A new international effort by astronomers is bringing sharper clarity to one of cosmology’s biggest questions—the rate at which the universe is expanding.
The expansion rate, known as the Hubble constant, has long been difficult to pin down despite increasingly precise observations. Data from NASA’s Hubble Space Telescope and the James Webb Space Telescope have improved accuracy, but different measurement methods continue to produce slightly varying results.
To address this, an international collaboration has developed a new framework that combines multiple measurement techniques into a single analysis. This approach reduces reliance on any one method and improves overall precision.
A key part of this process is the Cosmic distance ladder, which links different techniques used to measure distances across the universe. The new framework integrates various distance markers while accounting for their reliability, helping minimise uncertainties and deliver a more refined estimate.
The findings, published on April 10 in the journal Astronomy & Astrophysics, estimate the expansion rate of the nearby universe at around 73.5 kilometers per second per megaparsec. Previous estimates have typically ranged between 73 and 76 kilometers per second per megaparsec.
“The power of this work is that it doesn’t depend on any single method,” said Adam Riess of Johns Hopkins University and the Space Telescope Science Institute. “When multiple, independent measurements all point to the same answer, it strengthens the case that we’re seeing a real feature of the universe, not a flaw in one technique. Right now, those measurements suggest the universe today is expanding faster than we would expect based on how it looked shortly after the big bang.”
This ongoing mismatch between measurements of the early universe and the present-day expansion rate is known as the Hubble tension.
Looking ahead, NASA plans to further investigate this mystery with the launch of the Nancy Grace Roman Space Telescope, expected as early as this fall. The mission is set to provide extensive new data to refine distance measurements and help resolve one of the most persistent puzzles in modern cosmology.
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