Astronomers using the W. M. Keck Observatory and other telescopes have measured the universe’s expansion rate, intensifying the long-running ‘Hubble tension.’ Using time delay cosmography of strongly lensed quasars, the measurement has 4.5% precision and gives a present-day H0 higher than early-universe estimates. Cosmologists debate whether this split reflects measurement issues or a need to revise the standard cosmological model.
The W. M. Keck Observatory reports that spectroscopy from its Keck Cosmic Web Imager, combined with data from NASA’s James Webb and Hubble Space Telescopes and ESO’s Very Large Telescope, measured the masses of the lensing galaxies. The study also relied on long-term photometric monitoring from ESO in Chile, achieving about 4.5% precision on the Hubble constant using this independent lensing-based method.
“Many scientists hope this could signal a new cosmological model,” Tommaso Treu of UCLA told Maui Now. The result, from the international TDCOSMO collaboration, uses multiple instruments and decades of monitoring to verify its assumptions.
How Time Delay Cosmography Tightened The Numbers
Time delay cosmography uses a foreground galaxy to split the light of a variable quasar into multiple images, each arriving at slightly different times, allowing astronomers to measure cosmological distances. The new analysis added spatially resolved kinematics—detailed measurements of star motions inside the lens galaxies—to improve lens mass models and address the mass sheet degeneracy. The TDCOSMO preprint on arXiv details how data from KCWI, JWST, and VLT reduce a major source of uncertainty in lensing-based H0 estimates.
Early Universe Numbers Vs. Local Measurements
The tension shows up squarely in the numbers. Fits to the cosmic microwave background under the standard ΛCDM model favor a Hubble constant of about 67 km/s/Mpc, as reported in the Planck 2018 cosmological parameters paper. By contrast, Cepheid and supernova distance ladder measurements led by the SH0ES team find H0 near 73 km/s/Mpc (Riess et al.), a gap far larger than the quoted measurement errors and the central puzzle that the new lensing result reinforces. Planck 2018 (A&A) and Riess et al. (SH0ES) provide the standard modern benchmarks.
What This Could Mean
If those three independent approaches, CMB inference, the local distance ladder, and lensing time delays, keep disagreeing as the error bars shrink, the mismatch may be pointing to new physics instead of bad data. Ideas on the table range from an episode of early dark energy to extra relativistic particles or other tweaks to the universe’s history, and multiple groups are actively testing such models against the latest measurements. Research centers following the new work note that independent confirmations are crucial before anyone starts rewriting the standard cosmological model. IEEC
What’s Next
The collaboration describes the 4.5% result as a big milestone, not the final verdict. The next target is to push the precision below about 1.5% so the community can finally see whether the Hubble tension survives an even tougher test. Keck scientists point to continued KCWI observations, additional JWST spectroscopy, and more long baseline photometric monitoring as the path to tighter lens models and smaller systematic errors. The W. M. Keck Observatory adds that the project is a textbook example of why cosmologists insist on multiple, independent techniques before calling for major changes to our picture of the universe.
