A team led by the University of Chicago, using the James Webb Space Telescope (JWST), has made a measurement that could help settle one of the most pressing debates in modern cosmology: the rate at which the universe is expanding. This debate, known as the “Hubble tension,” has persisted for decades, with two primary methods of measurement yielding conflicting results.
The Hubble constant defines the universe’s rate of expansion. One method to determine H0 is through the CMB, relic light from the Big Bang. This method has always been giving a consistent value of H0: about 67.4 km/s/Mpc. Another method is through the measurement of local galaxy expansion, employing stars whose brightness is known. The latter method often gives a higher current value of H0: about 74 km/s/Mpc.
This difference has made scientists begin to speculate that there could be unknown factors disturbing their understanding of the evolution of the universe. The conflict has become one of the focuses of cosmological research and raises begging questions about possible changes in the universe over time.
Then enter the James Webb Space Telescope, launched in 2021, the successor to Hubble. JWST is allowing astronomers to access unprecedented data and sharp images of new insights into the cosmos. Using JWST, University of Chicago cosmologist Wendy Freedman and her team measured ten nearby galaxies in an attempt to pin down a more precise value for H0.
Freedman’s team checked their answers using three independent methods. One was based on what’s called Cepheid variable stars. Their brightness varies with a regular period, so astronomers know exactly how bright they are. A second method, called the “Tip of the Red Giant Branch,” is based on the way low-mass stars always have the same upper limit of brightness when they become red giants. A third method is based on carbon stars, all of which show very similar colors and brightnesses when seen in near-infrared wavelengths.
Quite amazingly, all three results returned values of H0 that were within the error margin from the estimate provided by the CMB method of 67.4 kilometers per second per megaparsec. “Getting good agreement from three completely different types of stars to us, is a strong indicator that we’re on the right track,” said Freedman.
A paper published in the Astrophysical Journal provides details of the measurements, which indicate the Hubble tension may not be as large as first thought. Coauthor Barry Madore said follow-up observations with JWST would be invaluable for either confirming or refuting the findings and, therefore, also would help evaluate the impact on cosmology.
The authors of the study are UChicago research scientist In Sung Jang; Taylor Hoyt of Lawrence Berkeley National Laboratory; and UChicago graduate students Kayla Owens and Abby Lee. Their collaboration is major progress to be had on the solution for one of cosmology’s most persistent mysteries.
As the scientific world further delves into discovering the dynamics of this ever-expanding universe, contributions by JWST will be among those shaping modern concepts of what’s out there. The resolution of the Hubble tension may have profound consequences on basic physics and modern cosmology and therefore change the way we understand the history, present, and future of the universe.