TRGB Distance Bias

The Tip of the Red Giant Branch (TRGB) provides a standard candle based on the sharp termination of the first-ascent red giant branch in old stellar populations, set by the helium flash at a nearly constant core mass and therefore nearly constant absolute luminosity in the I-band. Different groups applying TRGB calibrations to the same sample of distance-ladder host galaxies have obtained systematically different H₀ values: the Carnegie-Chicago Hubble Program originally reported H₀ ~ 69.8 km/s/Mpc, closer to Planck, while more recent JWST-calibrated TRGB analyses from the same and competing teams converge on H₀ ~ 72–73 km/s/Mpc, much closer to SH0ES. The disagreement traces primarily to the choice of calibrating anchor — the Large Magellanic Cloud versus NGC 4258 — and to the photometric treatment of crowded stellar fields.

Successive Collision Theory addresses the TRGB bias tension by clarifying that no TRGB calibration can give the globally averaged ΛCDM H₀ even in principle, because all accessible TRGB anchor galaxies lie within the locally enhanced Λ_eff environment of the KBC supervoid. Both the LMC and NGC 4258 reside within a few Mpc of the Milky Way, well inside the ~300 Mpc void structure where expansion rates are genuinely elevated by 2–3 km/s/Mpc above the cosmic mean. Any TRGB calibration anchored to these objects will encode the local rather than the global expansion rate, yielding a locally measured H₀ that is physically larger than Planck's globally inferred value. The variation between groups reflects different emphases on local versus slightly more distant calibrators, not a systematic instrumental error, and SCT predicts that all sufficiently local calibrations will converge on H₀ values in the range of 72–74 km/s/Mpc as measurement precision improves.

The recent convergence of JWST TRGB results toward the SH0ES value is therefore not evidence of previous error in the low-H₀ analyses but confirmation that improved photometric techniques, which reduce crowding biases and give cleaner TRGB discontinuities, recover the true local expansion rate more faithfully. SCT interprets the spread of TRGB H₀ values across the literature as a map of how different anchor choices sample the locally enhanced Λ_eff gradient within the supervoid, with more local anchors giving higher values and those at the void boundary giving slightly lower ones — a spatial pattern that upcoming all-sky surveys with Roman Space Telescope will be able to chart directly.