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Session 17 - CMB Radiation, Cosmology and Gravity.
Oral session, Monday, June 08

[17.06] An Accelerating Universe and Other Cosmological Implications from SNe Ia

A. G. Riess (UCB)

We present an empirical method to measure precise distances from type Ia supernova (SN Ia) light curves and apply it to estimate cosmological parameters. This multi-color light curve shape (MLCS) method derives the empirical relationship between light and color curve shape and luminosity to distinguish between SNe Ia which are distant, intrinsically dim, or obscured by dust. This method reduces the dispersion of the standard candle method from 0.50 mag to 0.15 mag and gives a Hubble constant of 65 \pm 6 km s^-1 Mpc^-1. A determination of the motion of the Galaxy with respect to the SN Ia frame is consistent with the cosmic microwave background; comparison of the observed peculiar motions of SNe Ia with the predictions of IRAS galaxies constrain the mass density parameter, \beta to 0.4 \pm 0.15. Building on our understanding of low-redshift SNe Ia, we have measured distances to 16 high-redshift SNe with 0.16 \leq z \leq 0.97. The distances of the high-redshift SNe Ia are, on average, 10% to 13% farther than expected in a low mass density (Ømega_M=0.2) Universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., Ømega_\Lambda > 0) and a current acceleration of the expansion (i.e., q_0 < 0). We estimate the likely effect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. None of these effects reconciles the data with Ømega_\Lambda=0 or current deceleration.

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