The Pantheon+ Analysis: Dependence of Cosmological Constraints on Photometric-Zeropoint Uncertainties of Supernova Surveys

Abstract

Type Ia supernovae (SNe Ia) measurements of the Hubble constant, H$_0$, the cosmological mass density, $\Omega_M$, and the dark energy equation-of-state parameter, $w$, rely on numerous SNe surveys using distinct photometric systems across three decades of observation. Here, we determine the sensitivities of the upcoming SH0ES+Pantheon+ constraints on H$_0$, $\Omega_M$, and $w$ to unknown systematics in the relative photometric zeropoint calibration between the 17 surveys that comprise the Pantheon+ supernovae data set. Varying the zeropoints of these surveys simultaneously with the cosmological parameters, we determine that the SH0ES+Pantheon+ measurement of H$_0$ is robust against inter-survey photometric miscalibration, but that the measurements of $\Omega_M$ and $w$ are not. Specifically, we find that miscalibrated inter-survey systematics could represent a source of uncertainty in the measured value of H$_0$ that is no larger than $0.2$ km s$^{-1}$ Mpc$^{-1}$. This modest increase in H$_0$ uncertainty could not account for the $7$ km s$^{-1}$ Mpc$^{-1}$ "Hubble Tension" between the SH0ES measurement of H$_0$ and the Planck $\Lambda$CDM-based inference of H$_0$. However, we find that the SH0ES+Pantheon+ best-fit values of $\Omega_M$ and $w$ respectively slip, to first order, by $0.04$ and $-0.17$ per $25$ mmag of inter-survey calibration uncertainty, underscoring the vital role that cross-calibration plays in accurately measuring these parameters. Because the Pantheon+ compendium contains many surveys that share low-$z$ Hubble Flow and Cepheid-paired SNe, the SH0ES+Pantheon+ joint constraint of H$_0$ is robust against inter-survey photometric calibration errors, and such errors do not represent an impediment to jointly using SH0ES+Pantheon+ to measure H$_0$ to 1% accuracy.