New Method for Measuring Universe’s Expansion Rate May Resolve Hubble Tension

For decades, astronomers have known the universe is expanding, but pinpointing the rate of that expansion has proven surprisingly difficult. Scientists calculate a value called the Hubble constant to quantify this rate. Multiple independent techniques are used to measure it, and due to the fact that they rely on the same underlying physics, they should produce matching results. Instead, measurements based on observations of the early universe conflict with those drawn from the more recent universe—a discrepancy known as the Hubble tension, and one of the most significant unresolved problems in modern cosmology.

A Novel Approach Using Gravitational Waves

A team of astrophysicists and cosmologists at the University of Illinois Urbana-Champaign and the University of Chicago has introduced a new way to calculate the Hubble constant using gravitational waves—tiny ripples in spacetime. Their approach improves the precision of earlier gravitational wave-based techniques. As detectors become more sensitive, this method could deliver even sharper measurements, potentially helping scientists close the gap behind the Hubble tension. The research, accepted for publication in Physical Review Letters, builds on the detection of gravitational waves by the LIGO-Virgo-KAGRA (LVK) Collaboration.

“This result is very significant—it’s important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension,” said Nicolás Yunes, Illinois Physics Professor and founding director of the Illinois Center for Advanced Studies of the Universe (ICASU). “Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.”

Daniel Holz, UChicago Professor of Physics and of Astronomy & Astrophysics, added, “It’s not every day that you come up with an entirely new tool for cosmology. We show that by using the background gravitational-wave hum from merging black holes in distant galaxies, we can learn about the age and composition of the universe. This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to assist constrain the Hubble constant, as well as other key cosmological quantities.”

How Scientists Measure Cosmic Expansion

Since the early 1900s, researchers have primarily relied on electromagnetic observations and, more recently, gravitational waves to measure cosmic expansion. A common electromagnetic method uses “standard candles,” such as supernovae, which are powerful stellar explosions. By understanding the intrinsic brightness of these events, astronomers can calculate their distance from Earth and how fast they are receding, revealing the universe’s expansion rate. NASA explains that the Hubble Space Telescope has been instrumental in refining these measurements, narrowing the estimated age of the universe to 13.8 billion years.

Gravitational waves, produced by colliding dense objects like black holes, offer another path. These ripples travel at the speed of light and are detected by the LVK Collaboration. The “standard siren” method uses gravitational waves to estimate distances, but determining the source’s recession speed due to cosmic expansion is challenging, often requiring light detection from the merger or identification of the host galaxy.

Ideally, all these techniques would converge on the same Hubble constant. The persistent disagreement suggests a need to revise our understanding of the early universe, potentially involving early dark energy, interactions between dark matter and neutrinos, or evolving dark energy behavior.

The Stochastic Siren Method and Gravitational-Wave Background

The Illinois and UChicago team developed a new way to estimate the Hubble constant by studying black hole collisions that current detectors cannot individually identify. These countless faint events create a “gravitational-wave background.”

“Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe. Based on those rates, we expect there to be a lot more events that we can’t observe, which is called the gravitational-wave background,” explains Bryce Cousins, a graduate student at the University of Illinois and lead author of the study.

The team demonstrated that a lower Hubble constant would result in a smaller observable universe volume, concentrating black hole collisions and increasing the gravitational-wave background’s strength. The absence of a detected background signal at a certain level would rule out slower expansion rates. This approach, termed the “stochastic siren method,” reflects the random nature of the contributing collisions.

Using current LVK data, the team tested their method and, even without directly detecting the gravitational-wave background, ruled out particularly slow expansion rates. Combining the stochastic siren method with existing measurements from individual black hole mergers yielded a more precise estimate of the Hubble constant, falling within the range of the Hubble tension.

As gravitational-wave observatories improve, this strategy is expected to become even more powerful. Scientists anticipate detecting the gravitational-wave background within approximately six years. Until then, increasingly stringent limits on the background signal will continue to refine the possible range of the Hubble constant.

“This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it,” says Cousins. “By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension.”

Research Support and Computing Resources

The analysis utilized the Illinois Campus Cluster, operated by the Illinois Campus Cluster Program in partnership with the National Center for Supercomputing Applications. Funding was provided by the NSF Graduate Research Fellowship Program and the NSF, as well as the Simons Foundation and NASA. Additional support came from the Eric and Wendy Schmidt AI in Science Postdoctoral Fellowship and the Kavli Institute for Cosmological Physics.