  JEDI will probe dark energy in three independent ways by measuring the
expansion history of the universe: (1) using type Ia supernovae as cosmological
standard candles over a range of distances, (2) using baryon oscillations as a
cosmological standard ruler over a range of cosmic epochs, (3) mapping the weak
gravitational lensing distortion by foreground galaxies of the images of
background galaxies at different distances. JEDI will unravel the nature of
dark energy with accuracy and precision.
  JEDI is a 2m-class space telescope with the unique ability of simultaneous
wide-field imaging (0.8-4.2 micron in five bands) and multi-slit spectroscopy
(0.8-3.2 micron) with a field of view of 1 square degree. What makes JEDI
efficient is its ability to simultaneously obtain high signal-to-noise ratio,
moderate resolution slit spectra for all supernovae and ~ 5000 galaxies in its
wide field of view, and to combine imaging and spectroscopy so that the
appropriate balance of time is devoted to each.
  JEDI will measure the cosmic expansion history H(z) as a free function to <
2% accuracy in redshift bins of 0.2-0.3. Assuming a flat universe and
\sigma(\Omega_m)=0.01 (0.03), JEDI could measure a departure from a vanilla
\Lambda CDM model (w_0=-1, w'=0) with \sigma(w_0)=0.013 (0.031) and
\sigma(w')=0.046 (0.063). JEDI will obtain the well-sampled lightcurves in Z,
J, H, K, L bands and spectra of ~ 14,000 type Ia supernovae with redshifts
ranging from 0 to 1.7; the redshifts of ~ 10-100 million galaxies to H ~ 23 and
z ~ 4 over 1000-10,000 square degrees; and measurements of the shapes of
galaxies over 1000-10,000 square degrees in Z,J,H,K,L for > 10^9 galaxies to H
\~ 25.
