  CMB observables have largely fixed the expansion history of the universe in
the deceleration regime and provided two self-calibrated absolute standards for
dark energy studies: the sound horizon at recombination as a standard ruler and
the amplitude of initial density fluctuations. We review these inferences and
expose the testable assumptions about recombination and reionization that
underly them. Fixing the deceleration regime with CMB observables, deviations
in the distance and growth observables appear most strongly at z=0 implying
that accurate calibration of local and CMB standards may be more important than
redshift range or depth. The single most important complement to the CMB for
measuring the dark energy equation of state at z~0.5 is a determination of the
Hubble constant to better than a few percent. Counterintuitively, with fixed
fractional distance errors and relative standards such as SNe, the Hubble
constant measurement is best achieved in the high redshift deceleration regime.
Degeneracies between the evolution and current value of the equation of state
or between its value and spatial curvature can be broken if percent level
measurement and calibration of distance standards can be made at intermediate
redshifts or the growth function at any redshift in the acceleration regime. We
compare several dark energy probes available to a wide and deep optical survey:
baryon features in galaxy angular power spectra and the growth rate from
galaxy-galaxy lensing, shear tomography and the cluster abundance.
