Hubble tension

Still disputed in the literature as of 2026.

Distance-ladder measurements of the local universe (Cepheid variables and Type Ia supernovae, refined by the SH0ES team) have increasingly disagreed with the Hubble constant inferred from Planck's cosmic microwave background data assuming standard LambdaCDM cosmology, with the discrepancy growing to roughly 5 standard deviations by the early 2020s. James Webb Space Telescope observations through 2024-2025 have largely confirmed the higher local value using multiple independent distance indicators (Cepheids, tip-of-the-red-giant-branch, and others), ruling out simple Cepheid-crowding systematic errors as the culprit, while DESI baryon acoustic oscillation data continues to support the lower early-universe value. Whether the resolution lies in new physics (e.g. early dark energy) or an unidentified systematic remains genuinely disputed as of 2026.

Two completely different ways of measuring how fast the universe is expanding — one looking at nearby stars, one looking at the afterglow of the Big Bang — stubbornly refuse to agree, and resolving why may require new physics.

Explanations that circulated

• That JWST observations resolved the tension by confirming one side was simply due to a measurement error — JWST has instead independently reproduced the local (SH0ES-like) high value with several methods, deepening rather than resolving the discrepancy as of 2025-2026
• That the tension is definitely new physics rather than an unidentified systematic error — this is still actively disputed among cosmologists

References

• Status Report on the Chicago-Carnegie Hubble Program (CCHP): Measurement of the Hubble Constant Using the Hubble and James Webb Space Telescopes, Chicago-Carnegie Hubble Program Collaboration, The Astrophysical Journal, 2025