'Collective hum' of black holes could mend our broken understanding of the universe, physicists say

Physicists may have a brand-new way to measure the expansion rate of the universe — one of the biggest outstanding mysteries in cosmology — using space-time ripples predicted by Einstein.

A new study suggests that the faint gravitational wave background produced by numerous 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 limits on the Hubble constant — a key quantity at the heart of one of modern cosmology's biggest puzzles.

If confirmed, the technique could settle the debate about whether we need to come up with new physics to explain the nature of the universe.

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 inferred from the leftover radiation from the Big Bang (known as the cosmic microwave background), disagree with measurements derived from more nearby objects, like flickering supernovas 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 Mingarelli, an assistant professor of physics at Yale University who was not involved in the new study, told Live Science via email. "Early-Universe and late-Universe measurements of the expansion rate disagree at over 5 sigma [the "gold standard" of statistical significance in physics], and we don't know why. Either there's an unidentified systematic error or new physics. Any genuinely independent measurement of the expansion rate is extremely valuable."

The new research, accepted for publication in the journal Physical Review Letters and available as a preprint, proposes 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," study co-author Nicolás Yunes, a professor of astrophysics at the University of Illinois Urbana-Champaign, said in a statement. "Our method is an innovative way to enhance 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 about the masses of the black holes involved and their distances from Earth.

"Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe," lead study author Bryce Cousins, a graduate student at the University of Illinois Urbana-Champaign, said in the statement. "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." This gravitational wave background, sometimes described as a stochastic (or random) signal, is the faint, collective effect of numerous distant mergers. Its overall strength depends on how quickly the universe is expanding. A slower expansion implies larger cosmic volumes and, therefore, more mergers contributing to the background.

"It's a clever 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 louder background. So even the non-detection of this background disfavors 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 present constraints are broad, the method establishes a new framework for cosmological inference.

A new tool for cosmology

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 you come up with an entirely new tool for cosmology," study co-author Daniel Holz, a 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 key cosmological quantities," Holz added.

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 uncertainties are still large, and the result depends on the assumed black hole population model. But the authors are upfront about this and show their choices are conservative."

Looking ahead, detector upgrades are expected to significantly improve sensitivity to the gravitational wave background.

"With planned detector upgrades, the background should be detected within a few years, turning this from a lower bound into a real measurement," Mingarelli said.

If successful, this stochastic siren method could become a powerful new tool for probing the expansion history of the universe and for investigating whether the Hubble tension signals new physics or hidden systematic errors in existing measurements.

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