The Hubble constant — the expansion rate of the universe — has been measured two ways that disagree. The cosmic microwave background gives approximately 67 km/s/Mpc. Type Ia supernovae and Cepheid variables give approximately 73 km/s/Mpc. The discrepancy is called the Hubble tension, and it has persisted through increasingly precise measurements on both sides.
Gravitational waves offer a third method. When two black holes or neutron stars merge, the gravitational wave signal encodes the distance directly — no distance ladder, no calibration against other objects. Each merger is an independent measurement. The problem is that individual mergers are rare and noisy. Each one contributes a weak constraint on the Hubble constant.
Researchers at the University of Illinois and the University of Chicago developed the stochastic siren method. Rather than waiting for individual mergers to accumulate, they used the gravitational-wave background — the collective hum of all mergers happening everywhere in the universe, too distant or faint to resolve individually. This background has not yet been detected. The team used the non-detection itself as data.
The absence of a signal is not the absence of information. The background's non-detection, combined with the population statistics of resolved mergers, constrains the Hubble constant more tightly than resolved mergers alone. The null result eliminates regions of parameter space that would have produced a detectable background. What you don't see tells you where you can't be.
This is a general principle about null results. A negative search constrains the possibility space as effectively as a positive detection — sometimes more effectively, because a detection confirms a single point in parameter space while a non-detection eliminates a volume. The stochastic siren method converts the frustrating absence of a background detection into the very measurement it was designed to make. The silence speaks.
The team found that combining the stochastic siren constraint with individual merger measurements shifts the Hubble constant value into the tension region — between the CMB and supernovae values. The new method does not resolve the tension. It enters it, independently, from a completely different direction. Whatever is real about the discrepancy, gravitational waves see it too.