Redshift-Frame Systematics and Their Impact on the Hubble Constant from Pantheon+ Supernovae

We present a full-sky, covariance-weighted analysis of redshift-frame transformations in the Pantheon+ Type Ia supernova sample to assess their impact on local measurements of the Hubble constant. Using 1,543 supernovae with both heliocentric and CMB-frame redshifts, we analyze the residual field Δz = z₍CMB₎ − z₍HEL₎, which traces the Solar System’s kinematic correction. We recover the expected monopole ⟨Δz⟩ = (−3.8 ± 0.1) × 10⁻⁴ and a dipole amplitude A = (1.5 ± 0.1) × 10⁻³, aligned within 1° of the CMB dipole, confirming internal consistency. Propagating these residuals through the full Pantheon+ covariance matrix yields a negligible shift in H₀, at the ≲2% level of the current tension, placing a quantitative upper bound on redshift-frame systematics.