  We use Type Ia supernovae studied by the High-Z Supernova Search Team to
constrain the properties of an energy component which may have contributed to
accelerating the cosmic expansion. We find that for a flat geometry the
equation of state parameter for the unknown component, alpha_x=P_x/rho_x, must
be less than -0.55 (95% confidence) for any value of Omega_m and is further
limited to alpha_x<-0.60 (95%) if Omega_m is assumed to be greater than 0.1 .
These values are inconsistent with the unknown component being topological
defects such as domain walls, strings, or textures. The supernova data are
consistent with a cosmological constant (alpha_x=-1) or a scalar field which
has had, on average, an equation of state parameter similar to the cosmological
constant value of -1 over the redshift range of z=1 to the present. Supernova
and cosmic microwave background observations give complementary constraints on
the densities of matter and the unknown component. If only matter and vacuum
energy are considered, then the current combined data sets provide direct
evidence for a spatially flat Universe with Omega_tot=Omega_m+Omega_Lambda =
0.94 +/- 0.26 (1-sigma).
