAGRA detectors—monitors these signals with a global network of over 2,000 scientists.
Gravitational waves also enable a “standard siren” distance measurement. Yet deciphering how fast the source is receding due to cosmic expansion is trickier. Typically, astronomers need concurrent light from the merger or knowledge of the galaxy where it occurred to pin down the actual distance and redshift.
Ideally, all methods should converge on the same Hubble constant, but they don’t. If this tension endures, it could prompt a revision of our early-universe models. Proposed explanations range from early dark energy and dark-matter–neutrino interactions to evolving dark energy properties over time.
A new twist: the gravitational-wave background method
In their latest work, Yunes, Cousins, and colleagues present a novel way to estimate the Hubble constant by studying black-hole collisions that current detectors can’t resolve individually. These innumerable faint events collectively create what researchers call the gravitational-wave background.
“By observing individual black-hole mergers, we can determine how often such events happen across the universe. From those rates, we expect many more events we can’t directly observe, which forms the gravitational-wave background,” Cousins explains.
The team showed that if the Hubble constant were smaller, the observable volume of the universe would be reduced. A tighter cosmic space would pack more black-hole collisions into a given volume, strengthening the overall gravitational-wave background. If this background isn’t detected at a certain level, that absence can rule out slower expansion rates.
They nickname their approach the stochastic siren method, highlighting the random, collective nature of the contributing mergers.
Using current LVK data, the researchers tested the method. Even without a direct detection of the gravitational-wave background, they could exclude very slow expansion rates. When they combined the stochastic siren approach with existing merger measurements, they achieved a more precise estimate of the Hubble constant. The result still sits within the range associated with the Hubble tension, but the method demonstrates real potential to sharpen future cosmological inferences.
Looking ahead, as gravitational-wave observatories grow more sensitive, this strategy should gain power. Scientists anticipate detecting the gravitational-wave background within roughly six years. In the meantime, tighter limits on the background are already narrowing the feasible values of the Hubble constant.
“This paves the way for applying the method as we improve sensitivity, constrain the background more tightly, and perhaps even detect it,” Cousins says. “By including that information, we expect to refine cosmological results and move closer to resolving the Hubble tension.”
Support and resources behind the research
The analysis benefited from the Illinois Campus Cluster, run by the Illinois Campus Cluster Program in partnership with the National Center for Supercomputing Applications.
Funding came from the NSF Graduate Research Fellowship Program (Grant Nos. DGE 21-46756 and DGE-1746047) and NSF awards PHY-2207650, and PHY-2110507. Additional support came from the Simons Foundation (Award No. 896696) and NASA (Grant No. 80NSSC22K0806). Other backing included the Eric and Wendy Schmidt AI in Science Postdoctoral Fellowship and Kavli Institute for Cosmological Physics through an endowment from the Kavli Foundation. The findings reflect the researchers’ views, not necessarily those of the funding agencies.
Public note: This material originates from Mirage News and has been edited for clarity, style, and length. The source also invites readers to view the full article at the provided link.
