SNAP is specifically designed to reveal the nature of the dark energy causing the acceleration of the expansion of the universe. It will characterize the dark energy density, equation of state, and time variation by precisely and accurately measuring the distance-redshift relation of Type Ia supernovae and carrying out a deep, wide area weak gravitational lensing survey.
A carefully calibrated, systematics-controlled sample of some 2000 supernovae over the full redshift range to z=1.7 will allow us to determine the expansion history of the universe over the last 10 billion years to 1% accuracy. Systematic uncertainties such as dust extinction, gray dust, population evolution, etc. will be tightly controlled by high signal to noise observations from optical to near infrared wavelengths, repeated measurements from soon after explosion through maximum light and in the decline phase, together with an energy spectrum and host galaxy redshift and imaging. The then "standardized candles" give a clear and direct measure of the expansion history a(t): the measured flux indicates their distance through the cosmological inverse square law, and hence the look back time t to the epoch when they exploded, while the redshift gives the scale factor a of the universe at that time.
As well, SNAP maps the geometric and dynamic effects of dark energy through the growth history of large scale structure using weak gravitational lensing. This detects the subtle shape distortions of background galaxy images by foreground mass concentrations. Space is the ideal locale for such observations due to the high resolution and stability of the imaging, as well as the ability to use infrared measurements to obtain accurate photometric redshifts and greater redshift depth.
The survey strategy brings together a deep, repeated, supernova oriented survey of 7.5 square degrees, achieving depths fainter than AB magnitude 30; a wide, lensing oriented survey of 1000-4000 square degrees, to AB mag 28; and possibly a panoramic survey of 7000-10000 square degrees to AB mag 27. The combination of supernovae and lensing data provides powerful complementarity and crosschecks, as well as independence, with no external priors needed. The matter density, dark energy density, and flatness of the universe can be determined at the 1% level, including systematic uncertainties, the dark energy equation of state to about 3% and its time variation characterized to within 10% of the Hubble expansion time.
SNAP will provide unprecedented measurements of the history and properties of the universe, and open the frontiers to new physics in cosmology, gravitation, and high energy physics.
SNAP represents a third generation experiment in supernova cosmology. The large supernova sample, broad redshift range, and high data quality - illustrated here on a Hubble diagram of magnitude vs. redshift - will measure the cosmological parameters with unprecedented accuracy and systematic controls.
There is strong and confirming evidence for the existence of a cosmological vacuum
density. Plotted are the 68% and 95% confidence regions of the matter density ΩM and vacuum energy density ΩL for current data from supernovae (Knop et al. 2003), cluster measurements (based on Allen et al. 2003), and CMB data with H_0 priors (outer contours: Lange et al. 2001, inner: Spergel et al. 2003). These results rule out a simple flat ΩM=1, ΩL=0 cosmology, and indeed the supernovae data rule out cosmologies without vacuum energy. Their consistent overlap is a strong indicator for dark energy dominating the universe with some 70% of the energy density. Also shown is the expected confidence region from just the SNAP supernova program, for ΩM=0.28, ΩL=0.72.