Cosmological constraints on polytropic gas model
  We study the polytropic gas scenario as the unification of dark matter and
dark energy. We fit the model parameters by using the latest observational data
including type Ia supernovae, baryon acoustic oscillation, cosmic microwave
background, and Hubble parameter data. At 68.3% and 95.4% confidence levels, we
find the best fit values of the model parameters as
$\tilde{K}=0.742_{-0.024}^{+0.024}(1\sigma)_{-0.049}^{+0.048}(2\sigma)$ and
$n=-1.05_{-0.08}^{+0.08}(1\sigma)_{-0.16}^{+0.15}(2\sigma)$. Using the best fit
values of the model, we obtain the evolutionary behaviors of the equation of
state parameters of the polytropic gas model and dark energy, the deceleration
parameter of the universe as well as the dimensionless density parameters of
dark matter and dark energy. We conclude that in this model, the universe
starts from the matter dominated epoch and approaches a de Sitter phase at late
times, as expected. Also the universe begins to accelerate at redshift $z_{\rm
t}=0.74$. Furthermore in contrary to the $\Lambda$CDM model, the cosmic
coincidence problem is solved naturally in the polytropic gas scenario.
