Leveraging SN Ia spectroscopic similarity to improve the measurement of $H_0$

Understand

Recent studies suggest spectroscopic differences explain a fraction of the variation in Type Ia supernova (SN Ia) luminosities after light-curve/color standardization.

• In this work, (i) we empirically characterize the variations of standardized SN Ia luminosities, and (ii) we use a spectroscopically inferred parameter, SIP, to improve the precision of SNe Ia along the distance ladder and the determination of the Hubble constant ($H_0$).
• First, we show that the \texttt{Pantheon+} covariance model modestly overestimates the uncertainty of standardized magnitudes by $\sim 7$%, in the parameter space used by the $\texttt{SH0ES}$ Team to measure $H_0$; accounting for this alone yields $H_0 = 73.01 \pm 0.92$ km s$^{-1}$ Mpc$^{-1}$.
• Furthermore, accounting for spectroscopic similarity between SNe~Ia on the distance ladder reduces their relative scatter to $\sim0.12$ mag per object (compared to $\sim 0.14$ mag previously).