Key Highlights

  • JWST has confirmed that earlier Hubble Space Telescope Cepheid measurements were accurate, eliminating crowding or dust bias as an explanation for the Hubble tension.
  • The SH0ES team's latest result places the local Hubble constant at 73.49 km/s/Mpc, roughly 6 sigma above the Planck CMB value of 67.4 km/s/Mpc.
  • DESI DR2 data provide compelling new evidence that dark energy is evolving over time, which may hold the key to resolving the tension.

The Disagreement That Will Not Disappear

For nearly a decade, two of the most precise methods for measuring the universe's expansion rate have produced answers that stubbornly refuse to agree. The Hubble constant, the number that quantifies how fast the cosmos is expanding, reads around 73 kilometres per second per megaparsec when measured through the local distance ladder, and around 67 when inferred from the cosmic microwave background. Both methods are highly precise. Both have been tested extensively. And the gap between them, now known as the Hubble tension, has grown sharper with every round of new data rather than dissolving.

The tension sits at approximately 5 to 6 sigma, a level of statistical significance that in particle physics defines a discovery. It is now widely regarded as the most consequential open problem in observational cosmology.

What JWST Has Settled, and What It Has Not

One of the leading candidate explanations for the tension was that Cepheid variable stars used in the local distance ladder were being measured inaccurately because of crowding in dense stellar environments and contamination from dust. The James Webb Space Telescope was expected to resolve that ambiguity.

It has done so decisively, but not in the direction that would dissolve the tension. The SH0ES team, led by Adam Riess, used JWST to observe Cepheids in 19 Type Ia supernova host galaxies with resolution far exceeding that of the Hubble Space Telescope. Published in The Astrophysical Journal in October 2025, the results showed no evidence of crowding bias between HST and JWST photometry at more than 8 sigma of confidence. Combining JWST measurements with HST data across 24 supernovae in 19 hosts, the team derived a Hubble constant of 73.49 plus or minus 0.93 km/s/Mpc. Including tip-of-the-red-giant-branch calibrations from 35 additional supernovae brought the combined value to 73.18 plus or minus 0.88 km/s/Mpc, approximately 6 sigma above the Planck satellite's CMB-derived value of 67.4.

A separate team, the Chicago-Carnegie Hubble Program led by Wendy Freedman, arrived at a lower local value of around 70.39 km/s/Mpc using JWST and alternative distance indicators. That value sits between the two camps and has led the CCHP to argue the tension may be overstated. Critics including astrophysicist Dan Scolnic of Duke University contend the CCHP analysis excludes supernovae that could be incorporated and that the tension remains real. At the American Astronomical Society meeting in January 2025, Scolnic declared the Hubble tension a crisis.

Dark Energy as the Wild Card

While instrument teams debate the distance ladder, the second family of evidence has introduced its own complications. Results from the Dark Energy Spectroscopic Instrument's second data release, published in The Astrophysical Journal in March 2026, provide compelling evidence that dark energy is not the simple cosmological constant assumed by the standard Lambda Cold Dark Matter model. Combining DESI DR2 baryon acoustic oscillation data with Planck CMB measurements and the Pantheon+ supernova catalogue, researchers found that dark energy evolves with cosmic time, favouring a phantom phase in the early universe and quintessence behaviour at late times.

If dark energy is dynamical rather than fixed, the inference of the Hubble constant from CMB data may be systematically shifted. A growing body of theoretical work, including proposals involving early dark energy and interacting dark sector components, is under active scrutiny. A conference proceedings from the Tensions in Cosmology 2025 meeting noted that the hypothesis of unknown systematic errors as the explanation for the tension is no longer the dominant view among practitioners.

Conclusion

The Hubble tension is no longer a statistical curiosity that might dissolve with better data. JWST has tested the primary instrument-based explanation and found it wanting. DESI has shown that the energy content of the universe may be more complex than assumed. The standard model of cosmology, Lambda Cold Dark Matter, has survived a generation of precision tests, but the persistence and growth of the Hubble tension suggests that it is incomplete. Whether the resolution lies in new physics governing the early universe, evolving dark energy at late times, or some undiscovered systematic in an unexpected place, the answer will redefine fundamental cosmology. Upcoming surveys including Euclid, the Nancy Grace Roman Space Telescope, and the Vera C. Rubin Observatory's Legacy Survey of Space and Time will provide the next decisive tests. For now, the universe is expanding. How fast remains, remarkably, an open question.