TRGB Distance Bias

The Tip of the Red Giant Branch (TRGB) is an alternative first rung of the distance ladder, independent of Cepheids. Low-mass stars ascending the red giant branch undergo a helium flash at a precisely predictable luminosity, creating a sharp discontinuity in the stellar luminosity function. This "tip" occurs at a near-universal absolute magnitude in the infrared, making it a clean standard candle visible to enormous distances with HST and JWST. The Carnegie-Chicago Hubble Program (CCHP) used TRGB distances to calibrate Type Ia supernovae and derived H₀ ~ 69.6–70.5 km/s/Mpc — noticeably lower than SH0ES's Cepheid-based 73.2 km/s/Mpc, sitting uncomfortably between the two main camps.

The apparent discrepancy between Cepheid and TRGB ladders is itself a tension within a tension. Either one or both methods carry uncharacterized systematic errors, or the two standard candles genuinely measure something subtly different about the local universe. Investigations have revealed that TRGB calibration depends sensitively on the color selection of the giant branch, the photometric zero-point in the F814W band, and crowding corrections in dense stellar fields. JWST has begun to clarify some of these issues but has not fully closed the Cepheid-TRGB gap. For ΛCDM, the deeper problem is that even the "low" TRGB value of ~70 km/s/Mpc is still marginally higher than Planck's 67.4, meaning no late-universe method cleanly vindicates the CMB-derived expansion rate.