- The stochastic siren measures the Hubble constant using undetected gravitational waves.
- Non-detection of the gravitational-wave background rules out slow expansion rates.
- Error bars remain wide - this is a proof of concept, not precision cosmology.
What a Missing Hum Tells Us About the Hubble Constant
Bryce Cousins analyzed 42 black hole collisions and found a conspicuous silence. The collisions were real, observed by LIGO and Virgo during their first three observing runs, each rippling across spacetime. The silence was what interested him more. It had something to say about the Hubble constant.
Somewhere beyond those 42 detected events, many more black hole pairs are spiraling together in distant galaxies. They are too far away for any detector to pick out individually.
Collectively, they should produce a faint gravitational hum, a cosmic background rumble stitched together from countless unresolved collisions. Nobody has heard it yet.
Cousins, a graduate student and NSF fellow at the University of Illinois, realized that this non-detection was itself a piece of data.
What is the Hubble tension?
Measurements of the universe's expansion rate disagree depending on where you look. Observations of the cosmic microwave background, light from the early universe, suggest space is expanding at about 67 km/s/Mpc. Measurements using nearby supernovae give roughly 73 km/s/Mpc. The gap has reached what some teams calculate as more than five standard deviations, and a decade of sharper instruments has only widened it.
The result, accepted for publication in Physical Review Letters, introduces what the team calls the "stochastic siren." It is a method for measuring the Hubble constant using a signal that hasn't been detected. By the authors' own framing, this is a proof of concept. But the logic beneath it turns a limitation into leverage.
The Argument From Quiet
The team's reasoning works like this. At lower values of the Hubble constant, the effective volume of space contributing to the background is smaller. The same number of black hole mergers packed into less space means a higher collision density, which would generate a louder gravitational-wave background.
Such low expansion rates would predict a background stronger than current upper limits allow.
The detectors haven't heard it. And that "quiet" carries information.
Cousins and his collaborators combined the 42 resolved black hole collisions with upper limits on the undetected background. This moved the peak value for the estimate from 57 to 72 km/s/Mpc, with the silence excluding slower expansion rates.
Key figure
72 km/s/Mpc (±37–44)
The stochastic siren's first measurement of the Hubble constant. Wide error bars, but the peak lands squarely in the Hubble tension zone.
The number deserves context. The error bars span from roughly 35 to 116 km/s/Mpc. That range is consistent with nearly every proposed value of the Hubble constant. The measurement demonstrates a method, not a result.
Nicolás Yunes, also at Illinois, and Daniel Holz at the University of Chicago are among the study's seven authors.
"It's not every day that you come up with an entirely new tool for cosmology," said Holz, who has some authority on the subject. In 2006, he coined the term "standard siren" to describe using gravitational-wave events as distance markers, the ripple-in-spacetime equivalent of a standard candle.
He co-authored the landmark 2017 measurement from GW170817, the first neutron star collision observed in both gravitational waves and light. The stochastic siren extends that program into territory where no individual event is visible at all.
"It's not every day that you come up with an entirely new tool for cosmology."
Daniel Holz, University of Chicago
A Third Voice in the Hubble Tension Argument
The Hubble tension has persisted, in part, because its two sides use fundamentally different physics. Early-universe estimates rely on the cosmic microwave background and a model of how the universe evolved. Late-universe estimates use supernovae and a chain of distance calibrations called the cosmic distance ladder. Each method is internally consistent. They simply disagree.
Can we improve our measurements for the Hubble Constant by listening to the silence of black hole mergers?
Gravitational waves offer something neither side has: a measurement that depends on neither electromagnetic light nor the distance ladder. The stochastic siren extracts cosmological information from the statistical properties of an entire population of mergers, not from any single event.
Nicolàs Yunes, who directs the Illinois Center for Advanced Studies of the Universe, framed the contribution in terms of independence. "This result is very significant - it's important to obtain an independent measurement," he said.
Whether the Hubble tension reflects new physics or some unidentified systematic error remains open. Candidates include early dark energy, dark matter–neutrino interactions, and an evolving dark energy equation of state.
The stochastic siren does not answer the question.
But it adds a 'third microphone' to a room where two have been picking up different sounds.
Waiting for the Hum
The gravitational-wave background is expected to be directly detected within roughly six years, according to the team's projections. A planned sensitivity upgrade known as LIGO A-sharp could reach the necessary threshold in under a year of observing. Next-generation upgrades like LIGO Voyager, with cryogenic silicon mirrors, are forecast to detect the background with high confidence within a few years of operation.
More Cosmic Discoveries
The Cosmic Expansion May Not Be Accelerating After All
A controversial finding by Korean cosmologists suggests that the expansion of the universe may be slowing down.
→Each year of continued non-detection, meanwhile, pushes the lower bound of the Hubble constant upward. The silence grows more informative with time.
Cousins put it plainly: "By including that information, we expect to get better cosmological results." He is describing a transition - from proof of concept to precision instrument. From an argument built on what we haven't heard to one grounded in a signal we can measure.
The method's value, for now, is not in its number but in its existence. Cosmology has a new way to listen. It just hasn't heard anything yet.
Sources
- Primary Research: The Stochastic Siren: Astrophysical Gravitational-Wave Background Measurements of the Hubble Constant (Cousins, Schumacher, Chung, Talbot, Callister, Holz & Yunes, 2026)
- Additional Context:
- Illinois and UChicago physicists develop a new method to measure the expansion rate of the universe (University of Illinois)
- Accepted paper in Physical Review Letters (American Physical Society)
- A gravitational-wave standard siren measurement of the Hubble constant (Abbott et al., Nature 2017)
Fact Check: Claim-by-Claim Verification Verified
Limits and uncertainties
Core method and logic strongly supported by peer-reviewed preprint and university release; H0 shift and independence clearly demonstrated.
Numbers slightly rounded in article but directionally accurate; wide errors reflect proof-of-concept stage, not precision measurement.
Relies on LVK GWTC-3 data and models; no hype beyond projections. Readers should note continued non-detections strengthen low-H0 exclusion.
GWB detection imminent with upgrades, transitioning method to higher precision.
Bottom line
Article accurately reports novel stochastic siren method from primary research, validating claims with minor rounding. Adds credible GW voice to Hubble tension debate.
Fact-checked by Perplexity Sonar Pro on 2026-03-01
