Distance Ladder Splits

The distance ladder community has been divided by the observation that H₀ values systematically depend on which calibrating anchor is chosen for the first rung. Using the LMC as the primary geometric anchor — as in the SH0ES program with eclipsing binaries — yields H₀ ~ 73 km/s/Mpc. Using the maser galaxy NGC 4258 as the primary anchor also yields H₀ ~ 73 km/s/Mpc in most analyses. However, certain combinations of TRGB calibrators with specific photometric pipelines initially yielded values closer to 70 km/s/Mpc, while Milky Way Cepheid parallax-based anchors give values intermediate between these. The existence of these splits — real or perceived — has been used to argue either that all anchors contain unresolved systematic errors, or that the discrepancy depends on which part of the local universe is sampled.

Successive Collision Theory transforms the interpretation of these splits from an embarrassment into a prediction. If Λ_eff varies spatially within the local cosmic environment, then anchors at different distances and directions should yield modestly different H₀ values even after all systematic errors are removed. The LMC at ~50 kpc, the Milky Way Cepheids at kiloparsec distances, and NGC 4258 at ~7 Mpc all reside within the innermost region of the KBC supervoid where Λ_eff is most strongly enhanced. The ~300 Mpc supervoid has a density gradient such that measurements anchored more deeply within the void should give slightly higher H₀ than those at the void periphery. The spread of H₀ values across different anchor choices is therefore a spatial map of the Λ_eff gradient, not evidence for uncontrolled systematics.

The SCT framework makes a specific and testable prediction about how the distance ladder splits should evolve as the anchor sample grows. As more geometric anchors are established across a wider range of distances and sky positions, the H₀ inferred from different anchors should show a statistically significant angular and distance dependence aligned with the orientation and profile of the local supervoid. Anchors on the far side of the void relative to the local maximum underdensity should give slightly lower H₀, while anchors within the deepest underdense region should give the highest values. This spatial pattern would be straightforwardly detectable with the full Gaia parallax program for Milky Way Cepheids combined with the next generation of eclipsing binary distance anchors from Roman Space Telescope and the Extremely Large Telescope.