Speaker
Description
The Cosmic Flows 4 (CF4) catalogue compiles ~56,000 extragalactic distance
measurements from eight independent methods — Type Ia supernovae,
Tully–Fisher, Fundamental Plane, surface brightness fluctuations, Type II
supernovae, TRGB, Cepheids, and masers. A pairwise consistency analysis
reveals significant inter-method tensions, particularly involving the
Tully–Fisher relation and TRGB, which propagate directly into inferred values
of H0. We present a Bayesian forward-modelling pipeline that simultaneously
recalibrates all methods via per-method zero-point offsets Δμ and
error-rescaling parameters α, anchored to the NGC 4258 megamaser distance.
Crucially, we account for Malmquist and selection bias through a joint
marginalisation over each galaxy's true distance modulus, weighted by the
comoving volume prior r^k and the selection probability across all methods
that observed it. This joint integral prevents double-counting of the bias
when a galaxy is measured by both magnitude- and redshift-limited surveys. We
explore three selection scenarios and find that the inferred H0 is sensitive
to assumptions about the survey completeness: a fully self-consistent
Malmquist correction under a uniform-in-volume prior (k=2) yields H0 = 69.8 ±
2.3 km/s/Mpc, while a bias-free reference (k=−1) gives H0 = 72.0 ± 2.4
km/s/Mpc. Trusting the CF4-precorrected flux-limited methods (SNIa, TF, FP,
SBF) as already de-biased raises the k=2 result to 72.6 ± 2.3 km/s/Mpc,
consistent with the k=−1 reference. The spread across scenarios quantifies the
systematic uncertainty due to selection modelling in distance-ladder
analyses.