Unified Dark Energy and Dark Matter from Dynamical Space Time
  A unification of dark matter and dark energy based on a dynamical space time
theory is suggested. By introducing a dynamical space time vector field
$\chi_\mu$ as a Lagrange multiplier, a conservation of an energy momentum
tensor $T^{\mu\nu}_{(\chi)}$ is implemented. This Lagrangian generalizes the
"Unified dark energy and dark matter from a scalar field different from
quintessence" [Phys.RevD 81, 043520 (2010)] which did not consider a Lagrangian
formulation. This generalization allows the solutions which were found
previously, but in addition to that also non singular bouncing solutions that
rapidly approach to the $\Lambda$CDM model. The dynamical time vector field
exactly coincides with the cosmic time for the a $\Lambda$CDM solution and
suffers a slight shift (advances slower) with respect to the cosmic time in the
region close to the bounce for the bouncing non singular solutions. In addition
we introduced some exponential potential which could enter into the
$T^{\mu\nu}_{(\chi)}$ stress energy tensor or coupled directly to the measure
$\sqrt{-g}$, gives a possible interaction between DE and DM and could explain
the coincidence problem.
