Scientists have known for nearly a century that the universe is expanding, with galaxies moving away from each other over time.
The speed of this expansion today is called the Hubble constant. But measuring it has turned into one of the biggest mysteries in modern astronomy.
Different methods produce different answers, a puzzle known as the “Hubble tension.”
Now, a team of researchers in the United States has developed a new way to measure the universe’s expansion using gravitational waves—tiny ripples in space and time caused by violent cosmic events.
The research team, based at the University of Illinois Urbana-Champaign and the University of Chicago, focused on gravitational waves produced when massive objects like black holes collide.
These collisions release enormous energy that travels across the universe as waves in spacetime, similar to ripples spreading across water after a stone is thrown in. Sensitive detectors on Earth, operated by an international collaboration called LIGO-Virgo-KAGRA, can measure these signals.
Scientists have already used individual gravitational-wave events to estimate the Hubble constant. However, that method has limitations because it can be difficult to determine exactly how fast the source of the waves is moving away from Earth.
The new approach looks instead at the faint “background hum” created by countless black hole mergers happening across the universe, most of which are too distant to detect individually.
Lead researcher Bryce Cousins explains that even though we cannot observe all these collisions directly, scientists can estimate how often they occur. If the universe were expanding more slowly, there would be less space overall, meaning the density of collisions would be higher and the background signal stronger.
If the expansion were faster, the opposite would be true. By analyzing whether this background signal is present or absent in current data, researchers can narrow down the possible values of the Hubble constant.
The team calls their technique the “stochastic siren method,” referring to the random nature of these distant collisions. When they applied the method to existing gravitational-wave observations, they found that the absence of a detectable background signal rules out some of the slower expansion scenarios.
Combining this new method with previous measurements from individual black hole collisions produced a more precise estimate of the universe’s expansion rate.
Researchers say this independent measurement is important because it uses completely different physics from traditional methods, such as observing supernova explosions in distant galaxies.
If multiple independent approaches eventually agree, scientists may finally resolve the Hubble tension. If they continue to disagree, it could signal that our understanding of the universe is incomplete, possibly pointing to unknown forms of energy or new physics from the early universe.
The new method’s power will grow as gravitational-wave detectors become more sensitive. Scientists expect to detect the background signal within the next several years. Once that happens, the technique could provide one of the most reliable measurements yet of how fast the universe is expanding, as well as clues about its age and composition.
For now, the study offers an exciting new tool for cosmology. By “listening” to the universe’s quiet background vibrations, scientists are finding fresh ways to answer one of the most fundamental questions about our cosmos: how fast space itself is stretching—and why.
