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 \approx 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 \pm 0.26 ( 1 \sigma ) .