  It is generally assumed that the two dark components of the energy density of
the universe, a smooth component called dark energy and a fluid of
nonrelativistic weakly interacting particles called dark matter, are
independent of each other and interact only through gravity. In this paper, we
consider a class of models in which the dark matter and dark energy interact
directly. The dark matter particle mass is proportional to the value of a
scalar field, and the energy density of this scalar field comprises the dark
energy. We study the phenomenology of these models and calculate the luminosity
distance as a function of redshift and the CMB anisotropy spectrum for several
cases. We find that the phenomenology of these models can differ significantly
from the standard case, and current observations can already rule out the
simplest models.
