Models of universe with a polytropic equation of state: III. The phantom
  universe
  We construct models of universe with a generalized equation of state
$p=(\alpha \rho+k\rho^{1+1/n})c^2$ having a linear component and a polytropic
component. The linear equation of state $p=\alpha\rho c^2$ with $-1\le
\alpha\le 1$ describes radiation ($\alpha=1/3$), pressureless matter
($\alpha=0$), stiff matter ($\alpha=1$), and vacuum energy ($\alpha=-1$). The
polytropic equation of state $p=k\rho^{1+1/n} c^2$ may be due to Bose-Einstein
condensates with repulsive ($k>0$) or attractive ($k<0$) self-interaction, or
have another origin. In this paper, we consider the case where the density
increases as the universe expands. This corresponds to a "phantom universe" for
which $w=p/\rho c^2<-1$ (this requires $k<0$). We complete previous
investigations on this problem and analyze in detail the different
possibilities. We describe the singularities using the classification of [S.
Nojiri, S.D. Odintsov, S. Tsujikawa, Phys. Rev. D {\bf 71}, 063004 (2005)]. We
show that for $\alpha>-1$ there is no Big Rip singularity although $w\le -1$.
For $n=-1$, we provide an analytical model of phantom bouncing universe
"disappearing" at $t=0$. We also determine the potential of the phantom scalar
field and phantom tachyon field corresponding to the generalized equation of
state $p=(\alpha \rho+k\rho^{1+1/n})c^2$.
