Abstract
We propose a cosmological scenario involving a scalar field, φ, that is a source of Dark Matter as well as of Dark Energy. Besides φ, the Lagrangian of the field theory envisaged in our scenario contains a second field χ, for simplicity assumed to be a scalar, too. For fixed values of χ, the potential term decays exponentially at large positive values of φ. While φ is not coupled to Standard Model fields, χ is assumed to be coupled to them, and the Green functions of χ depend on the cosmological redshift in the expanding universe. We assume that the term in the Lagrangian coupling χ to φ is such that, at redshifts z larger than some critical redshift zc, φ is trapped near φ=0, and oscillations of φ about φ=0 describing massive scalar particles give rise to Dark Matter. At redshifts below zc, the field φ is no longer trapped near the origin and starts to "roll" towards large field values. A homogenous component of φ emerges that acts as Dark Energy. Within over-dense regions, such as galaxies and galaxy clusters, the redshifting of χ stops, and φ therefore remains trapped near φ=0 as long as zc is smaller than the redshift when structures on galactic scales decouple from the Hubble flow. Thus, at the present time, φ describes both Dark Energy and Dark Matter.