Dark Energy Probes in Light of the CMB
  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. 
Wayne Hu (KICP, U Chicago)