‘An entirely new tool for cosmology’: The gravitational wave background could mend our broken understanding of the universe
Physicists may have a whole new way to measure the rate of expansion of the universe – one of cosmology’s greatest mysteries – using space-time ripples predicted by Einstein.
A new study suggests that the faint background of gravitational waves produced by the merger of many black holes across the universe can be used to independently measure the rate at which space is expanding. Even without directly detecting this background “hum,” the researchers show that it already imposes limits on the Hubble constant – a key quantity at the heart of one of the greatest puzzles of modern cosmology.
An independent test of the Hubble constant
The expansion rate of the universe, encoded in the Hubble constant, has become the focus. intense debate in recent years. Measurements based on the early Universe, such as those deduced from residual radiation from the Universe Big Bang (known as the cosmic microwave background), which disagrees with measurements derived from closer objects, such as supernovas and flickering galaxies. This gap, 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 Mingarelliassistant professor of physics at Yale University who was not involved in the new study, told Live Science via email. “Measurements of the expansion rate at the beginning and end of the Universe disagree by more than 5 sigma. [the “gold standard” of statistical significance in physics]and we don’t know why. Either there is an unidentified systematic error or new physics. Any truly independent measure of the rate of expansion is extremely valuable. »
The new research, accepted for publication in the journal Physical Review Letters and available as preprintproposes such an independent method based almost entirely on gravitational waves – subtle ripples in the fabric of space-time predicted by Einstein’s theory of general relativity.
“This result is very significant”, co-author of the study Nicholas Yunesprofessor of astrophysics at the University of Illinois at Urbana-Champaign, said in a statement statement. “Our method is an innovative way to improve the accuracy of Hubble constant inferences using gravitational waves.”
Listen to the 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 mergers of individual black holes via gravitational waves. Each merger provides information on the masses of the black holes involved and their distances from Earth.
“As we observe individual black hole collisions, we can determine the rates of these collisions occurring across the universe,” said the study’s lead author. Bryce Cousinsa graduate student at the University of Illinois at Urbana-Champaign, said in the release. “Based on these rates, we expect there to be many more events that we cannot observe, the so-called gravitational wave background.” This background of gravitational waves, sometimes described as a stochastic (or random) signal, is the weak collective effect of many distant mergers. Its overall strength depends on how quickly the universe is expanding. Slower expansion implies larger cosmic volumes and, therefore, more background-contributing mergers.
“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. Slower expansion means larger volumes, more mergers, and a stronger background. So even failure to detect this background disadvantages low values of the Hubble constant.”
Using current data from gravitational wave detectors, the team showed that the absence of a detected background already excludes some lower values of the Hubble constant. Although the current constraints are broad, the method establishes a new framework for cosmological inference.
The approach builds on the concept of “standard sirens,” in which individual gravitational wave events act as distance markers. But instead of relying on single bright events, the new method exploits the entire unresolved population of colliding black holes.
“It’s not every day that we propose an entirely new tool for cosmology,” co-author of the study Daniel Holzprofessor of physics and astronomy at the University of Chicago, said in the release. “We show that using the hum of background gravitational waves from merging black holes in distant galaxies, we can learn more 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 data sets to help constrain the Hubble constant, as well as other key cosmological quantities,” Holz added.
Although the new method is promising, Mingarelli also highlighted its current limitations. “The main strength is that it is a measurement based almost entirely on gravitational waves, independent of the electromagnetic distance scale and the cosmic microwave background,” Mingarelli said. “The limitation is that the uncertainties are still large and the outcome depends on the assumed black hole population model. But the authors are frank about this and show that their choices are conservative.”
Looking to the future, detector upgrades should significantly improve the sensitivity to gravitational wave background.
“With planned detector upgrades, background noise should be detected within a few years, turning this lower limit into a real-world measurement,” Mingarelli said.
If successful, this stochastic siren method could become a powerful new tool for probing the expansion history of the universe and determining whether the Hubble voltage signals new physics or systematic errors hidden in existing measurements.
Bryce Cousins, Kristen Schumacher, Adrian Ka-Wai Chung, Colm Talbot, Thomas Callister, Daniel E. Holz, Nicolás Yunes. (2026). Stochastic siren: background measurements of astrophysical gravitational waves of the Hubble constant. Physical Examination Letters. https://doi.org/10.1103/4lzh-bm7y
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