JWST H₀ Prior Contamination

The James Webb Space Telescope has revolutionized distance ladder calibration by providing infrared photometry of Cepheid variable stars in nearby galaxies at unprecedented precision, dramatically reducing the systematic uncertainty from crowding, blending, and photometric contamination that plagued HST optical measurements. Early JWST results confirmed the SH0ES Cepheid-based calibration and yielded H₀ ~ 73 km/s/Mpc with reduced systematic uncertainty, strengthening the case that the Hubble tension is not an artifact of Cepheid photometry. However, some analyses have argued that the interpretation of JWST photometry still involves prior assumptions — about stellar population models, the treatment of outlier Cepheids, and the choice of period-luminosity relation slope — and that different prior choices can shift the inferred H₀ by 1–2 km/s/Mpc. The question of whether JWST analyses are sufficiently prior-independent to claim a definitive H₀ measurement remains a point of contention.

Successive Collision Theory addresses the JWST prior contamination debate by clarifying that the fundamental tension is physical rather than methodological. In SCT, the local universe expansion rate is genuinely higher than the globally averaged value by approximately 4–6 km/s/Mpc due to the combined effect of the KBC supervoid spatial enhancement and the temporal growth of Λ_eff since decoupling. This physical offset is so large that no plausible prior contamination — which shifts H₀ by at most 1–2 km/s/Mpc — can explain it away. If JWST analyses were prior-free and perfectly calibrated, they would still yield H₀ ~ 72–74 km/s/Mpc because that is the true local expansion rate. Prior contamination debates are therefore addressing a secondary level of precision within the high-H₀ camp rather than bridging the fundamental gap to Planck.

The SCT framework does, however, motivate a specific recommendation for JWST Cepheid analyses: account for the hereditary time transmission correction when converting Cepheid pulsation periods to stellar ages in the distance-ladder host galaxies. Because Cepheids in galaxies embedded at different depths within the local gravitational hierarchy tick at slightly different proper time rates, their pulsation periods are modestly shifted relative to what a freely coasting standard clock would indicate. This is the same GPS-correction mechanism extended to every gravitational potential level, and it introduces a systematic offset in the Cepheid period-luminosity zero-point that varies with the local gravitational environment of each calibrator galaxy. The effect is at the sub-percent level but grows coherently with environmental depth, making it a potential source of the mild scatter observed in the period-luminosity residuals across different calibrator galaxies.