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