The Silence Between the Bells
Every black hole merger in the universe rings a bell.
Most of these bells are too faint for our instruments to catch. LIGO and its partner detectors have picked up around a hundred individual mergers since 2015 — including GW250114, the loudest one ever recorded, announced just weeks ago. But for every merger we hear, thousands more are happening beyond our reach: distant, quiet collisions of black holes in galaxies we will never resolve, each one sending its own ripple through spacetime.
Those ripples don't vanish. They accumulate. Layer upon layer, merger upon merger, stretching back through cosmic time, they should be building into a faint, constant hum — a gravitational-wave background permeating all of spacetime, the collective murmur of every black hole collision the universe has ever hosted.
We haven't heard it yet.
And a team of physicists at the University of Illinois and the University of Chicago has just shown that this silence is not empty. It is information.
The Hubble Tension
To understand why silence matters, you need to understand the biggest open wound in modern cosmology.
The universe is expanding. This has been known for nearly a century. The rate of that expansion is described by a single number: the Hubble constant, H₀. In principle, every method of measuring it should converge on the same value. In practice, they don't.
Measurements based on the early universe — the cosmic microwave background, baryon acoustic oscillations, the faint afterglow of the Big Bang — consistently yield around 67–68 km/s/Mpc. Measurements based on the nearby universe — supernovae, the cosmic distance ladder, gravitationally lensed quasars — consistently yield around 72–74 km/s/Mpc. The disagreement now exceeds 5σ. This is not a rounding error. This is two pictures of the universe that refuse to be the same picture.
This is the Hubble tension, and it has been tormenting cosmologists for years. Either one set of measurements is systematically wrong, or our model of the universe is missing something fundamental.
Gravitational waves were supposed to help. A merging pair of compact objects emits gravitational waves whose amplitude directly encodes the distance to the source — no distance ladder required, no calibration chain to worry about. If you can also determine the source's redshift, you can calculate H₀. This is the "standard siren" method, and it was one of the great promises of gravitational-wave astronomy.
The problem is precision. The most recent standard siren measurement, using 42 binary black hole mergers from LIGO-Virgo-KAGRA's first three observing runs, yielded H₀ = 46⁺⁴⁹₋₂₆ km/s/Mpc. The error bars are enormous. The answer is technically consistent with both sides of the tension — and therefore useless for resolving it.
Listening to the Choir
This is where Bryce Cousins, Nicolás Yunes, Daniel Holz, and their colleagues enter the story. Their insight is deceptively simple: stop trying to hear individual voices. Listen to the choir.
The gravitational-wave background — the accumulated hum of all those unresolved mergers — depends on how many mergers are happening across the universe and how much volume they're spread across. Both of those quantities depend on the Hubble constant.
Here is the key: if H₀ is smaller (the universe is expanding more slowly), then the comoving volume of space is larger. More volume means more mergers contributing to the background. More mergers mean a louder hum. Conversely, if H₀ is larger, the volume shrinks, there are fewer mergers per unit volume, and the hum is quieter.
So: the louder the expected background, the easier it should be to detect. And if you don't detect it, that tells you the background can't be too loud — which means the universe can't have too much volume — which means H₀ can't be too small.
Silence becomes a lower bound on the expansion rate.
The team calls this the stochastic siren — a cosmological measurement tool built not from individual gravitational-wave events, but from the statistical properties of the entire unresolved population. Standard sirens use the foreground. The stochastic siren uses the background. And critically, it works even before the background is detected.
What the Silence Says
Applying their method to data from LIGO-Virgo-KAGRA's first three observing runs, the team combined the population inference from 42 resolved binary black hole mergers with the current non-detection of the gravitational-wave background. The result: H₀ = 72⁺⁴⁴₋₃₇ km/s/Mpc.
Compare this to the foreground-only measurement from the same data: 57⁺⁴³₋₃₅ km/s/Mpc. The error bars are still wide — this is early days — but the peak of the distribution has shifted substantially, landing squarely in the range where the Planck and SH0ES values disagree. The stochastic siren nudges the gravitational-wave measurement closer to the electromagnetic measurements, improving consistency across methods.
And there's an asymmetry built into the method that makes it particularly interesting for the tension. As observing runs continue without detecting the background, the lower bound on H₀ progressively tightens. Each additional year of silence squeezes out the low end of the distribution. The stochastic siren naturally probes the gap between the early-universe value (~67) and the late-universe value (~73), precisely where the tension lives.
The Coming Hum
The gravitational-wave background is expected to be detected within the next several years. The upcoming A# detector upgrades to LIGO could achieve it with a signal-to-noise ratio of 8 in less than a year of operation. When that happens, the stochastic siren will flip from using the absence of a signal to using the strength of it, yielding even sharper constraints on H₀.
What I find beautiful about this work is the conceptual inversion. In gravitational-wave astronomy, we've spent a decade chasing the loudest events — the most massive mergers, the closest collisions, the signals with the highest signal-to-noise ratio. GW250114, the bell that rang louder than any before it, was the culmination of that pursuit. But Cousins and colleagues are saying: the real cosmological power might not be in the soloists. It might be in the chorus.
There is a deep elegance in the idea that the expansion rate of the universe is encoded not in any single event, but in the collective whisper of all the events we cannot individually hear. That the answer to one of cosmology's biggest questions might come not from listening harder, but from learning to interpret the silence between the bells.
Source: Cousins, B., Schumacher, K., Chung, A.K.-W., Talbot, C., Callister, T., Holz, D.E. & Yunes, N. (2026). The Stochastic Siren: Astrophysical Gravitational-Wave Background Measurements of the Hubble Constant. Physical Review Letters (March 11, 2026). arXiv:2503.01997 | DOI:10.1103/4lzh-bm7y