Interacting quintessence from a variational approach Part I: algebraic
  couplings
  We present a new approach to build models of quintessence interacting with
dark or baryonic matter. We use a variational approach for relativistic fluids
to realize an effective description of matter fields at the Lagrangian level.
The coupling is introduced directly in the action by considering a single
function mixing the dynamical degrees of freedom of the theory. The resulting
gravitational field equations are derived by variations with respect to the
independent variables. New interesting phenomenology can be obtained at both
small scales, where new screening mechanisms for scalar fields can be realized,
and large scales, where one finds an original and rich class of interacting
quintessence models. The background cosmology of two of these models is studied
in detail using dynamical system techniques. We find a variety of interesting
results: for instance, these models contain dark energy dominated late time
attractors and scaling solutions, both with early time matter dominated epochs
and a possible inflationary origin. In general this new approach provides the
starting point for future in depth studies on new interacting quintessence
models.
