Is the present expansion of the Universe really accelerating?

Abstract

The current observations are usually explained by an accelerating expansion of the present Universe. However, with the present quality of the supernovae Ia data, the allowed parameter space is wide enough to accommodate decelerating models as well. This is shown by considering a particular example of the dark energy equation of state w Φ ≡ p Φ /ρ Φ = -1/3, which is equivalent to modifying the geometrical curvature index k of the standard cosmology by shifting it to (k - a), where a is a constant. The resulting decelerating model is consistent with the recent cosmic microwave background observations made by WMAP, as well as with the high-redshift supernovae Ia data including SN 1997ff at z = 1.755. It is also consistent with the newly discovered supernovae SN 2002dc at z = 0.475 and SN 2002dd at z = 0.95, which have a general tendency to improve the fit.