ESSENCE/FORS1 High-z Type Ia Supernovae

The ESSENCE Survey was initiated to determine the nature of the "dark energy" that causes the accelerated expansion of the universe by determining its equation-of-state parameter, w. It aims to answer a simple, but very important, question: is the dark energy of the universe consistent with a cosmological constant (w=-1)? If not, this means the dark energy must be a more general energy field such as "quintessence." Over the six-year period of the survey (2002-2007) we have discovered and followed-up ~200 Type Ia supernovae (SNe Ia) over a large redshift range (out to z~0.8). To properly identify these SNe Ia and determine their redshift, we have carried out optical long-slit spectroscopy using the FORS1 instrument of the VLT at ESO's Cerro Paranal Observatory, Chile.