Researchers developed a "stochastic standard siren" method using the gravitational-wave background to measure the universe's expansion rate.
The technique offers a new, independent approach to address the "Hubble tension," a major discrepancy in cosmology.
Analysis of current data shows the non-detection of the background signal argues against slower expansion rates.
The method will become increasingly powerful as next-generation observatories detect the gravitational-wave background directly.
📖 Full Retelling
A collaborative team of astrophysicists and cosmologists from the University of Illinois Urbana-Champaign and the University of Chicago has proposed a novel method for measuring the rate of the universe's expansion, known as the Hubble constant, by analyzing the faint, collective hum of gravitational waves from countless distant black hole mergers. Their study, published in the journal *Physical Review Letters* on January 16, 2026, introduces the "stochastic standard siren" technique as a potential new tool to resolve the long-standing "Hubble tension," a significant discrepancy between the two primary existing methods for calculating cosmic expansion.
The Hubble tension represents one of cosmology's most pressing mysteries, where measurements from the early universe's Cosmic Microwave Background and those from the local universe's "cosmic distance ladder" yield conflicting values for the expansion rate. The new method leverages the gravitational-wave background (GWB), a persistent, stochastic signal created by the cumulative effect of astrophysical collisions too faint for current observatories like LIGO-Virgo-KAGRA (LVK) to detect individually. As lead author Bryce Cousins explained, the expected rate of observable black hole mergers allows scientists to infer the properties of this unseen background, which in turn contains information about cosmic geometry and expansion.
In a proof-of-concept analysis using existing LVK data, the team demonstrated that the current non-detection of the GWB actually provides evidence against slower cosmic expansion rates. They combined this constraint with data from individual gravitational-wave events to refine the Hubble constant measurement. The principle hinges on volume: a slower-expanding universe implies a smaller cosmic volume, which would concentrate merger events and amplify the GWB signal to a potentially detectable level. With major gravitational-wave observatories scheduled for upgrades, a direct detection of the background is anticipated within the next six years, at which point this method could deliver even more precise cosmological parameters.
Study co-author Daniel Holz highlighted the innovation, noting it opens an entirely new direction for cosmology. The stochastic siren method not only offers a third, independent avenue to measure the Hubble constant but also allows scientists to place meaningful constraints on its value even before the GWB is definitively detected. This work paves the way for future, more sensitive observations to finally reconcile the conflicting expansion rates and test fundamental theories about dark energy and the composition of the universe.
Hubble's law, officially the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther a galaxy is from the Earth, the faster it moves away. A galaxy's recessional velocity is typically...
This development is crucial for resolving one of the most enduring mysteries in cosmology—the Hubble Tension—where measurements from two primary methods (CMB and Cosmic Distance Ladder) yield inconsistent values for the universe’s expansion rate. A unified, gravitational-wave-based approach could provide an independent validation of cosmic expansion, potentially reconciling discrepancies and advancing our understanding of dark energy, dark matter, and fundamental physics beyond Einstein’s General Relativity.
Context & Background
The Hubble Tension persists despite decades of research, with tensions exceeding 3σ, indicating a possible flaw in current cosmological models or new physics influencing expansion rates.
Gravitational waves from black hole mergers have already been used to probe cosmic distances, but detecting the stochastic gravitational-wave background (GWB) remains challenging due to low signal-to-noise ratios.
The UChicago and Illinois collaboration leverages both individual GW events and the anticipated GWB to refine Hubble constant estimates, offering a complementary method to existing techniques.
Future advancements in LIGO-Virgo-KAGRA’s sensitivity could detect the GWB within six years, enabling real-time cosmological constraints.
What Happens Next
The stochastic siren method will be tested against future LVK data sets to refine upper bounds on the Hubble constant. If the GWB is detected within the next decade, this study’s approach could become a cornerstone for resolving tensions by providing direct measurements of cosmic expansion via gravitational-wave astronomy.
Frequently Asked Questions
What is the Hubble Tension?
The Hubble Tension refers to the discrepancy between two primary cosmological methods measuring the universe’s expansion rate (Hubble constant): CMB-based estimates (~70 km/s/Mpc) and Distance Ladder-based estimates (~75 km/s/Mpc), with a 3σ gap suggesting unresolved physics.
How does gravitational-wave background measurement improve Hubble constant accuracy?
By analyzing stochastic GW signals from distant black hole mergers, the method quantifies collision rates across cosmic volumes, indirectly constraining expansion history and dark energy dynamics beyond individual events.
When could this method detect the gravitational-wave background?
With upgraded LVK detectors, the GWB is expected to be detectable within six years, enabling direct measurement of its spectral properties for cosmological inference.
Why is resolving Hubble Tension important beyond just the constant?
A resolved tension could validate or refute dark energy models (e.g., evolving dynamics), test modified gravity theories, and confirm whether neutrino masses or new particles influence cosmic expansion.
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Original Source
Illinois and UChicago Physicists Develop a New Method for Measuring Cosmic Expansion By Matthew Williams - March 05, 2026 12:03 AM UTC | Cosmology For about a century, scientists have known that the Universe is in a state of constant expansion. In honor of the scientists who definitively showed this, this expansion has come to be known as the Hubble Constant (or Hubble-Lemaitre Constant). Today, scientists use two main techniques to measure the rate of expansion: the Cosmic Microwave Background and the Cosmic Distance Ladder. The former relies on redshift measurements of the CMB, the relic radiation left over from the Big Bang, while the latter relies on parallax and redshift measurements using variable stars and supernovae (aka "standard candles"). The only problem is that the two methods don't agree, leading to what is known as the "Hubble Tension." This problem is considered one of the greatest cosmological mysteries facing scientists today. Luckily, new methods are emerging that could help resolve this "tension" and bring order to the Standard Model of Cosmology. In a recent study , a team of astrophysicists, cosmologists, and physicists from the University of Illinois and the University of Chicago has proposed a new method using the tiny ripples in spacetime known as gravitational waves . The study was led by Bryce Cousins, an NSF Graduate Research Fellow from the Institute of Gravitation and the Cosmos at the University of Illinois Urbana-Champaign. He was joined by multiple colleagues from the IGC, as well as researchers from the Kavli Institute for Cosmological Physics and the Enrico Fermi Institute at the University of Chicago. Their study, " Stochastic Siren: Astrophysical gravitational-wave background measurements of the Hubble constant ," appeared on Jan. 16th in the Physical Review Letters. Scientists hoping to resolve the Hubble Tension have proposed several solutions, ranging from Early Dark Energy and interactions between Dark Matter and neutrinos to...
