New parameterization for unified dark matter and dark energy
  In this paper we investigate a new phenomenological parameterization for
unified dark matter and dark energy based on the polynomial expansion of the
barotropic equation of state parameter $w$. Our parameterization provides
well-behaving evolution of $w$ for both small and big redshifts as well as in
the far future. The dark fluid described by our parameterization behaves for
big redshifts like a dark matter. Therefore one can parameterize dark energy
and dark matter using a single dark fluid, like in the case of the Chaplygin
gas. Within this parameterization we consider 2 models: one with DE barotropic
parameter fixed to be $-1$ and the second one, where $w \neq -1$ is chosen to
match the best fit to the data. We study main cosmological properties of these
models at the expansion and perturbation levels. Based on Markov chain Monte
Carlo method with currently available cosmic observational data sets, we
constrain these models to determine the cosmological parameters at the level of
background and clustering of matter. We consider the interaction between DM and
DE which directly affects the evolution of matter and its clustering. Our model
appears to be perfectly consistent with the $\Lambda$CDM model, while providing
unification of DE and DM.
