What Drives Our Accelerating Universe?

Abstract

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.