The universe may be filled with a faint, constant hum — the collective gravitational wave signal from countless black hole mergers too distant to detect individually. A new study argues that this background noise can serve as an independent tool to measure how fast the universe is expanding.
The research, accepted for publication in Physical Review Letters, proposes using the gravitational wave background to constrain the Hubble constant — the number at the center of one of cosmology’s most stubborn disputes.
The Tension That Won’t Go Away
Two separate methods of measuring the universe’s expansion rate keep producing different answers. Calculations drawn from the cosmic microwave background — leftover radiation from the Big Bang — consistently disagree with measurements derived from nearby objects like supernovas and galaxies. The gap has now reached over 5 sigma, the standard threshold for statistical significance in physics.
“The Hubble tension is one of the most important open problems in cosmology,” said Chiara Mingarelli, an assistant professor of physics at Yale University. “Early-Universe and late-Universe measurements of the expansion rate disagree at over 5 sigma, and we don’t know why. Either there’s an unidentified systematic error or new physics.”
She was not involved in the new research.
What the Hum Can Do
Since 2015, detectors including LIGO, the Virgo interferometer, and the KAGRA detector have captured dozens of individual black hole mergers. Each event carries data about the masses involved and their distance from Earth.
The new study goes further. It shows that even without directly detecting the gravitational wave background, its existence already places constraints on the Hubble constant. The signal doesn’t need to be heard clearly to be useful.
“This result is very significant,” said study co-author Nicolás Yunes, a professor of astrophysics at the University of Illinois Urbana-Champaign. “Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.”
The approach is grounded in Einstein’s theory of general relativity, which predicts that massive colliding objects ripple the fabric of space-time. Those ripples carry information about the geometry of the universe — including how fast it is expanding.
According to the announcement, the method is nearly independent of the electromagnetic observations that underpin existing Hubble constant measurements. That independence is precisely what makes it significant: it sidesteps the systematic errors suspected of distorting current estimates.
If the technique is refined and applied to future detectors — including the European Space Agency‘s planned LISA mission — it could produce measurements precise enough to determine whether the Hubble tension points to a flaw in existing data or evidence of physics beyond the current standard model.
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