A team of astrophysicists from the University of Illinois Urbana-Champaign and the University of Chicago has developed a new technique to measure the rate at which the universe is expanding, leveraging the faint hum of gravitational waves produced by merging black holes across the cosmos. The research, accepted for publication in Physical Review Letters, introduces what the team calls the "stochastic siren" method — an approach that could help resolve one of the most persistent puzzles in modern cosmology.
A New Tool for Cosmology
The Hubble constant, which quantifies the universe's present-day expansion rate, has been measured using two broad approaches: observations of the early universe, such as the cosmic microwave background, and observations of the nearby universe, such as supernovae. These approaches yield values that disagree — roughly 67 km/s/Mpc from early-universe data versus approximately 73 km/s/Mpc from late-universe measurements — a discrepancy that has reached a statistical confidence exceeding five sigma.
The stochastic siren method offers an entirely independent path. Rather than relying solely on individually detected black hole mergers or electromagnetic observations, it incorporates information from the gravitational-wave background — a collective signal from the many distant black hole collisions too faint for current detectors to resolve individually.
"Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe," said Bryce Cousins, a physics graduate student at Illinois and lead author of the study. "Based on those rates, we expect there to be a lot more events that we can't observe, which is called the gravitational-wave background."
The method's logic rests on a relationship between the expansion rate and spatial volume. A slower expansion rate implies a larger cosmic volume, which means more mergers and a stronger background signal. The current non-detection of the background therefore rules out the lowest expansion rates. When the team combined this constraint with data from resolved mergers recorded during the first three observing runs of the LIGO-Virgo-KAGRA Collaboration, they produced a more precise estimate of the Hubble constant than resolved mergers alone could provide.
A Promising Path Forward
Daniel Holz, a professor at the University of Chicago and co-author, called the technique "an entirely new tool for cosmology," adding that it opens "an exciting and completely new direction" for constraining the Hubble constant and other cosmological quantities.
The gravitational-wave background is expected to be detected within the next six years as detector sensitivity improves. Even before that milestone, the stochastic siren method would progressively tighten the lower bound on the Hubble constant with each successive observing run that fails to detect the background — gradually probing the heart of the Hubble tension.
"This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it," Cousins said. "By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension."
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