The researchers drew on data from Keck's Cosmic Web Imager together with observations from NASA's James Webb Space Telescope, the Hubble Space Telescope, and ESO's Very Large Telescope to derive the Hubble constant, H0, using a method that does not depend on early-universe assumptions. Their result confirms that the expansion rate measured in the nearby universe does not match the value inferred from measurements of the universe when it was much younger, reinforcing the evidence that the standard cosmological model may be incomplete.

The Hubble constant, introduced by Edwin Hubble in 1929, describes how quickly galaxies recede from one another and encodes information about the age and evolution of the cosmos. Early-universe probes that rely on cosmological models favor an expansion rate of about 67 kilometers per second per megaparsec, while measurements of the local universe point to roughly 73 kilometers per second per megaparsec, a discrepancy known as the Hubble tension.

If this difference is not due to unrecognized measurement errors, it could indicate new physics such as additional types of particles or a brief early dark energy phase that altered the expansion history after the Big Bang. Because such implications would require revising key aspects of current theory, astronomers emphasize the need for multiple, fully independent methods to test the result.

"What many scientists are hoping is that this may be the beginning of a new cosmological model," said Tommaso Treu, Distinguished Professor of Physics and Astronomy at the University of California Los Angeles and one of the authors of the study published in Astronomy and Astrophysics. "This is the dream of every physicist. Find something wrong in our understanding so we can discover something new and profound," added Simon Birrer, Assistant Professor of Physics at the Stony Brook University and one of the corresponding authors of the study.

"This is significant in that cosmology as we know it may be broken," said John O'Meara, Chief Scientist and Deputy Director of Keck Observatory. "If it is true that the Hubble Tension isn't a mistake in the measurements, we will have to come up with new physics."

To obtain the new value of H0, the team used time-delay cosmography, which exploits gravitational lensing by massive galaxies that split the light from more distant galaxies and quasars into multiple images. When the background source brightens or dims, astronomers can measure how long it takes those changes to appear in each image; these time delays act as cosmic yardsticks that allow them to determine distances and derive the expansion rate.

KCWI's spectroscopy was central to the analysis because it measures the motions of stars in the lensing galaxies, revealing how massive they are and how strongly they bend light. This information reduces a major source of uncertainty in lensing studies known as the mass-sheet degeneracy and, when combined with more than 20 years of photometric monitoring from ESO facilities in Chile, yields precise time delays for the lensed systems.

"The key breakthrough relied on the motion of stars in the lens galaxies as measured via Keck/JWST/VLT spectroscopy to address the main source of uncertainty, known as the mass-sheet degeneracy. The result also relies on long-term collaborative work between observatories including time delay measurements from 20 years of photometric data obtained at ESO in Chile," said Anowar Shajib, postdoctoral fellow at the University of Chicago, and a corresponding author of the study.

The team currently achieves about 4.5 percent precision on the Hubble constant, placing this work among the most accurate late-time measurements but still short of the level needed to rule out remaining systematics beyond doubt. The next objective is to push the uncertainty below 1.5 percent by expanding the sample of lensed systems, improving time-delay measurements, and refining models of the lens galaxies, a level of accuracy the researchers note would exceed how precisely most people know their own height.

Research Report:Astronomy and Astrophysics

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