Distance-Ladder Measurements of the Hubble Constant: Recent Progress, Systematics, and Prospects

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Original abstract

The Hubble constant, \(H_0\), links the nearby distance scale to the present cosmic expansion rate. Local distance-ladder measurements now reach percent-level precision and remain more than \(5σ\) higher than the value inferred from cosmic microwave background (CMB) observations in base-\(Λ\)CDM, making the reliability of the local ladder a central issue in the Hubble tension. We describe the ladder as a covariance network connecting level-0 geometric anchors, level-1 stellar distance indicators, and level-2 Hubble-flow probes. The Cepheid--Type Ia supernova (SN Ia) route remains the most precise single local ladder, but independent indicators including the tip of the red giant branch (TRGB), J-region asymptotic giant branch (JAGB) stars, Mira variables, surface-brightness fluctuations (SBF), the Tully--Fisher relation, and Type II supernovae (SNe II) now test shared and method-specific systematics. In a compact seven-route covariance summary, combining the Cepheid--SN Ia route with three level-1 alternatives (TRGB, JAGB, and Mira) and three level-2 alternatives (SBF, Tully--Fisher, and SNe II) gives \(H_0=73.30\pm0.92~{\rm km~s^{-1}~Mpc^{-1}}\), still \(5.6σ\) above Planck base-\(Λ\)CDM. JWST has already tested Cepheid crowding and is making independent TRGB-based \(H_0\) measurements increasingly feasible. Over the next five years, a reliable one-percent local \(H_0\) requires larger calibrator samples, cross-validated level-1 zero points, explicit covariance propagation, and AI-assisted, reproducible, pre-specified selection criteria for distance-indicator measurements.

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