The Universe is continually expanding, and scientists measure its expansion rate, the Hubble Constant, using two main methods: electromagnetic methods (using light from stars, galaxies, or supernovae) and gravitational-wave methods (using ripples in spacetime from black hole collisions).
Gravitational waves come from the collision of two massive black holes. They spread out like ripples on water and travel at the speed of light until they reach detectors on Earth. These detectors are part of the LIGO-Virgo-KAGRA (LVK) Collaboration, a global network with thousands of scientists.
Measuring the Hubble Constant using gravitational waves is similar to measuring it using supernovae. Distances to black hole collisions can be calculated with the standard siren method. However, astronomers also need to know how fast the collision site is moving away from Earth (its recessional velocity) due to the Universe’s expansion. To find this, they must either detect light from the merger or identify the galaxy where it happened.
Ideally, all techniques would give the same value for the Hubble Constant. But they don’t; this disagreement is called the Hubble tension. If the tension persists, it may point to new physics about the early Universe.
Webb verifies Hubble’s measurement of the universe’s expansion rate as accurate
Scientists at the University of Illinois Urbana-Champaign and the University of Chicago have found a brand-new way to measure how fast the Universe is expanding. Instead of depending on light from stars or galaxies, they focused on the faint “hum” of gravitational waves.
The study published in Physical Review Letters is the first to measure the Hubble constant using the stochastic gravitational-wave background.
Through this study, scientists introduce a new tool for probing the expansion history of the Universe, improving upon the accuracy of prior methods.
Illinois Physics Professor Nicolás Yunes said, “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.”
The stochastic gravitational-wave background is the faint, persistent hum produced by countless unresolved black hole mergers across cosmic time. Unlike individually detected mergers, this background encodes information about the Universe’s expansion in a statistical way.
UChicago Professor of Physics and of Astronomy & Astrophysics and co-author on the new research, Daniel Holz, comments, “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 new method has been nicknamed the ‘stochastic siren’. When they applied the method to data from the current LVK Collaboration, they found something surprising: even the absence of a gravitational-wave background carries meaning. It rules out the idea of a very slowly expanding Universe, giving evidence that expansion must be faster than those lower-rate scenarios.
To sharpen their results, the team then combined this background analysis with measurements of the Hubble constant from individual black hole collisions. Together, these two approaches confirmed that the new gravitational-wave method is not only viable, but also shifts the measured value of the Hubble constant into the region of the ongoing Hubble tension.
New precise measurement of the universe’s expansion rate
As gravitational-wave detectors become more sensitive, scientists will be able to use this new method to measure the Universe’s expansion with even greater accuracy. Within the next six years, the faint gravitational-wave background, the cosmic “hum” from countless black hole collisions, is expected to be detected directly.
Until that moment arrives, the stochastic siren method remains an important tool. Each time the background remains undetected, the upper limits improve, and the method pushes the measured value of the Hubble constant higher.
Journal Reference:
- Bryce Cousins, Kristen Schumacher, Adrian Ka-Wai Chung, Colm Talbot, Thomas Callister, Daniel E. Holz and Nicolás Yunes. Stochastic Siren: Astrophysical gravitational-wave background measurements of the Hubble constant. Phys. Rev. Lett. DOI: 10.1103/4lzh-bm7y
