JWST H₀ Prior Contamination

The James Webb Space Telescope was widely expected to resolve or sharpen the Hubble tension by providing superior photometry of Cepheids in supernova-host galaxies, eliminating crowding and blending concerns that plagued HST measurements. Early JWST results confirmed the SH0ES Cepheid distances in the key calibrating galaxies NGC 4258, the Large Magellanic Cloud, and several supernova hosts, finding H₀ ~ 72–73 km/s/Mpc with no significant downward revision from the HST values. However, a separate concern has emerged: when JWST data are analyzed, the choice of prior on H₀ used in the analysis pipeline — even a supposedly uninformative prior — can subtly bias the result. JWST calibration products and distance analysis software chains often include implicit assumptions about cosmological parameters that feed back into the distance fitting.

The contamination tension is methodological rather than purely observational: ΛCDM-based priors permeate the data reduction and analysis steps in ways that are not always transparent. Anchors for the Lyman-break galaxy photometry used to validate JWST zero-points, assumptions about stellar population models, and the cosmological model used to transform observed redshifts into velocities for Hubble flow membership all carry implicit prior information. Critics argue that a fully prior-free analysis of JWST Cepheid data would have larger uncertainties and a less definitive H₀ value. Conversely, if JWST data are analyzed with a Planck-cosmology prior and still return H₀ ~ 73, the tension only deepens. The struggle to perform a genuinely model-independent measurement of H₀ from any dataset — including the most advanced space telescope ever built — underscores the depth of the problem that ΛCDM faces.