A collaborative team of astrophysicists and cosmologists from The Grainger College of Engineering at the University of Illinois Urbana-Champaign and the University of Chicago has unveiled a groundbreaking technique for calculating the Hubble constant using gravitational waves--minute ripples in spacetime. This innovative approach enhances the precision of previous methods that relied on gravitational waves, and as detection technology advances, it promises even more accurate measurements, potentially bridging the gap in the ongoing Hubble tension debate.
Nicolás Yunes, a professor at Illinois Physics, emphasized the significance of this research, stating, "Obtaining an independent measurement of the Hubble constant is crucial to resolving the current Hubble tension. Our method offers a novel way to improve the accuracy of Hubble constant estimations through gravitational waves." Yunes is also the founding director of the Illinois Center for Advanced Studies of the Universe (ICASU).
Co-author Daniel Holz, a professor of Physics and Astronomy & Astrophysics at UChicago, remarked, "It's rare to develop a completely new tool for cosmology. By harnessing the background gravitational-wave hum from merging black holes in distant galaxies, we can gain insights into the universe's age and composition. This exciting direction holds great promise for future datasets that will refine our understanding of the Hubble constant and other essential cosmological metrics."
The research team includes graduate students Bryce Cousins and Kristen Schumacher, both NSF Graduate Research Fellows, along with postdoctoral researchers Ka-wai Adrian Chung, Colm Talbot, and Thomas Callister. Their findings are set to be published in Physical Review Letters on March 11, with the full paper already accessible on arXiv.
Understanding Cosmic Expansion Measurement
For over a century, scientists have utilized two primary methods to measure the expansion of the cosmos. One involves electromagnetic observations, while the other employs gravitational waves. A prominent electromagnetic technique utilizes "standard candles," such as supernovae, which allow astronomers to calculate their distance and velocity, revealing the universe's expansion rate.
Recent advancements in gravitational wave detection have opened a new avenue for exploration. These waves are generated when dense objects like black holes collide, creating ripples that traverse space at light speed. The LIGO-Virgo-KAGRA (LVK) Collaboration, comprising over 2,000 members, plays a crucial role in detecting these signals.
Gravitational waves also facilitate distance estimation through a method known as standard sirens, although measuring the speed of the source due to cosmic expansion presents challenges. Typically, astronomers must detect light from the merger or identify the originating galaxy.
Ideally, all measurement techniques should converge on the same Hubble constant. However, discrepancies exist, indicating that a deeper understanding of the early universe may be necessary. Proposed theories range from early dark energy to interactions between dark matter and neutrinos.
Innovative Gravitational Wave Background Method
The researchers introduced a novel approach to estimate the Hubble constant by analyzing black hole collisions that current detectors cannot individually identify, collectively forming a gravitational-wave background.
Cousins explained, "By observing individual black hole collisions, we can determine their rates across the universe. We anticipate many more events that remain unobserved, contributing to the gravitational-wave background." Their findings suggest that if the Hubble constant were lower, the observable universe's volume would also decrease, leading to a denser concentration of black hole collisions.
This stochastic siren method reflects the random nature of these collisions. The team tested their approach using existing LVK data, successfully ruling out slower expansion rates and achieving a more precise estimate of the Hubble constant, indicative of its relevance to the Hubble tension.
As gravitational-wave observatories enhance their capabilities, this method is expected to gain even greater efficacy, with scientists anticipating the detection of the gravitational-wave background within six years. Until then, ongoing research will refine the limits on this background signal, narrowing the range of the Hubble constant.
Cousins concluded, "This methodology lays the groundwork for future applications as we enhance sensitivity and possibly detect the gravitational-wave background, leading to improved cosmological insights and a clearer resolution to the Hubble tension."
Research Support and Resources
The analysis utilized the Illinois Campus Cluster, operated in collaboration with the National Center for Supercomputing Applications. Funding was provided by multiple NSF grants and additional support from various foundations.