Supernova Limits on the Cosmic Equation of State

We use Type Ia supernovae studied by the High-z Supernova Search Team to constrain the properties of an energy component that may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation-of-state parameter for the unknown component, α_x = P_x/ρ_x, must be less than -0.55 (95% confidence) for any value of Ω_m, and it is further limited to α_x < -0.60 (95% confidence) if Ω_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 (SN) data are consistent with a cosmological constant (α_x = -1) or a scalar field that 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. SN 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 Ω_tot = Ω_m + Ω_Λ = 0.94 ± 0.26 (1 σ).