## Abstract

We use Type la 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_{iot} = Ω_{m} + Ω_{Δ} = 0.94 ± 0.26 (1 σ).

Original language | English (US) |
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Pages (from-to) | 74-79 |

Number of pages | 6 |

Journal | Astrophysical Journal |

Volume | 509 |

Issue number | 1 PART I |

DOIs | |

State | Published - 1998 |

## All Science Journal Classification (ASJC) codes

- Astronomy and Astrophysics
- Space and Planetary Science