Scientists Use Black Hole "Hum" to Measure Universe's Expansion Rate

A Revolutionary Approach to Cosmic Measurement

Scientists at the University of Illinois and University of Chicago have discovered a groundbreaking way to measure how fast the universe is expanding, using something that sounds like science fiction: the faint gravitational-wave "hum" created by countless black hole mergers across the cosmos . This innovative technique, called the "stochastic siren" method , represents an entirely new tool for cosmology that could help resolve one of physics' most perplexing mysteries.

The breakthrough centers on a cumulative, faint hum produced by innumerable unresolved black hole mergers throughout the universe . Unlike traditional methods that rely on observing individual cosmic events, this approach harnesses the collective background noise from billions of distant catastrophes. Gravitational waves are generated by the energetic collisions of compact astrophysical objects such as black holes , creating spacetime ripples analogous to concentric waves that spread out across the surface of a pond after a stone is thrown into the water .

The research team, led by Illinois physics graduate student Bryce Cousins, has achieved improved accuracy over prior gravitational-wave methods of measuring the Hubble constant . Their findings have been accepted for publication in Physical Review Letters , marking a significant milestone in cosmological research.

Solving the Hubble Tension Crisis

The new method tackles what scientists call the "Hubble tension," considered to be one of the most significant open questions in cosmology . The problem is straightforward yet troubling: different measurement methods are internally consistent and based on the same physics, so all observed values of the Hubble constant should agree . However, those that come from early-universe datasets disagree with those that come from late-universe datasets .

The disagreement has reached alarming proportions. Measurements tied to the early universe land in the high 60s when expressed in kilometers per second per megaparsec, while late-universe measurements tend to land higher, in the low-to-mid 70s . The mismatch is now described as being in more than 5-sigma conflict, a level that keeps theorists awake at night .

Illinois Physics Professor Nicolás Yunes emphasized the significance of their independent approach: "This result is very significant—it's important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension" . The beauty of gravitational waves lies in their fundamental difference from electromagnetic observations, potentially revealing hidden biases in traditional methods.

The Science Behind the Cosmic Hum

The "stochastic siren" method exploits a fascinating relationship between the universe's expansion rate and the strength of gravitational-wave background noise. The strength of an astrophysical gravitational-wave background depends on how many mergers occurred over cosmic time and how they are spread across the available volume, which depends on the universe's expansion history .

The logic is elegantly simple: a smaller Hubble constant corresponds to larger comoving volumes at a given redshift, meaning more mergers contribute to the background and the energy density in gravitational waves goes up . Conversely, a lower value means a smaller observable universe with higher density of mergers and stronger background signal, while a higher Hubble constant means more volume, lower density, and weaker signal .

University of Chicago Professor Daniel Holz captured the excitement of the discovery: "By using the background gravitational-wave hum from merging black holes in distant galaxies, we can learn about the age and composition of the universe" . The team has already applied their method to existing data from the LIGO-Virgo-KAGRA collaboration, which has caught hundreds of gravitational wave events .

Future Implications and Cosmic Revelations

While current detectors haven't yet directly observed the gravitational-wave background, the absence itself provides valuable constraints. If the background is not detected, values of the Hubble constant that would have produced an overly strong background become less plausible, with continued non-detection pushing against the slow-expansion end of the range . This unique feature allows the method to naturally raise the lower bound of the Hubble constant as limits on the background improve .

The timing couldn't be better. Detector upgrades are coming, and the gravitational-wave background is expected to be detectable within roughly the next six years . As detection limits improve, so does the constraint on the Hubble constant—the stochastic siren gets more precise even