The findings offer a crucial cross-check for previous Hubble Space Telescope measurements of the universe's mysterious expansion
New observations from the James Webb Space Telescope suggest that a new feature in the universe—not a flaw in telescope measurements—may be behind the decadelong mystery of why the universe is expanding faster today than it did in its infancy billions of years ago.
The new data confirms Hubble Space Telescope measurements of distances between nearby stars and galaxies, offering a crucial cross-check to address the mismatch in measurements of the universe's mysterious expansion. Known as the Hubble tension, the discrepancy remains unexplained even by the best cosmology models.
"The discrepancy between the observed expansion rate of the universe and the predictions of the standard model suggests that our understanding of the universe may be incomplete," said Nobel laureate and lead author Adam Riess, a Bloomberg Distinguished Professor and professor of Physics and Astronomy at Johns Hopkins University. "With two NASA flagship telescopes now confirming each other's findings, we must take this [Hubble tension] problem very seriously—it's a challenge but also an incredible opportunity to learn more about our universe."
Published in The Astrophysical Journal, the research builds on Riess' Nobel Prize–winning discovery that the universe's expansion is accelerating owing to a mysterious "dark energy" permeating vast stretches of space between stars and galaxies.
Riess' team used the largest sample of Webb data collected over its first two years in space to verify the Hubble telescope's measure of the expansion rate of the universe, a number known as the Hubble constant. They used three different methods to measure distances to galaxies that hosted supernovae, focusing on distances previously gauged by the Hubble telescope and known to produce the most precise "local" measurements of this number. Observations from both telescopes aligned closely, revealing that Hubble's measurements are accurate and ruling out an inaccuracy large enough to attribute the tension to an error by Hubble.
Still, the Hubble constant remains a puzzle because measurements based on telescope observations of the present universe produce higher values compared to projections made using the "standard model of cosmology," a widely accepted framework of how the universe works calibrated with data of cosmic microwave background, the faint radiation left over from the big bang.
While the standard model yields a Hubble constant of about 67-68 kilometers per second per megaparsec, measurements based on telescope observations regularly give a higher value of 70 to 76, with a mean of 73 km/s/Mpc. This mismatch has perplexed cosmologists for more than a decade because a 5-6 km/s/Mpc difference is too large to be explained simply by flaws in measurement or observational technique. (Megaparsecs are huge distances. Each is 3.26 million light-years, and a light-year is the distance light travels in one year: 9.4 trillion kilometers, or 5.8 trillion miles.)
Since Webb's new data rules out significant biases in Hubble's measurements, the Hubble tension may stem from unknown factors or gaps in cosmologists' understanding of physics yet to be discovered, Riess' team reports.
"The Webb data is like looking at the universe in high definition for the first time and really improves the signal-to-noise of the measurements,'' said Siyang Li, a graduate student working at Johns Hopkins University on the study.
The new study covered roughly a third of Hubble's full galaxy sample, using the known distance to a galaxy called NGC 4258 as a reference point. Despite the smaller dataset, the team achieved impressive precision, showing differences between measurements of under 2%—far smaller than the approximately 8-9% size of the Hubble tension discrepancy.