Do Type Ia Supernovae Provide Direct Evidence for Past Deceleration of the Universe?

Abstract

Observations of SN 1997ff at z~1.7 favor the accelerating universe interpretation and provide some direct evidence that the universe was once decelerating. We show that the strength of this conclusion depends upon the nature of the dark energy causing the present acceleration. Only for a cosmological constant is the SNe evidence definitive. Using a new test which is independent of the contents of the universe, we show that the SN data favor recent acceleration (z<0.5) and past deceleration (z>0.5).