Constraints on Cosmological Models from [ITAL]Hubble Space Telescope[/ITAL] Observations of High-[CLC][ITAL]z[/ITAL][/CLC] Supernovae

We have coordinated Hubble Space Telescope (HST) photometry with ground-based discovery for three supernovae: Type Ia supernovae near z~0.5 (SN 1997ce, SN 1997cj) and a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. We use these light curves and relations between luminosity, light-curve shape, and color calibrated from low-z samples to derive relative luminosity distances that are accurate to 10% at z~0.5 and 20% at z=1. When the HST sample is combined with the distance to SN 1995K (z=0.48), analyzed by the same precepts, we find that matter alone is insufficient to produce a flat universe. Specifically, for Ωm+ΩΛ=1, Ωm is less than 1 with more than 95% confidence, and our best estimate of Ωm is -0.1+/-0.5 if ΩΛ=0. Although this result is based on a very small sample whose systematics remain to be explored, it demonstrates the power of HST measurements for high-redshift supernovae.