The High-Z Supernova Search: Measuring Cosmic Deceleration and Global Curvature of the Universe Using Type Ia Supernovae^*

The High-Z Supernova Search is an international collaboration to discover and monitor Type Ia supernovae (SNe Ia) at z > 0.2 with the aim of measuring cosmic deceleration and global curvature. Our collaboration has pursued a basic understanding of supernovae in the nearby universe, discovering and observing a large sample of objects and developing methods to measure accurate distances with SNe Ia. This paper describes the extension of this program to z ≥ 0.2, outlining our search techniques and follow-up program. We have devised high-throughput filters that provide accurate two-color rest frame B and V light curves of SNe Ia, enabling us to produce precise, extinction-corrected luminosity distances in the range 0.25 < z < 0.55. Sources of systematic error from K-corrections, extinction, selection effects, and evolution are investigated, and their effects estimated. We present photometric and spectral observations of SN 1995K, our program's first supernova (SN), and use the data to obtain a precise measurement of the luminosity distance to the z = 0.479 host galaxy. This object, when combined with a nearby sample of SNe, yields an estimate for the matter density of the universe of Ω_M=-0.2_−0.8^+1.0 if Ω_Λ = 0. For a spatially flat universe composed of normal matter and a cosmological constant, we find Ω_M=0.4_−0.4^+0.5, Ω=0.6_−0.5^+0.4. We demonstrate that with a sample of ~30 objects, we should be able to determine relative luminosity distances over the range 0 < z < 0.5 with sufficient precision to measure Ω_M with an uncertainty of ±0.2.