**AAS Meeting #193 - Austin, Texas, January 1999**

*Session 39. Cosmology/Large Scale Structure*

Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibits Hall 1
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## [39.05] Supernova Limits on the Cosmic Equation of State

*P. M. Garnavich, S. Jha, P. M. Challis (CfA), A. Clocchiatti (U.Cat\'olica), A. Diercks (U.Wash.), A. V. Filippenko (UCB), R. L. Gilliland (STScI), C.J. Hogan (U.Wash.), R. P. Kirshner (CfA), B. Leibundgut (ESO), M.M. Phillips (CTIO), D. J. Reiss (U.Wash.), A.G. Riess (UCB), B.P Schmidt (MSSSO), R.A. Schommer (CTIO), R.C. Smith (U.Mich.), J. Spyromilio (ESO), C. Stubbs (U.Wash.), N.B. Suntzeff (CTIO), J. Tonry (U.Hawaii), S.M. Carroll (ITP)*

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_{\rm x}=P_{\rm x}/\rho_{\rm x}, must be less than
-0.55 (95% confidence) for any value of \Omega_{\rm m}
and is further limited to \alpha_{\rm x} <-0.60 (95%) if
\Omega_{\rm 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_{\rm 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_{\rm tot}=\Omega_{\rm m}+\Omega_{\}Lambda = 0.94 ±
0.26 (1\sigma).

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