Papers › On the homogeneity of SnIa absolute magnitude in the Pantheon+ sample
On the homogeneity of SnIa absolute magnitude in the Pantheon+ sample
Leandros Perivolaropoulos, Foteini Skara
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We have analysed the Pantheon+ sample using a new likelihood model that replaces the single SnIa absolute magnitude parameter M used in the standard likelihood model of Brout et. al. with two absolute magnitude parameters M_<, M_> and a transition distance d_(crit) 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 ΛCDM background by Δχ²=-19.6. The tension between the M_< and M_> best fit values is at a level more than 3σ with a best fit d_(crit) very close to 20Mpc. 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 well known systematic effect called 'volumetric redshift scatter bias' which is due to asymmetric peculiar velocity variations at redshifts z<0.01 induced by unequal projected volumes at lower and higher distances compared to a given distance and 2. A mild signal for a change of intrinsic SnIa luminosity at about 20Mpc. This interpretation of the results is confirmed by truncating the z<0.01 Hubble diagram data from Pantheon+ 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 a Monte Carlo simulation comparing the SnIa absolute luminosities Mᵢ of SnIa+Cepheid hosts, with the anticipated luminosities in the context of a homogeneous single absolute magnitude M. This simulation shows that the maximum significance of the SnIa luminosity transition (Σ≡(|M_>-M_<|)/(√(σ_(M_>)²+σ_(M_<)²))) in the real data, is larger than the corresponding maximum significance of 94% of the corresponding homogeneous simulated samples.
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