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An illustration inspired by the European Space Agency\u2019s upcoming LISA detector, with gravitational waves rippling through the background. Studying the faint hum of gravitational waves across the universe could help solve the Hubble tension, one of the biggest nagging problems in physics. | Credit: ESA<\/p>\n
Physicists may have a brand-new way to measure the expansion rate of the universe \u2014 one of the biggest outstanding mysteries in cosmology \u2014 using space-time ripples predicted by Einstein<\/a>.<\/p>\n A new study suggests that the faint gravitational wave background<\/a> 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 \u2014 a key quantity at the heart of one of modern cosmology’s biggest puzzles.<\/p>\n 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.<\/p>\n An independent test of the Hubble constant<\/p>\n The expansion rate of the universe, encoded in the Hubble constant, has become the focus of intense debate in recent years<\/a>. Measurements based on the early universe, such as those inferred from the leftover radiation from the Big Bang<\/a> (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.<\/p>\n “The Hubble tension is one of the most important open problems in cosmology,” Chiara Mingarelli<\/a>, 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<\/a> 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.”<\/p>\n The new research, accepted for publication in the journal Physical Review Letters and available as a preprint<\/a>, proposes such an independent method based almost entirely on gravitational waves \u2014 subtle ripples in the fabric of space-time predicted by Einstein’s theory of general relativity<\/a>.<\/p>\n “This result is very significant,” study co-author Nicol\u00e1s Yunes<\/a>, a professor of astrophysics at the University of Illinois Urbana-Champaign, said in a statement<\/a>. “Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.”<\/p>\n Listening to the background hum of black holes<\/p>\n 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<\/a> involved and their distances from Earth.<\/p>\n Gravitational waves are released when two massive objects, like black holes, collide (illustrated here). Physicists believe that the universe is humming with a faint background noise from countless black hole collisions that are too faint to detect \u2014 a feature called the gravitational wave background. | Credit: NASA Goddard<\/p>\n “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>, 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.<\/p>\n “It’s a clever idea,” Mingarelli said. “The gravitational-wave background \u2014 the collective hum of distant black hole mergers too faint to detect individually \u2014 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.”<\/p>\n 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.<\/p>\n A new tool for cosmology<\/p>\n 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.<\/p>\n “It’s not every day that you come up with an entirely new tool for cosmology,” study co-author Daniel Holz<\/a>, 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.<\/p>\n An illustration of gravitational waves emitted by a black hole collision. | Credit: NASA\/C. Henze<\/p>\n “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.<\/p>\n 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 \u2014 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.”<\/p>\n RELATED STORIES<\/p>\n \u2014Physicists detect rare ‘second-generation’ black holes that prove Einstein right… again<\/a><\/p>\n \u2014‘Impossible’ black hole collision pushed relativity to its breaking point \u2014 and scientists finally understand how<\/a><\/p>\n \u2014Scientists think they detected the first known triple black hole system in the universe \u2014 and then watched it die<\/a><\/p>\n![\"An](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/03\/2e149a82563bda5aa831ceb742927d79.jpeg\"\/)
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