The property of dark energy and the physical reason for acceleration of the present universe are two of the most difficult problems in modern cosmology .
The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernova ( SNe Ia ) and anisotropy of cosmic microwave background ( CMB ) .The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift , implying the existence of a nearly uniform component of dark energy with negative pressure .
We use the “ gold ” sample containing 157 SNe Ia and two recent well-measured additions , SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift .
For a flat universe with the cosmological constant , we measure \Omega _ { M } = 0.28 _ { -0.05 } ^ { +0.04 } , which is consistent with Riess et al .
The transition redshift is z _ { T } = 0.60 _ { -0.08 } ^ { +0.06 } .
We also discuss several dark energy models that define the w ( z ) of the parameterized equation of state of dark energy including one parameter and two parameters ( w ( z ) being the ratio of the pressure to energy density ) .
Our calculations show that the accurately calculated transition redshift varies from z _ { T } = 0.29 _ { -0.06 } ^ { +0.07 } to z _ { T } = 0.60 _ { -0.08 } ^ { +0.06 } across these models .
We also calculate the minimum redshift z _ { c } at which the current observations need the universe to accelerate .