Metric-Independent Spacetime Volume-Forms and Dark Energy/Dark Matter Unification

Abstract

The method of non-Riemannian (metric-independent) spacetime volumeforms (alternative generally-covariant integration measure densities) is applied to construct a modified model of gravity coupled to a single scalar field providing an explicit unification of dark energy (as a dynamically generated cosmological constant) and dust fluid dark matter flowing along geodesics as an exact sum of two separate terms in the scalar field energy-momentum tensor. The fundamental reason for the dark species unification is the presence of a non-Riemannian volume-form in the scalar field action which both triggers the dynamical generation of the cosmological constant as well as gives rise to a hidden nonlinear Noether symmetry underlying the dust dark matter fluid nature. Upon adding appropriate perturbation breaking the hidden "dust" Noether symmetry we preserve the geodesic flow property of the dark matter while we suggest a way to get growing dark energy in the present universe' epoch free of evolution pathologies. Also, an intrinsic relation between the above modified gravity + single scalar field model and a special quadratic purely kinetic "k-essence" model is established as a weak-versus-strong-coupling duality.