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

TL;DR

This study analyzes redshift-frame systematics in the Pantheon+ supernova sample to evaluate their effect on local Hubble constant measurements, finding negligible impact within current tension levels.

Contribution

It provides a comprehensive, covariance-weighted analysis of redshift-frame transformations and quantifies their minimal influence on H0 estimates.

Findings

Residual redshift differences are consistent with expected kinematic corrections.

Redshift-frame systematics cause less than 2% shift in H0.

Results confirm internal consistency of supernova redshift measurements.

Abstract

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 heliocentric and CMB-frame redshifts, we study the residual field delta z = zCMB - zHEL, which traces the Solar System's kinematic correction. We recover the expected monopole <delta z> = (-3.8 +/- 0.1) x 10^-4 and a dipole amplitude A = (1.5 +/- 0.1) x 10^-3 aligned within 1 degree 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.