Bright High z SnIa: A Challenge for LCDM?

Abstract

It has recently been pointed out by Kowalski et. al. (arxiv:0804.4142) that there is `an unexpected brightness of the SnIa data at z>1'. We quantify this statement by constructing a new statistic which is applicable directly on the Type Ia Supernova (SnIa) distance moduli. This statistic is designed to pick up systematic brightness trends of SnIa datapoints with respect to a best fit cosmological model at high redshifts. It is based on binning the normalized differences between the SnIa distance moduli and the corresponding best fit values in the context of a specific cosmological model (eg LCDM). We then focus on the highest redshift bin and extend its size towards lower redshifts until the Binned Normalized Difference (BND) changes sign (crosses 0) at a redshift z_c (bin size N_c). The bin size N_c of this crossing (the statistical variable) is then compared with the corresponding crossing bin size N_{mc} for Monte Carlo data realizations based on the best fit model. We find that the crossing bin size N_c obtained from the Union08 and Gold06 data with respect to the best fit LCDM model is anomalously large compared to N_{mc} of the corresponding Monte Carlo datasets obtained from the best fit LCDM in each case. In particular, only 2.2% of the Monte Carlo LCDM datasets are consistent with the Gold06 value of N_c while the corresponding probability for the Union08 value of N_c is 5.3%. Thus, according to this statistic, the probability that the high redshift brightness bias of the Union08 and Gold06 datasets is realized in the context of a (w_0,w_1)=(-1,0) model (LCDM cosmology) is less than 6%. The corresponding realization probability in the context of a (w_0,w_1)=(-1.4,2) model is more than 30% for both the Union08 and the Gold06 datasets.