Determination of the deceleration parameter from supernovae data

Supernovae searches have shown that a simple matter-dominated and decelerating universe should be ruled out. However, a determination of the present deceleration parameter q₀ through a simple kinematical description is not exempt of possible drawbacks. We show that, with a time dependent equation of state for the dark energy, a bias is present for q₀: models which are very far from the so-called concordance model can be accommodated by the data, and a simple kinematical analysis can lead to wrong conclusions. We present a quantitative treatment of this bias and we present our conclusions when a possible dynamical dark energy is taken into account.