To be published in "Supernovae & Gamma Ray Bursts", K. Weiler, Ed., Springer, Lecture Notes in Physics.

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Saul Perlmutter 1 and Brian P. Schmidt 2

1 Physics Division, Lawrence Berkeley National Laboratory,
University of California, Berkeley, CA 94720, USA
2 Research School of Astronomy and Astrophysics,
The Australian National University, via Cotter Rd,
Weston Creek, ACT 2611, Australia

Abstract. Over the past decade, supernovae have emerged as some of the most powerful tools for measuring extragalactic distances. A well developed physical understanding of type II supernovae allow them to be used to measure distances independent of the extragalactic distance scale. Type Ia supernovae are empirical tools whose precision and intrinsic brightness make them sensitive probes of the cosmological expansion. Both types of supernovae are consistent with a Hubble Constant within ~ 10% of H0 = 70 km s-1 Mpc-1. Two teams have used type Ia supernovae to trace the expansion of the Universe to a look-back time more than 60% of the age of the Universe. These observations show an accelerating Universe which is currently best explained by a cosmological constant or other form of dark energy with an equation of state near w = p / rho = - 1. While there are many possible remaining systematic effects, none appears large enough to challenge these current results. Future experiments are planned to better characterize the equation of state of the dark energy leading to the observed acceleration by observing hundreds or even thousands of objects. These experiments will need to carefully control systematic errors to ensure future conclusions are not dominated by effects unrelated to cosmology.

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