2023ApJ...944..188B

Type Ia supernova (SN) measurements of the Hubble constant, H₀; cosmic mass density, Ω_M; and dark energy equation-of-state parameter, w, rely on heterogeneous SN surveys across three decades of observation. These distinct surveys may have undiagnosed, relative photometric zero-point errors. We determine the sensitivities of the SH0ES+Pantheon+ cosmological constraints to unknown gray systematics in the photometric zero-point calibration between the 19 surveys that comprise the Pantheon+ SN compendium. Varying the surveys' gray zero-points simultaneously with cosmological parameters, we determine that the SH0ES+Pantheon+ measurement of H₀ is robust against gray intersurvey photometric miscalibration. Specifically, uncalibrated intersurvey systematics could represent a source of uncertainty no larger than 0.39 km s^–1 Mpc^–1 for H₀. This modest increase in H₀ uncertainty could not account for the 7 km s^–1 Mpc^–1 “Hubble tension” between the SH0ES measurement of H₀ and the Planck ΛCDM-based inference of H₀. However, the SH0ES+Pantheon+ best-fit values of Ω_M and w are not robust against gray zero-point error, slipping by up to 0.16 and -0.63. Because measurements of Ω_M and w depend on intrasurvey cross-band calibration, the hypothetical gray miscalibration underestimates the sensitivity of these measurements to zero-point miscalibration. Because the Pantheon+ compendium contains many surveys that share low-z Hubble flow (HF) and Cepheid-paired SNe, intersurvey photometric calibration errors do not significantly impede the joint use of SH0ES and Pantheon+ to measure H₀ to 1% accuracy. However, H₀ constraints that rely on one HF survey but numerous galactic distance calibration surveys are susceptible to intersurvey photometric miscalibration.