Physicists may have a brand new way to measure the expansion rate of the universe – one of the greatest outstanding mysteries in cosmology – using space-time ripples predicted by Einstein.
A new study suggests that the weak gravitational wave background produced by many merging black holes across the universe can be used to independently measure how fast space is expanding. Even without detecting this background “hum” directly, the researchers show that it already places constraints on the Hubble constant – a key quantity at the heart of one of modern cosmology’s biggest puzzles.
An independent test of the Hubble constant
The expansion rate of the universe, encoded in the Hubble constant, has become the focus of intense debate in recent years. Measurements based on the early universe, such as those derived from radiation remnants from Big Bang (known as the cosmic microwave background), disagrees with measurements derived from more nearby objects, such as flickering supernovae and galaxies. This discrepancy, known as the Hubble tension, has now reached high statistical significance.
“The Hubble tension is one of the most important open problems in cosmology,” Chiara Mingarellian assistant professor of physics at Yale University who was not involved in the new study told LiveScience via email. “Early Universe and Late Universe measurements of the expansion rate disagree at above 5 sigma (the “gold standard” of statistical significance in physics), and we don’t know why. Either it’s an unidentified systematic error or new physics. Any genuinely independent measurement of the rate of expansion is extremely valuable.”
The new research, accepted for publication in the journal Physical Review Letters and available as a preprintpropose such an independent method based almost entirely on gravitational waves – subtle ripples in the fabric of space-time predicted by Einstein’s general theory of relativity.
“This result is very important,” co-author of the study Nicolás Yunesa professor of astrophysics at the University of Illinois Urbana-Champaign, said in a statement. “Our method is an innovative way to improve the accuracy of Hubble constant inferences using gravitational waves.”
Listening to the background hum of black holes
Since 2015, detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Virgo interferometer, and the Kamioka Gravitational Wave Detector (KAGRA) have observed dozens of individual black hole mergers through gravitational waves. Each merger provides information on the masses of black holes involved and their distances from Earth.
“Because we observe individual black hole collisions, we can determine the frequency of these collisions occurring across the universe,” lead study author Bryce Cousinsa graduate student at the University of Illinois Urbana-Champaign, said in the statement. “Based on these rates, we expect that there will be many more events that we cannot observe, which are called the gravitational wave background.” This gravitational wave background, sometimes described as a stochastic (or random) signal, is the weak, collective effect of a series of distant mergers. Its overall strength depends on how fast the universe is expanding. A slower expansion implies larger cosmic volumes and therefore more mergers that contribute to the background.
“It’s a smart idea,” Mingarelli said. “The gravitational wave background – the collective hum of distant black hole mergers too faint to detect individually – depends on the expansion rate. A slower expansion means larger volumes, more mergers and a higher background. So even non-detection of this background is unfavorable for low values of the Hubble constant.”
Using current data from gravitational wave detectors, the team showed that the absence of a detected background already rules out some lower values of the Hubble constant. While the current limitations are broad, the method establishes a new framework for cosmological inference.
The approach is based on the concept of “standard sirens”, where individual gravitational wave events act as distance markers. But instead of relying on single bright events, the new method exploits the entire unsolved population of colliding black holes.
“It’s not every day you come up with a brand new tool for cosmology,” study co-author Daniel Holza professor of physics and astronomy at the University of Chicago, said in the statement. “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 important cosmological quantities,” added Holz.
While the new method shows promise, Mingarelli also emphasized the current limitations. “The main strength is that this is an almost entirely gravitational wave-based measurement — independent of the electromagnetic distance ladder and the cosmic microwave background,” Mingarelli said. “The limitation is that the uncertainty is still large, and the result depends on the assumed population model for black holes. But the authors are clear about this and show that their choices are conservative.”
Looking ahead, detector upgrades is expected to significantly improve the sensitivity to the gravitational wave background.
“With planned detector upgrades, the background should be detected within a few years, making this a lower limit to a real measurement,” Mingarelli said.
If successful, this stochastic siren method could become a powerful new tool for probing the universe’s expansion history and for investigating whether the Hubble voltage signals new physics or hidden systematic errors in existing measurements.
Bryce Cousins, Kristen Schumacher, Adrian Ka-Wai Chung, Colm Talbot, Thomas Callister, Daniel E. Holz, Nicolás Yunes. (2026). Stochastic Siren: Astrophysical Gravitational Wave Background Measurements of the Hubble Constant. Physical review letters. https://doi.org/10.1103/4lzh-bm7y
