One of the most enduring enigmas in modern cosmology centers on the exact rate at which the universe is expanding. New observations from the James Webb Space Telescope (JWST) have added a fresh layer of intrigue, confirming earlier measurements that suggest the universe's expansion rate has fluctuated over different epochs in its history.
These findings, published in The Astrophysical Journal, align closely with data from the Hubble Space Telescope, representing a crucial step forward in solving this cosmic riddle—a puzzle that has long defied even the most advanced cosmological models.
James Webb Telescope arrives at its destination
(Approximation: NASA)
“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. 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,” said Professor Adam Riess of Johns Hopkins University. Riess, a Jewish-American astrophysicist and Nobel laureate, was honored in 2011 for his pivotal role in discovering the universe's accelerating expansion.
The study builds on the groundbreaking research of Riess and his fellow Nobel laureates, Professor Saul Perlmutter (UC Berkeley) and Professor Brian Schmidt (Australian National University). Their work revealed that the universe's accelerated expansion is driven by a mysterious force known as "dark energy," which permeates the vast voids between stars and galaxies.
Riess and his team utilized the largest dataset to date from the JWST, collected over its first two years of operation, to cross-check Hubble’s measurements of the universe’s expansion rate—known as the Hubble Constant. Using three independent methods to measure distances to galaxies hosting supernovae, the researchers found that the Hubble Constant remains as enigmatic as ever. Observations of the present-day universe yield higher values for the Hubble Constant than predictions from the “standard cosmological model,” which relies on Einstein’s general relativity to describe gravity on cosmic scales.
The standard model predicts a Hubble Constant of approximately 41.6–42.3 miles per second per megaparsec (a unit of distance equivalent to 3.26 million light-years). However, telescope-based observations consistently produce values between 43.5 and 47.2 miles per second per megaparsec. This discrepancy—an excess of about 3–3.7 miles per second per megaparsec—has baffled cosmologists for over a decade. The gap is far too large to be explained away by errors in measurement techniques or observational data.
The new study relied on a subset of Hubble’s galaxy observations, with the galaxy NGC 4258 (located 15 million light-years from Earth) serving as a reference point. Despite using a smaller dataset, the team achieved striking precision. Alongside analyzing Cepheid variable stars—giant stars whose predictable cycles of brightness make them reliable “cosmic yardsticks”—the researchers incorporated measurements based on carbon-rich stars and the brightest red giant stars in the same galaxies. Across the galaxies observed by the JWST and their accompanying supernovae, the team calculated a Hubble Constant of 45.1 miles per second per megaparsec, closely matching Hubble’s measurement of 45.2 miles per second per megaparsec for the same galaxies.
While the Hubble Constant has no direct bearing on the solar system, Earth, or day-to-day life, it provides critical insights into the universe’s large-scale evolution. Scientists rely on this value to map the cosmos and deepen their understanding of its development over the 13–14 billion years since the Big Bang.
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The findings by Professor Riess and his team could offer new perspectives on other discrepancies that have emerged between the standard cosmological model and recent observations. For example, the model fails to fully explain the nature of dark matter and dark energy—mysterious components that are estimated to make up approximately 96% of the universe’s composition. These elusive forces are likely responsible for the accelerated expansion observed since the universe’s earliest moments following the Big Bang.
The study serves as a poignant reminder that even with cutting-edge technology and revolutionary discoveries, the universe continues to guard its deepest secrets. Yet, with tools like the JWST and Hubble working in tandem, humanity edges closer to peeling back the cosmic veil.