Cosmological parameter extraction and biases from type Ia supernova magnitude evolution ^*

¹ Centre de Physique Théorique, Université de Provence, CNRS de Luminy case 907, 13288 Marseille Cedex 9, France e-mail: linden@cpt.univ-mrs.fr

² Centre de Physique des Particules de Marseille, Université de la Mediterranée, CNRS de Luminy case 907, 13288 Marseille Cedex 9, France

Received: 2 July 2009
Accepted: 11 August 2009

Abstract

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 the best-fit values imply supernova magnitude evolution such that high-redshift supernovae appear some percent brighter than would be expected in a standard cosmos with a dark energy component. However, the errors on the evolution parameters are of the same order, and data are consistent with nonevolving magnitudes at the  level, except for 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 nonevolving SN magnitude is not detectable, and for which biases on the fitted cosmological parameters are introduced at the same time. 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 show up to be not too dangerous when neglected in the fitting procedure, late epoch models with have high chances of undetectably biasing the fit results.