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\, 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\, 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 (|M -₌_ | ₌_{ ²+ ₌_{ ²}}) as obtained from the real SH0ES data is larger than the corresponding maximum significance of 94{\ per\ cent} of the corresponding homogeneous simulated samples.