Intrinsic tension in the supernova sector of the local Hubble constant measurement and its implications

Understand

We reanalyse observations of type Ia supernovae (SNe) and Cepheids used in the local determination of the Hubble constant and find strong evidence that SN standardisation in the calibration sample (galaxies with observed Cepheids) require a steeper slope of the colour correction than in the cosmological sample (galaxies in the Hubble flow).

• The colour correction in the calibration sample is consistent with being entirely due to an extinction correction due to dust with properties similar to that of the Milky Way (R_B~4.6+/-0.4) and there is no evidence for intrinsic scatter in the SN peak magnitudes.
• An immediate consequence of this finding is that the local measurement of the Hubble constant becomes dependent on the choice of SN reference colour, i.e., the colour of an unreddened SN.
• Specifically, the Hubble constant inferred from the same observations decreases gradually with the reference colour assumed in the SN standardisation.