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The most recent SH0ES measurement of the Hubble constant, based on type Ia supernovae from the Pantheon+ compilation, employs corrections of supernova peak magnitudes which effectively accounts for extinction in the supernova host galaxies. These corrections are estimated using a probabilistic model which is trained on Hubble flow (z>0. 03) supernovae and extrapolated to the calibration galaxies (those with observed Cepheid distances). By comparing the corrected peak magnitudes to distance moduli from Cepheids, we show that this standard approach underestimates the brightness of reddened supernovae in the high stellar-mass (M_>10^10M_) calibration galaxies. This can be traced back to the fact that for these galaxies, a low total-to-selective extinction coefficient (RB~3) is assumed, while for the low stellar-mass analogues a more standard RB~4 is assumed. We propose a minimalistic modification of the Pantheon+ extinction model in order to alleviate this systematic effect. The modification is twofold and it involves: (i) the same, Milky Way-like distribution of RB in all calibration galaxies (with mean RB of 4. 3 -- consistent with the extinction curve used for colour corrections of the Cepheids -- and scatter 0. 4) and (ii) a modified shape of the E (B-V) reddening distribution while keeping the same effective slope of the supernova peak magnitude-colour relation and the same mean E (B-V) reddening as measured for supernovae in the Hubble flow. We show that this new approach yields a significantly better fit (BIC=-11) to the calibration data and results in a lower value of the derived Hubble constant through a stronger extinction correction of supernovae in the calibration galaxies. Our result is H₀=70. 51 km/s/Mpc implying a reduction of the tension with the Planck H₀ measurement assuming a flat LCDM cosmology from 5. 2 to 2. 8.
Wojtak et al. (Fri,) studied this question.