  We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space
Telescope (HST) and have used them to provide the first conclusive evidence for
cosmic deceleration that preceded the current epoch of cosmic acceleration.
These objects, discovered during the course of the GOODS ACS Treasury program,
include 6 of the 7 highest-redshift SNe Ia known, all at z>1.25, and populate
the Hubble diagram in unexplored territory. The luminosity distances to these
and 170 previous SNe Ia are provided. A purely kinematic interpretation of the
SN Ia sample provides evidence at the > 99% confidence level for a transition
from deceleration to acceleration or similarly, strong evidence for a cosmic
jerk. Using a simple model of the expansion history, the transition between the
two epochs is constrained to be at z=0.46 +/- 0.13. The data are consistent
with the cosmic concordance model of Omega_M ~ 0.3, Omega_Lambda~0.7
(chi^2_dof=1.06), and are inconsistent with a simple model of evolution or dust
as an alternative to dark energy. For a flat Universe with a cosmological
constant. When combined with external flat-Universe constraints we find w=-1.02
+ 0.13 - 0.19 (and $<-0.76 at the 95% confidence level) for an assumed static
equation of state of dark energy, P = w\rho c^2. Joint constraints on both the
recent equation of state of dark energy, $w_0$, and its time evolution, dw/dz,
are a factor of ~8 more precise than its first estimate and twice as precise as
those without the SNe Ia discovered with HST. Our constraints are consistent
with the static nature of and value of w expected for a cosmological constant
(i.e., w_0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution
of dark energy. We address consequences of evolving dark energy for the fate of
the Universe.
