Researchers from the University of Illinois Urbana-Champaign and the University of Chicago have developed a novel approach to calculate the Hubble constant using gravitational waves from black hole collisions. This technique, known as the stochastic siren method, analyzes the background hum of faint mergers to potentially resolve the Hubble tension. The findings, accepted for publication in Physical Review Letters, offer improved precision with current data.
Astronomers have debated the rate of the universe's expansion for decades, relying on the Hubble constant to quantify it. Measurements from early universe observations conflict with those from more recent cosmic events, creating the Hubble tension—a key puzzle in cosmology.
A team led by Illinois physics graduate student Bryce Cousins has introduced the stochastic siren method, which studies the gravitational-wave background produced by countless undetected black hole mergers. These ripples in spacetime, detected by the LIGO-Virgo-KAGRA Collaboration, form a faint cosmic hum that reveals collision rates across the universe. By linking these rates to the observable volume, the method infers expansion speed: a lower Hubble constant would compress events into a smaller space, strengthening the background signal.
Using existing LIGO-Virgo-KAGRA data, the researchers ruled out very slow expansion rates without directly detecting the background. Combining this with data from individual mergers yielded a more precise Hubble constant estimate, aligning with the tension's disputed range. As detectors improve, the approach promises sharper results; scientists anticipate background detection in about six years.
Nicolás Yunes, Illinois physics professor and founding director of the Illinois Center for Advanced Studies of the Universe, stated, "This result is very significant—it's important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension. Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves."
Daniel Holz, University of Chicago professor of physics and astronomy, added, "It's not every day that you come up with an entirely new tool for cosmology. We show that 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. This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to help constrain the Hubble constant, as well as other key cosmological quantities."
The study involved collaborators including Kristen Schumacher, Ka-wai Adrian Chung from Illinois, and Colm Talbot and Thomas Callister from Chicago. Funding came from the NSF, Simons Foundation, NASA, and others. The paper is set for the March 11 issue of Physical Review Letters and is available on arXiv.