Gravitational lens time delays sharpen the Hubble constant measurement
by Riko Seibo
Tokyo, Japan (SPX) Dec 09, 2025
A growing team of astronomers is testing an alternative route to gauge the universe’s expansion rate by exploiting time delays in gravitationally lensed images of distant quasars. This approach aims to address the long-standing disagreement known as the Hubble tension, which arises between local measurements and early-universe inferences of the Hubble constant.
The Hubble constant describes how fast galaxies recede from one another as a function of distance. Local observations suggest objects about one megaparsec away (roughly 3.3 million light-years) recede at about 73 kilometers per second. Traditional methods rely on distance ladders built from Type Ia supernovae and Cepheid variable stars to infer distances in distant galaxies, but lingering concerns about systematic errors have spurred independent techniques that do not rely on these calibrators.
In a new study, a team led by Project Assistant Professor Kenneth Wong and postdoctoral researcher Eric Paic at the University of Tokyo’s Research Center for the Early Universe reports findings from time-delay cosmography applied to eight strong gravitational lens systems. Each system features a massive foreground galaxy that bends light from a background quasar.
“To measure the Hubble constant with time-delay cosmography, a truly massive galaxy is required to act as a lens,” Wong explains.
“Gravity from the lensing galaxy deflects light from objects behind it, producing distorted images. When the geometry is favorable, multiple distorted images appear, each following a slightly different path and arriving at different times. By identifying identical fluctuations in these images that are offset in time, we can determine the time delay between light paths.”
Pairing this time-delay data with estimates of the lensing galaxy’s mass distribution enables a more precise determination of the expansion rate. The resulting Hubble constant aligns with other late-universe estimates and sits apart from early-universe values derived from the cosmic microwave background.
Each of the eight lens systems includes a central lensing galaxy surrounded by multiple lensed images of a background quasar, which may appear as bright points arranged in rings or arcs around the lens. Since light travels along different routes through warped spacetime near the lens, brightness variations in the quasar reach observers at slightly different times. Those delays, together with models of the lens’s mass distribution, provide an independent path to inferring the Hubble constant.
The new measurement broadly agrees with other nearby, late-universe estimates and diverges from the early-universe value near 67 kilometers per second per megaparsec. “Our Hubble constant estimate is consistent with current-day observations and less compatible with early-universe measurements. This discrepancy hints that the Hubble tension could reflect real physics beyond standard models, rather than merely hidden systematic errors,” says Wong. “Crucially, our result is independent of other methodologies, so any systematic biases in those methods should not bias this measurement.”
This work contributes to the Hubble tension—the gap between expansion rates derived from local distance ladders and those inferred from the cosmic microwave background. If the disagreement persists as measurements improve, it could signal that the standard cosmological model is incomplete or that new physical processes influenced the universe’s expansion history.
“The main objective here was to refine the methodology, and now the goal is to expand the sample size to improve precision and decisively address the Hubble tension,” Paic notes. “Right now, the precision is about 4.5%. To firmly confirm whether a tension exists, measurements need to reach roughly 1–2% precision.”
The current analysis uses eight time-delay lens systems with background quasars, incorporating new data from both ground-based observatories and space-based facilities, including the James Webb Space Telescope. Plans are to enlarge the lens sample, enhance models of mass distribution within lens galaxies, and continue to hunt for residual systematic errors that could influence the inferred expansion rate.
“One of the largest uncertainties is the exact mass distribution within the lens galaxies. While simple mass profiles are often assumed to match observations, such assumptions may not capture reality precisely, and this uncertainty can directly affect the results,” Wong explains. “The Hubble tension matters because it could herald a new era in cosmology with new physics. This project embodies decades of collaboration across multiple observatories and research teams, underscoring the importance of international cooperation in science.”
Research Report: TDCOSMO 2025: Cosmological constraints from strong lensing time delays (https://doi.org/10.1051/0004-6361/202555801)
Related Links
- The University of Tokyo (https://www.u-tokyo.ac.jp/en/index.html)
- Understanding Time and Space (https://www.spacedaily.com/TimeAndSpace.html)
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