A team of scientists has proposed using gravitational waves to measure the universe's expansion rate, potentially resolving the "Hubble tension"—a discrepancy in calculations of this rate.

The Hubble Tension

Scientists have observed since 1998 that the universe's expansion rate is accelerating, a phenomenon attributed to "dark energy."

A central issue is the "Hubble constant," which measures this expansion rate. Different values are obtained depending on whether calculations start from the local, modern universe (using Type 1a supernovas) or the distant, ancient cosmos (using the standard model of cosmology).

Researchers from the University of Illinois Urbana-Champaign and the University of Chicago suggest that gravitational waves could offer a third, independent method to measure the Hubble constant.

Nicolas Yunes, founding director of Urbana's Illinois Center for Advanced Studies of the Universe (ICASU), stated that this result is significant for obtaining an independent measurement to resolve the Hubble tension, highlighting the innovative nature of their method in enhancing accuracy.

Gravitational Waves as a Measurement Tool

Gravitational waves are ripples in spacetime, predicted by Albert Einstein's theory of general relativity, which radiate outward at the speed of light when massive objects accelerate. Humanity first detected these waves in 2015 via the Laser Interferometer Gravitational-Wave Observatory (LIGO), stemming from the merger of two black holes. Since then, LIGO, along with Virgo and KAGRA, has detected waves from various cosmic mergers, including black holes and neutron stars.

While gravitational waves have been previously suggested for measuring the Hubble constant, limitations in accuracy existed. The current team believes their novel approach offers the necessary precision, which is expected to improve with enhanced detector sensitivity.

Daniel Holz of the University of Chicago commented that this new tool for cosmology allows learning about the universe's age and composition by using the background gravitational-wave hum from merging black holes in distant galaxies.

The Stochastic Siren Method

To utilize gravitational waves for measuring the Hubble constant, scientists must determine the recession speed of events that generate these waves, not solely their distance. This requires multi-messenger astronomy, combining gravitational wave data with electromagnetic radiation from these events or their host galaxies.

Comparing measurements from electromagnetic radiation alone with those combining electromagnetic radiation and gravitational waves could either confirm or persist the Hubble tension, indicating potential differences between the early and modern universe.

The team's proposed technique, called the stochastic siren method, uses background gravitational waves—the collective "hum" from numerous distant collision events. The reasoning is that a lower Hubble constant value implies a smaller volume of space for collisions, leading to a higher collision density and a stronger gravitational wave background signal. Conversely, an undetectable background could suggest a higher Hubble constant.

Though current detectors like LIGO-Virgo-KAGRA are not yet sensitive enough to detect the gravitational wave background, the team applied the stochastic siren method to existing data. Their findings indicated higher Hubble constant values and a more rapid universal expansion rate.

This application serves as a proof of concept.

Over the next six years, as gravitational wave detector sensitivity increases and the gravitational wave background becomes detectable, the stochastic siren method is expected to provide an independent measure of the Hubble constant, potentially resolving the Hubble tension.