Astronomers have long known the universe is expanding—but exactly how fast remains one of the biggest mysteries in cosmology. Different techniques for measuring the Hubble constant stubbornly disagree, creating the so-called “Hubble tension.” Now researchers at the University of Illinois Urbana-Champaign and the University of Chicago have unveiled a bold new way to weigh in on the debate using gravitational waves—the faint ripples in spacetime produced by colliding black holes.