  The homogeneous expansion history H(z) of our universe measures only
kinematic variables, but cannot fix the underlying dynamics driving the recent
acceleration: cosmographic measurements of the homogeneous universe, are
consistent with either a static finely-tuned cosmological constant or a dynamic
`dark energy' mechanism, which may be material Dark Energy or modified gravity
(Dark Gravity). Resolving the composition and foreground noise to reduce their
large systematic errors.dynamics of either kind of `dark energy', will require
complementing the homogeneous expansion observations with observations of the
growth of cosmological fluctuations. Because the 'dark energy' evolution is at
least quasi-static, any dynamical effects on the fluctuation growth function
g(z) will be minimal. They will be best studied in the weak lensing convergence
of light from galaxies at 0<z<5, from neutral hydrogen at 6<z<20, and
ultimately from the CMB last scattering surface at z=1089. Galaxy clustering
also measures g(z), but requires large corrections for baryonic composition and
foreground noise. Projected observations potentially distinguish static from
dynamic `dark energy', but distinguishing dynamic Dark Energy from Dark Gravity
will require a weak lensing shear survey more ambitious than any now projected.
Low-curvature modifications of Einstein gravity are also, in principle,
observable in the solar system or in isolated galaxy clusters. The cosmological
constant can only be fine-tuned, at present. The Cosmological Coincidence
Problem, that we live when the ordinary matter density approximates the
'gravitational vacuum energy', on the other hand, is a material problem,
calling for an understanding of the observers' role in cosmology.
