Unified description of galactic dynamics and the cosmological constant

We explore the phenomenology of a two-fluid cosmological model, where the field equations of general relativity (GR) are sourced by baryonic and cold dark matter. We find that the model allows for a unified description of small and large scale, late-time cosmological dynamics. Specifically, in the static regime we recover the flattening of galactic rotation curves by requiring the matter density profile to scale as $1/r^2$. The same behavior describes matter inhomogeneities distribution at small cosmological scales. This traces galactic dynamics back to structure formation. At large cosmological scales, we focus on back reaction effects of the spacetime geometry to the presence of matter inhomogeneities. We find that a cosmological constant with the observed order of magnitude, emerges by averaging the back reaction term on spatial scales of order 100 Mpc and it is related in a natural way to matter distribution. This provides a resolution to both the cosmological constant and the coincidence problems and shows the existence of an intriguing link between the small and large scale behavior in cosmology.