IS COSMOLOGICAL ACCELERATION DRIVEN BY CLASSICAL SPACE–TIME GEOMETRY?

Abstract

The homogeneous expansion history H(z) of our universe (Hubble diagram) measures only kinematic variables, it cannot fix the underlying dynamics driving the recent acceleration: cosmographic measurements of the homogeneous universe are consistent with either a static fine-tuned cosmological constant or a dynamic "dark energy" mechanism, which itself may be either material dark energy or low-curvature modifications of Einstein gravity (dark gravity). This dark energy/dark gravity degeneracy in the homogeneous expansion observations can only be resolved by observing the growth of the cosmological fluctuations. However, because the "dark energy" evolution is now quasi-static at most, any dynamical effects on the fluctuation growth function g(z) will be minimal. Projected observations may 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. Dark gravity is also, in principle, observable in the solar system or in isolated galaxy clusters. The cosmological constant problem — that quantum material vacuum fluctuations apparently do not gravitate — suggests identifying gravitational "vacuum energy" with classical intrinsic space–time curvature, divorcing it from any quantum material property. This empty space–time curvature appears cosmologically and about isolated sources and 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. A particularly restrictive weak anthropic principle, that dark energy and dark gravity be indistinguishable, selects static "dark energy" (ΛCDM) and rejects any dynamical effects in the growth of fluctuations.