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

Abstract

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 (RB ≈ 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. We recover the Hubble constant measured by SH0ES for the standard choice of reference colour (SALT2 colour parameter c = 0) while for a reference colour which coincides with the blue end of the observed SN colour distribution (c ≈ −0.13), the Hubble constant from Planck observations of the CMB (assuming a flat ΛCDM cosmological model) is recovered. These results are intriguing in that they may provide an avenue for resolving the Hubble tension. However, since there is no obvious physical basis for the differences in colour corrections in the two SN samples, the origin of these require further investigations.

A reanalysis of the latest SH0ES data for $H_0$: Effects of new degrees of freedom on the Hubble tension

“…This transition model is also strongly selected over the baseline SH0ES model with the constraint, by both model selection criteria AIC and BIC despite the penalty imposed by AIC and especially BIC on the additional parameter involved in the new modeling. This behavior along with other recent studies [113] hints towards the need of more detailed Cepheid+SnIa calibrating data at distances D 50Mpc i.e. at the high end of rung 2 on the distance ladder.…”

“…This point is further enhanced by the recent study of Ref. [113] indicating that SnIa in the Hubble flow (at distances D > 90Mpc) appear to have different color calibration properties than SnIa in Cepheid hosts (at distances D < 75Mpc).…”

“…In addition other degrees of freedom that could be excited by the data could involve the modeling parameters that have been absorbed in the R21 SH0ES data like the Cepheid dust extinction parameter R W and the color and stretch SnIa calibration parameters β and α. These degrees of freedom may also be probed and excited by the fit to the data if allowed by the modeling [113].…”

“…In addition, a more recent analysis has allowed the parameter β to have a different value in Hubble flow SnIa (β = β HF for z > 0.02) compared to calibrating SnIa (β = β cal for z < 0.01). A reanalysis allowing for this new degree of freedom has indicated a tension between the the two best fit values (β HF and β cal ) at a level of up to 3.8σ [113].…”

“…The third approach to the resolution of the Hubble tension is based on a search for possible unaccounted systematic effects including possible issues in modelling the Cepheid data such as non-standard dust induced color correction [109], the impact of outliers [110][111][112], blending effects, SnIa color properties [113], robustness of the constancy of the SnIa absolute magnitude in the Hubble flow [62,[114][115][116][117][118][119][120][121][122][123][124][125][126][127][128][129][130] etc. There is currently a debate about the importance of these potential systematic effects [131][132][133][134].…”