Searching for Bumps in the Cosmological Road: Do Type Ia Supernovae with Early Excesses Have Biased Hubble Residuals?

Abstract Flux excesses in the early-time light curves of Type Ia supernovae (SNe Ia) are predicted by multiple theoretical models and have been observed in a number of nearby SNe Ia over the last decade. However, the astrophysical processes that cause these excesses may affect their use as standardizable candles for cosmological parameter measurements. We perform a systematic search for early-time excesses in SNe Ia observed by the Zwicky Transient Facility (ZTF) to study whether SNe Ia with these excesses yield systematically different Hubble residuals. We analyze two compilations of SN Ia light curves from ZTF’s first year of operations: 127 high-cadence light curves from Y. Yao et al. and 305 light curves from the ZTF cosmology data release of S. Dhawan et al. We detect significant early-time excesses for 17 SNe Ia in these samples and find that the excesses have a median g − r color of 0.10 ± 0.11 mag; we do not find a clear preference for blue excesses as predicted by several models. Using the SALT3 model, we measure Hubble residuals for these two samples, finding that excess-having SNe Ia may have lower Hubble residuals (HR) after correcting for shape, color, and host-galaxy mass, at ∼2–3 σ significance; our baseline result is ΔHR = −0.056 ± 0.026 mag (2.2 σ ). We compare the host-galaxy masses of excess-having and no-excess SNe Ia and find they are consistent, though at marginal significance excess-having SNe Ia may prefer lower-mass hosts. Additional discoveries of early excess SNe Ia will be a powerful way to understand potential biases in SN Ia cosmology and probe the physics of SN Ia progenitors.