For decades, astronomers have known that the universe is expanding. To determine how fast it is growing today, scientists calculate a value called the Hubble constant. Multiple independent techniques are used to measure it, and because 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. This mismatch is known as the Hubble tension, and it stands as one of the most important unresolved problems in modern cosmology.
A group of astrophysicists and cosmologists at The Grainger College of Engineering at the University of Illinois Urbana-Champaign and at the University of Chicago has introduced a new way to calculate the Hubble constant using gravitational waves, which are 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.
Illinois Physics Professor Nicolás Yunes said, “This result is very significant — it’s important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension. Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.” Yunes is the founding director of the Illinois Center for Advanced Studies of the Universe (ICASU) on the Urbana campus.
Daniel Holz, UChicago Professor of Physics and of Astronomy & Astrophysics and a co author of the research, said, “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 help constrain the Hubble constant, as well as other key cosmological quantities.”
The research team also includes Illinois physics graduate student Bryce Cousins, an NSF Graduate Research Fellow and lead author of the study; Illinois physics graduate student Kristen Schumacher, an NSF Graduate Research Fellow; Illinois physics postdoctoral research associate Ka-wai Adrian Chung; and University of Chicago postdoctoral researchers Colm Talbot and Thomas Callister, both Kavli Institute for Cosmological Physics Postdoctoral Fellows. The findings have been accepted for publication in Physical Review Letters and will appear in the March 11 issue. The full paper is already available on arXiv.
How Scientists Measure the Universe’s Expansion
Since the early 1900s, researchers have relied on two main strategies to measure cosmic expansion. One approach uses electromagnetic observations, while the other uses gravitational waves. A well known electromagnetic method involves “standard candles,” such as supernovae, which are powerful stellar explosions. Because astronomers understand how bright these events truly are, they can calculate both their distance from Earth and how fast they are moving away. Combining those numbers reveals the universe’s expansion rate.
In recent years, gravitational waves have opened another path. These waves are produced when extremely dense objects like black holes collide. The ripples move through space at the speed of light, similar to the circular waves that spread across water after a stone is dropped into a pond. On Earth, the LIGO-Virgo-KAGRA (LVK) Collaboration, a global network with more than 2,000 members, detects these signals.
Gravitational waves can also be used to estimate distances through what is known as the standard siren method. However, determining how fast the source is receding due to cosmic expansion is more difficult. To measure that speed, astronomers typically need to detect light from the merger or identify the galaxy where it occurred.
Ideally, all these techniques would point to the same Hubble constant. Instead, they disagree. If the tension persists, it could signal that scientists need to revise their understanding of the early universe. Proposed explanations include early dark energy, interactions between dark matter and neutrinos, or changes in how dark energy behaves over time.
A New Gravitational Wave Background Method
In their latest work, Yunes, Cousins, and their colleagues describe a new way to estimate the Hubble constant by studying black hole collisions that current detectors cannot individually pick up. Together, these countless faint events create what is called the 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 Cousins.
The team showed that if the Hubble constant were lower, the total observable volume of the universe would also be smaller. That would mean black hole collisions are packed into a tighter space, increasing the overall strength of the gravitational-wave background. If this background signal is not detected at a certain level, it rules out slower expansion rates.
The researchers call their approach the stochastic siren method, reflecting the random nature of the collisions that contribute to the gravitational-wave background.
Using current LVK data, the team tested their method. Even without detecting the gravitational-wave background directly, they were able to rule out particularly slow expansion rates. When they combined the stochastic siren method with existing measurements from individual black hole mergers, they achieved a more precise estimate of the Hubble constant. Their result falls within the range associated with the Hubble tension, showing the method’s potential to sharpen future measurements.
As gravitational-wave observatories improve, this strategy should become even more powerful. Scientists expect the gravitational-wave background to be detected within about six years. Until then, increasingly strict limits on the background signal will continue to narrow 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 relied on the Illinois Campus Cluster, operated by the Illinois Campus Cluster Program in partnership with the National Center for Supercomputing Applications.
Funding came from the NSF Graduate Research Fellowship Program under Grant No. DGE 21-46756 and Grant No. DGE-1746047 and the NSF under award PHY-2207650, PHY-2207650, and PHY2110507. Additional support was provided by the Simons Foundation through Award No. 896696 and NASA through Grant No. 80NSSC22K0806. Support also came from the Eric and Wendy Schmidt AI in Science Postdoctoral Fellowship and the Kavli Institute for Cosmological Physics through an endowment from the Kavli Foundation and its founder Fred Kavli. The findings presented are those of the researchers and not necessarily those of the funding agencies.
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