New Gravitational Wave Method Narrows Search for Universe’s Expansion Rate | Hubble Tension Update

For decades, the rate at which the universe is expanding has been a subject of intense study, and a persistent discrepancy between different measurement methods has emerged as one of cosmology’s biggest puzzles. Now, a novel approach leveraging the faint “hum” of gravitational waves – ripples in spacetime – offers a potential new tool for resolving this “Hubble tension.” Researchers at the University of Illinois Urbana-Champaign and the University of Chicago have developed a technique to estimate the Hubble constant, the value that describes the universe’s expansion rate, by analyzing the gravitational-wave background created by merging black holes.

The Hubble Tension: A Cosmic Disagreement

The Hubble constant is fundamental to our understanding of the universe’s age, size, and evolution. Edwin Hubble first observed that galaxies are receding from us, and the farther away they are, the faster they move [NASA]. Determining the precise value of the Hubble constant is therefore crucial. However, measurements derived from observing the early universe – based on the cosmic microwave background radiation – consistently differ from those obtained from studying the late universe, using techniques like observing supernovae and Cepheid variable stars. This discrepancy, known as the Hubble tension, suggests a potential gap in our understanding of fundamental physics.

Traditional methods for measuring the Hubble constant rely on electromagnetic radiation – light. One common technique uses “standard candles,” like Type Ia supernovae, which have a known intrinsic brightness. By comparing this known brightness to their observed brightness, astronomers can calculate their distance and, the expansion rate. Gravitational waves offer an independent method, but traditionally required identifying the host galaxy of the merger to determine the recession speed.

Gravitational Waves and the Stochastic Siren Method

Gravitational waves, predicted by Einstein’s theory of general relativity, are disturbances in the fabric of spacetime caused by accelerating massive objects. The Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, and KAGRA (LVK) Collaboration have detected gravitational waves from numerous black hole and neutron star mergers. The new research, detailed in a paper accepted for publication in Physical Review Letters, focuses not on individual merger events, but on the collective “hum” of countless, undetectable black hole collisions – the gravitational-wave background.

“Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe,” explains Bryce Cousins, a physics graduate student at the University of Illinois and lead author of the study. “Based on those rates, we expect there to be a lot more events that we can’t observe, which is called the gravitational-wave background.”

The team’s approach, dubbed the “stochastic siren method,” leverages the relationship between the Hubble constant and the strength of this background signal. If the Hubble constant were lower, the observable volume of the universe would be smaller, meaning black hole collisions would be more densely packed, resulting in a stronger gravitational-wave background. By setting limits on the strength of the background signal, researchers can rule out slower expansion rates. What we have is a significant departure from previous gravitational wave methods, which often relied on identifying the host galaxy of the merger event.

How the New Method Improves Accuracy

The researchers tested their method using existing data from the LVK Collaboration. Even without directly detecting the gravitational-wave background, they were able to constrain the possible values of the Hubble constant. Combining the stochastic siren method with existing measurements from individual black hole mergers yielded a more precise estimate, falling within the range of the ongoing Hubble tension. [University of Illinois]. This suggests the method holds promise for refining future measurements and potentially resolving the discrepancy.

“This result is very significant—it’s important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension,” says Nicolás Yunes, Illinois Physics Professor and founding director of the Illinois Center for Advanced Studies of the Universe (ICASU). “Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves.”

Implications and Future Directions

The Hubble tension isn’t merely a disagreement over a number; it could indicate that our current cosmological model – the Lambda-CDM model – is incomplete. Possible explanations include the existence of early dark energy, interactions between dark matter and neutrinos, or a changing behavior of dark energy over time. [Keck Observatory]. An independent and accurate measurement of the Hubble constant is therefore crucial for guiding theoretical developments.

Daniel Holz, UChicago Professor of Physics and Astronomy & Astrophysics, emphasizes the novelty of the approach: “It’s not every day that you come up with an entirely new tool for cosmology. 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.”

As gravitational-wave observatories become more sensitive – with planned upgrades to LIGO, Virgo, and KAGRA, and the development of new detectors like the Einstein Telescope and Cosmic Explorer – the gravitational-wave background is expected to be directly detected within the next six years. Until then, increasingly stringent limits on the background signal will continue to refine the possible range of the Hubble constant. The team anticipates that incorporating the detected background signal will further improve the accuracy of their method and bring scientists closer to resolving the Hubble tension. The analysis was supported by the Illinois Campus Cluster, operated by the Illinois Campus Cluster Program and the National Center for Supercomputing Applications.

This research represents a significant step forward in our ability to probe the universe’s expansion history and potentially unlock new insights into the fundamental laws governing its evolution. The stochastic siren method offers a promising, independent path toward resolving one of the most challenging problems in modern cosmology.