Physicists at the University of Illinois Urbana-Champaign and the University of Chicago have proposed a new way to measure how fast the universe is expanding, and it relies on something detectors have not yet picked up: the faint collective hum of gravitational waves from countless black hole mergers. Their technique, called the “stochastic siren” method, uses the absence of that signal to constrain the Hubble constant, a value at the center of one of modern cosmology’s most persistent disagreements.
What the Hubble Tension Actually Is
The Hubble constant, often written as H0, represents the local expansion rate of the universe and sets its overall scale. It tells scientists how quickly galaxies are receding from one another. Two independent ways of measuring it, however, produce answers that refuse to agree. Observations of the cosmic microwave background, the afterglow of the Big Bang, point to a value near 67 kilometers per second per megaparsec. Direct measurements of nearby supernovae and variable stars, by contrast, yield a figure closer to 73 km/s/Mpc, meaning galaxies move away 73 km per second faster for each additional 3.3 million light-years of distance. That gap, now statistically significant after years of refined data, is the Hubble tension. Either one camp’s measurements contain a hidden systematic error, or the standard model of cosmology is incomplete.
Turning Silence Into a Measurement
The new paper, accepted in Physical Review Letters, introduces a creative workaround. Rather than relying on a single dramatic event like a neutron star collision with a visible counterpart, the stochastic siren method combines two pieces of information. First, it draws on the growing catalog of individually resolved binary black hole mergers detected by LIGO and Virgo. Second, it incorporates the current upper limits on the stellar-mass stochastic gravitational-wave background, the collective rumble that should arise from the sum of all mergers too faint or distant to be individually identified.
The logic works like this: the Hubble constant determines how distances and volumes in the universe relate to one another. A lower value of H0 would pack more mergers into a given volume of space, producing a louder gravitational-wave background. Because current detectors have not yet heard that background hum, the non-detection itself carries information. It disfavors the lower end of the H0 range, nudging the estimate upward and potentially toward the higher value favored by local measurements. According to the Illinois team, this connection between a quieter-than-expected background and a faster expansion rate is the core insight behind the method.
How It Differs From Earlier Gravitational-Wave Cosmology
Gravitational waves have been used to estimate H0 before. The landmark 2017 detection of a binary neutron star merger, GW170817, provided the first “standard siren” measurement by combining the gravitational-wave signal with electromagnetic observations of the same event. More recently, the LIGO-Virgo-KAGRA collaboration used events from the third gravitational-wave transient catalog, GWTC-3, to infer cosmological parameters through approaches including redshifted masses and galaxy catalogs. Those results offered quantitative constraints but remained sensitive to assumptions about the binary black hole mass distribution and how those systems form and evolve.
The stochastic siren method sidesteps some of those assumptions by adding a new observable: the amplitude of the unresolved background itself. Instead of asking “what does each individual merger tell us about distance?” it asks “given how many mergers we see and how loud the leftover hum should be, what expansion rate is consistent with both?” That reframing opens a channel of information that previous analyses left on the table. In practice, the method folds in the observed rate of detected mergers, models for the merger population, and the stringent limits on any diffuse background to carve out a region of allowed H0 values.
A Different Hum at a Different Frequency
The concept of a gravitational-wave background is not new. The NANOGrav collaboration reported evidence for such a background at nanohertz frequencies using 15 years of pulsar timing data, with results supported by quantitative Bayesian analyses. That signal, likely generated by supermassive black hole binaries orbiting each other over millions of years, occupies a completely different frequency band from the stellar-mass background targeted by the stochastic siren method. LIGO and Virgo search for signals with durations from a few milliseconds up to around 10 seconds, corresponding to much higher frequencies than pulsar timing arrays probe.
The distinction matters because it means the two backgrounds carry independent cosmological information. NANOGrav’s low-frequency signal tests the population of supermassive black holes and the dynamics of galaxies over cosmic time. The stellar-mass background, in contrast, is tied to the life cycles of massive stars and the merger history of black holes a few to a few dozen times the Sun’s mass. The stochastic siren method proposes to extract precise cosmological parameters from this higher-frequency background, even before it is directly detected, by treating the current non-detection as a meaningful data point.
Why the Non-Detection Cuts Both Ways
Using a non-detection as a cosmological probe comes with caveats. The same quiet background that rules out very low H0 values could also point to gaps in our understanding of black hole populations or their environments. If, for example, stellar-mass black holes merge less frequently in the distant universe than current models predict, the background would be weaker regardless of the true expansion rate. Conversely, if there are more mergers hidden below the detection threshold than expected, the background could be louder, tightening the constraints on H0 once it is finally measured.
The authors address this by explicitly folding astrophysical uncertainties into their analysis. They explore a range of plausible merger rates, mass distributions, and redshift evolutions, then ask which combinations are compatible with both the catalog of detected events and the stringent stochastic limits. In regions of parameter space where no reasonable merger history can reconcile a low H0 with a quiet background, the method effectively rules out those cosmological scenarios. Where degeneracies remain, the technique highlights exactly which astrophysical inputs most strongly affect the result.
Crucially, this approach does not depend on any single spectacular event. As gravitational-wave detectors improve and accumulate more observing time, both the catalog of resolved mergers and the sensitivity to a diffuse background will grow. Each additional run will either push down the upper limits on the background or, eventually, produce a first detection. In either case, the stochastic siren framework can be updated, steadily sharpening the inferred value of the Hubble constant and providing an independent cross-check on other methods.
ArXiv’s Quiet Role in a Noisy Universe
Behind the scenes, much of this work circulates first through the preprint server arXiv, where the stochastic siren study and related gravitational-wave analyses are posted for the community. The platform is maintained by a network of institutional members and supported by researchers who contribute financially to keep it freely accessible. Its mission, described in arXiv’s about pages, is to accelerate the sharing of scientific ideas, and its help resources guide authors and readers through everything from submission policies to subject classifications.
That open infrastructure has been especially important for fast-moving fields like gravitational-wave astronomy, where collaborations span continents and new data releases can quickly reshape the landscape. By the time a paper appears in a journal such as Physical Review Letters, it has often already been scrutinized, extended, or challenged by follow-up work visible on arXiv. The stochastic siren proposal is part of this broader ecosystem: a novel idea that leverages both cutting-edge detectors and an open preprint culture to tackle one of cosmology’s most stubborn puzzles.
Whether the method ultimately tips the scales in the Hubble tension debate remains to be seen. Future observing runs by LIGO, Virgo, and KAGRA, as well as planned third-generation detectors, will be decisive in determining how quickly the stellar-mass background emerges from the noise. If the hum arrives louder or softer than expected, the stochastic siren framework will help translate that sound, or its continued absence, into sharper clues about the universe’s expansion rate, the lives of black holes, and the physics that links them.
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*This article was researched with the help of AI, with human editors creating the final content.
