Gravitational Wave Background Non-Detection Narrows the Hubble Constant's Possible Range
The universe is expanding. That much has been known since the early 20th century. What remains stubbornly unclear is the precise rate of that expansion - a number called the Hubble constant, measured in kilometers per second per megaparsec, that sets the scale for cosmic distances and the age of the universe itself. Two independent families of measurement techniques give answers that do not agree, and the gap between them has widened as measurement precision has improved. That disagreement is the Hubble tension, widely considered one of the most significant unresolved questions in modern cosmology.
A team at the University of Illinois Urbana-Champaign and the University of Chicago has now developed a new method for measuring the Hubble constant using gravitational waves - specifically, using the non-detection of a gravitational wave background to exclude certain values. The approach, accepted for publication in Physical Review Letters for the March 11, 2026 issue, adds an independent probe to a crowded but consequential measurement landscape.
How gravitational waves currently measure cosmic expansion
Gravitational waves - ripples in spacetime produced when massive objects collide - carry information about how far their source is from Earth in a way that does not depend on the visual properties of stars or galaxies. This makes them useful for measuring distances independently of the electromagnetic methods that anchor the traditional cosmic distance ladder.
Existing gravitational-wave methods for measuring the Hubble constant work by combining the distance derived from the wave signal with the recession velocity of the source galaxy - how fast that galaxy is moving away from Earth due to cosmic expansion. The distance is determined precisely from the gravitational wave amplitude. The recession velocity, however, requires either finding an electromagnetic counterpart to the merger or identifying the host galaxy in a catalog. Both approaches carry limitations and uncertainties that current detectors cannot fully overcome.
The stochastic siren: what is not heard
The new method takes a different approach. The gravitational-wave detectors operated by the LIGO-Virgo-KAGRA (LVK) Collaboration are sensitive enough to detect individual black hole collisions above a certain distance threshold. Below that threshold, collisions still happen but their signals are too faint to resolve individually. Summed together, those unresolved events should produce a continuous, low-level hum - the gravitational-wave background.
That background has not yet been detected. But its non-detection is informative. The expected strength of the background depends on how many black hole mergers occur within a given volume of space, which in turn depends on how large that volume is - which depends on the Hubble constant. At lower values of the Hubble constant, the universe expands more slowly, spacetime is more compressed, and the same number of mergers pack into a smaller volume, producing a stronger background signal.
"Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe. 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," explained lead author Bryce Cousins, a physics graduate student at Illinois and NSF Graduate Research Fellow.
The team named their approach the stochastic siren method: stochastic because the background collisions occur randomly across cosmic time and space, siren because gravitational-wave sources are called standard sirens by analogy to the standard candles of electromagnetic cosmology.
What applying the method to current data shows
Applied to current LVK data, the stochastic siren method's non-detection of the background was able to provide evidence against low values of the Hubble constant. Combined with measurements from individual detected black hole mergers, the method produced a Hubble constant estimate that falls within the Hubble tension region - the range where the disagreement between early-universe and late-universe methods currently sits - rather than clearly resolving it.
That is not a failure. The current result is a proof of principle. The gravitational-wave background is expected to be detected within six years as detector sensitivity improves. Until and after that detection, the stochastic siren method will continue to tighten constraints on the Hubble constant in increments, contributing to the eventual convergence - or confirmed divergence - of cosmological measurements.
"This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it," said Cousins. "By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension."
The method's power will grow with detector improvements. The LIGO network is currently undergoing upgrades that will extend its reach to larger volumes of space. The proposed next-generation Cosmic Explorer and Einstein Telescope detectors would be sensitive enough to detect individual mergers at much greater distances, reducing the amplitude of the unresolved background and making the non-detection constraint significantly more stringent.