The Hubble Constant from Blue Type Ia Supernovae

There is a persistent tension of about $5\sigma-6\sigma$ between the value of the Hubble constant, as derived from the local distance ladder vs. the cosmic microwave background, signaling either unaccounted for systematics in the measurements or `new physics', such as early dark energy. Determining the Hubble constant using Type Ia supernovae requires nontrivial and accurate corrections for dust extinction. To circumvent this obstacle, we here determine the Hubble constant from blue, and hence presumably unextinguished, supernovae. For two different sets of Type Ia supernova data and lightcurve fitting methods, we find that when using blue supernovae only, the derived Hubble constant is consistently lower by $\sim$ 3 km s$^{-1}$ Mpc$^{-1}$ ($70 \pm 2.1$ and $70.3 \pm 3.0$ km s$^{-1}$ Mpc$^{-1}$), and within 1$\sigma$ of the cosmic microwave background measurement, compared to when using all supernovae. This is consistent with the hypothesis that systematic effects in dust corrections may affect standard supernova cosmology. However, the number of blue calibrating Type Ia supernovae is small (about six), and values of the Hubble constant for a range of different supernova colors are consistent at the 1.2$\sigma$ level. Upcoming major transient surveys will discover numerous unextinguished SNe~Ia, and thus be able to increase the precision of the Hubble constant measured from blue SNe~Ia, heralding a promising path toward resolving the Hubble constant tension.