Evidence for possible overestimation of systematic reliability of Type Ia Supernovae for measuring extragalactic distances

Abstract

We investigate systematic reliability of Type Ia Supernovae (SNe Ia) as an extragalactic distance measurement method. First we review the evidence for potential sources of underestimated uncertainty and bias in SNe Ia distance measurements. Then we invite attention to the fact that most estimates of systematic uncertainty in SNe Ia distances do not take into account the uncertainties in the other distance measurement methods on which the SNe Ia distance scale is calibrated. With this backdrop, we analyse 17,371 reported SNe Ia distances for 3,743 galaxies compiled in the NED Redshift Independent Distances database. We find that any two reported SNe Ia distance measurements for a galaxy differ by 2.84 times the reported uncertainty on average. Among all the ordered pairs of multiple reported SNe Ia distances for the same galaxy, over all the galaxies, we find that 37.65\% of the ordered pairs differ by more than twice the reported uncertainty. This points towards possible underestimation of the reported uncertainties in SNe Ia distances. We also find that this percentage of ordered pairs of SNe Ia distances differing by more than twice the reported uncertainty shows a monotonically increasing trend from 1999 to 2018, pointing towards increasing underestimation of uncertainties with the period of publication. Using data from the Sloan Digital Sky Survey II, we find that the uncertainty introduced by the choice of MLCS2k2 versus SALT2 light curve models is 2.41 times the uncertainty reported in an individual measurement on average.