SNAP: Supernova / Acceleration Probe. An Experiment to Measure the Properties of the Accelerating Universe

A ~2-meter satellite telescope with a 1-square-degree optical imager, a small near-IR imager, and a three-channel near-UV-to-near-IR spectrograph can discover over 2000 Type Ia supernovae in a year at redshifts between z=0.1 and 1.7, and follow them with high-signal-to-noise calibrated light-curves and spectra. The resulting data set can determine the cosmological parameters with precision: mass density Ω _M to +/-0.02, vacuum energy density Ω _Λ to +/-0.05, and curvature Ω _k to +/-0.06. The data set can test the nature of the ``dark energy'' that is apparently accelerating the expansion of the universe. In particular, a cosmological constant dark energy can be differentiated from alternatives such as ``quintessence,'' by measuring the ratio of the dark energy's pressure to its density to +/-0.05, and by studying this ratio's time dependence. The large numbers of supernovae across a wide range of redshifts are necessary but not sufficient to accomplish these goals; the controls for systematic uncertainties are primary drivers of the design of this space-based experiment. These systematic and statistical controls cannot be obtained with other ground-based and/or space-based telescopes, either currently in construction or in planning stages.