Exploring Dark Energy with Next-Generation Photometric Redshift Surveys
  The coming decade will be an exciting period for dark energy research, during
which astronomers will address the question of what drives the accelerated
cosmic expansion as first revealed by type Ia supernova (SN) distances, and
confirmed by later observations.
  The mystery of dark energy poses a challenge of such magnitude that, as
stated by the Dark Energy Task Force (DETF), "nothing short of a revolution in
our understanding of fundamental physics will be required to achieve a full
understanding of the cosmic acceleration." The lack of multiple complementary
precision observations is a major obstacle in developing lines of attack for
dark energy theory. This lack is precisely what next-generation surveys will
address via the powerful techniques of weak lensing (WL) and baryon acoustic
oscillations (BAO) -- galaxy correlations more generally -- in addition to SNe,
cluster counts, and other probes of geometry and growth of structure. Because
of their unprecedented statistical power, these surveys demand an accurate
understanding of the observables and tight control of systematics.
  This white paper highlights the opportunities, approaches, prospects, and
challenges relevant to dark energy studies with wide-deep multiwavelength
photometric redshift surveys. Quantitative predictions are presented for a
20000 sq. deg. ground-based 6-band (ugrizy) survey with 5-sigma depth of
r~27.5, i.e., a Stage 4 survey as defined by the DETF.
