Physicists from the University of Illinois and the University of Chicago have introduced a new method called the “stochastic siren” to measure how fast the universe is expanding. Published in Physical Review Letters on March 11, 2026, the technique flips a longstanding assumption on its head: instead of relying on what gravitational-wave detectors can see, it draws information from what they have failed to detect. The approach arrives at a moment when cosmology’s most persistent disagreement, the so‑called Hubble tension, has resisted resolution for over a decade.
Turning Silence Into a Cosmic Ruler
Most attempts to pin down the Hubble constant, the number that describes the universe’s expansion rate, depend on observing specific objects at known distances. Supernovae that reach a roughly uniform brightness have served as “standard candles” for decades. Gravitational-wave astronomers developed their own version, called “standard sirens,” by pairing a detected black hole or neutron star merger with the galaxy that hosted it. The stochastic siren departs from both traditions. Rather than matching individual events to host galaxies, it uses the collective hum, or lack thereof, produced by thousands of unresolved binary black hole mergers spread across the cosmos.
The key insight is that the strength of this gravitational-wave background depends on the Hubble constant. A lower expansion rate would pack more mergers into a given volume of space, producing a stronger background signal. Because the LIGO–Virgo–KAGRA detector network has not yet picked up that signal, the non-detection itself sets a floor on the Hubble constant, effectively ruling out values that are too low. The research team combined this constraint with data from individually resolved mergers cataloged in the collaboration’s latest release to sharpen the measurement further, turning the absence of a background into a quantitative lower bound on how quickly space is stretching.
How the Method Differs From Standard Sirens
Conventional standard-siren analyses face a practical bottleneck: they need an electromagnetic counterpart or a galaxy catalog to determine a merger’s redshift, which tells astronomers how far away the event occurred. Only one event so far, the 2017 neutron star collision known as GW170817, came with a clear optical counterpart. The stochastic siren sidesteps this limitation entirely. As Nicolas Yunes and Daniel Holz emphasized in public comments, the new technique relies on population-level statistics rather than individual event identification, allowing it to exploit every merger that contributes to the unresolved background, not just the rare ones with bright flashes of light.
The team drew on the GWTC‑4.0 catalog, which was released in late August 2025 and covers the first part of the fourth observing run, known as O4a. A separate collaboration analysis used 142 gravitational-wave sources from that same catalog to measure the Hubble constant through “spectral sirens” and “dark sirens,” two techniques that exploit mass-spectrum features and galaxy-catalog associations, respectively. The stochastic siren adds a third, independent line of evidence from the same detector data, which means it can be cross-checked against those other approaches without introducing new instruments or observing campaigns, and it naturally dovetails with ongoing efforts to characterize the full gravitational-wave sky.
A New Way to Probe the Hubble Tension
The disagreement at the center of this work is simple to state and stubbornly difficult to resolve. Measurements based on the early universe, particularly the cosmic microwave background, point to a slower expansion rate, while measurements anchored in the nearby universe, using Cepheid variable stars and supernovae, consistently return a faster one. A widely cited NASA overview captured the community’s frustration years ago, noting that astronomers initially suspected hidden problems in the data rather than a genuine cosmic discrepancy. As error bars have shrunk, the mismatch has become harder to dismiss, prompting a wave of proposals that range from subtle systematic effects to entirely new physics.
More recent work has complicated the picture rather than clarifying it. A team led by UChicago scientist Wendy Freedman used the James Webb Space Telescope and reported little sign of a serious conflict in the Hubble constant, suggesting earlier local measurements may have been skewed by systematic errors in Cepheid calibrations. Other groups disagree, maintaining that the discrepancy is real and may signal unknown ingredients in the cosmos. The stochastic siren enters this debate from an entirely different direction, one that does not depend on light-based distance measurements at all. By tying its constraint to the gravitational-wave background, it offers a way to test whether the preferred expansion rates from early- and late-universe methods are compatible with the merger population implied by current detectors.
Inside the Stochastic Siren Analysis
To translate gravitational-wave silence into a cosmological statement, the researchers built models of how many binary black holes should be merging throughout the universe and how loudly their combined signal should ring in the detectors. Those models incorporate assumptions about the mass distribution of black holes, the rate at which they form and merge over cosmic time, and how their signals are stretched and dimmed by the expanding universe. For each assumed value of the Hubble constant, the team computed the expected strength of the stochastic background and compared it to the upper limits reported by the LIGO–Virgo–KAGRA collaboration. Values of the Hubble constant that would have produced a detectable background are ruled out by the fact that no such background has yet been observed.
Crucially, the same catalog of individually resolved mergers informs both sides of the calculation. The GWTC‑4.0 events help pin down the merger-rate density and mass distribution, while the non-detection of a background constrains how those mergers can be distributed across cosmic volume. According to an Illinois Physics summary, this self-consistency is a key strength: the stochastic siren does not require any additional astrophysical inputs beyond what the detectors already provide. Instead, it leverages the interplay between loud, individually resolvable events and the quiet, cumulative background they would create if they were more numerous or more distant than the data allow.
Stronger Constraints on the Horizon
The current power of the stochastic siren is limited by detector sensitivity. LIGO, Virgo, and KAGRA have not yet detected the gravitational-wave background, so the method can only exclude certain values of the Hubble constant rather than pinpoint it precisely. But that limitation is temporary. As outlined in a Newswise briefing, upgrades to the interferometers during the ongoing observing runs are expected to improve their sensitivity to the stochastic background by factors of a few. If a detection occurs, the same framework that now sets a lower bound on the Hubble constant could be inverted to measure it directly, turning the background from a null result into a standard siren of its own.
Looking further ahead, the stochastic siren concept is flexible enough to migrate to next-generation observatories. Planned facilities such as the Einstein Telescope and Cosmic Explorer aim to probe a far larger volume of the universe and to observe mergers out to much higher redshifts, dramatically enhancing the expected background signal. A recent technical preprint on future gravitational-wave cosmology argues that combining multiple siren techniques (standard, spectral, dark, and stochastic) could eventually deliver percent-level precision on the Hubble constant without relying on traditional distance ladders. If that vision is realized, the quiet hiss of countless unseen black holes may become one of the most powerful tools for resolving the universe’s most persistent expansion mystery.
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*This article was researched with the help of AI, with human editors creating the final content.
