Large evidence for dark energy from a study of type Ia supernovae

Type Ia supernovae have been observed to be very good "Standard Candles" after suitable calibration. In the Supernova Cosmology Project we have observed over 80 such distant supernovae with redshifts between 0.18 and 1.2. In a study of 42 of these supernovae and comparison with 18 "near by" supernovae we have obtained evidence that at the present epoch of the expansion the universe is accelerating. This implies the presence of a positive cosmological constant or perhaps some other form of "Dark Energy". Our data are strongly inconsistent with the previously preferred cosmology with Omega(M) = I and Omega(Lambda) = 0. I show that a time stretching or contracting parameter, s, alows a single "composite" curve to fit all the SNe for 0.3 < z < 0.7 as well as the nearby SNe. I also give a direct test that the red-shift is due to the expansion of the universe rather than some other process, such as "tired light". I will also briefly describe the plans of the SNAP collaboration [29] to design, build and launch a satellite. The aim is to discover and critically measure 2000 SNe per year with redshifts from 0.1 to 1.7 to tightly controll systematics. For example SNAP will allow us to explore possible effects due to "grey" dust, and/or evolution on our measurements. Highlights are a 2 meter mirror and a one square degree field of view involving about 150 high resistivity CCD's and 3-arm spectrometer covering the region from near-UV to near-IR.