On the homogeneity of SnIa absolute magnitude in the Pantheon+ sample

ABSTRACT We have analysed the Pantheon+ sample using a new likelihood model that replaces the single Type Ia supernovae (SnIa) absolute magnitude parameter M used in the standard likelihood model of Brout et. al. with two absolute magnitude parameters (M< and M>) and a transition distance (dcrit) that determines the distance at which M changes from M< to M>. The use of this likelihood dramatically changes the quality of fit to the Pantheon+ sample for a Lambda cold dark matter background by Δχ2 = −19.6. The tension between the M< and M> best-fitting values is at a level more than 3σ with a best-fitting dcrit very close to $20\, \mathrm{Mpc}$. The origin of this improvement of fit and M<–M> tension is that the new likelihood model successfully models two signals hidden in the data: (1) the volumetric redshift scatter bias systematic and (2) a mild signal for a change of intrinsic SnIa luminosity at about $20\, \mathrm{Mpc}$. This interpretation of the results is confirmed by truncating the z < 0.01 Hubble diagram data from the Pantheon+ data where the above systematic is dominant and showing that the M<–M> tension decreases from above 3σ to a little less than 2σ. It is also confirmed by performing a Monte Carlo simulation, which shows that the maximum significance of the SnIa luminosity transition ($\Sigma \equiv \frac{|M_{\gt} -M_{\lt} |}{\sqrt{\sigma _{M_{\gt} }^2+\sigma _{M_{\lt} }^2}}$) as obtained from the real SH0ES data is larger than the corresponding maximum significance of $94{{\ \rm per\ cent}}$ of the corresponding homogeneous simulated samples.