Cosmological parameter extraction and biases from type Ia supernova magnitude evolution

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We study different one-parametric models of type Ia Supernova magnitude evolution on cosmic time scales. Constraints on cosmological and Supernova evolution parameters are obtained by combined fits on the actual data coming from Supernovae, the cosmic microwave background, and baryonic acoustic oscillations. We find that data prefer a magnitude evolution such that high-redshift Supernova are brighter than would be expected in a standard cosmos with a dark energy component. Data however are consistent with non-evolving magnitudes at the one-sigma level, except special cases. We simulate a future data scenario where SN magnitude evolution is allowed for, and neglect the possibility of such an evolution in the fit. We find the fiducial models for which the wrong model assumption of non-evolving SN magnitude is not detectable, and for which at the same time biases on the fitted cosmological parameters are introduced. Of the cosmological parameters the overall mass density has the strongest chances to be biased due to the wrong model assumption. Whereas early-epoch models with a magnitude offset z² show up to be not too dangerous when neglected in the fitting procedure, late epoch models with magnitude offset sqrt(z) have high chances to bias the fit results.