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Models of universe with a polytropic equation of state: III. The phantom universe

We construct models of universe with a generalized equation of state $p=(αρ+kρ^{1+1/n})c^2$ having a linear component and a polytropic component. The linear equation of state $p=αρc^2$ with $-1\le α\le 1$ describes radiation ($α=1/3$), pressureless matter ($α=0$), stiff matter ($α=1$), and vacuum energy ($α=-1$). The polytropic equation of state $p=kρ^{1+1/n} c^2$ 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 $w=p/ρc^2<-1$ (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 $α>-1$ 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 $p=(αρ+kρ^{1+1/n})c^2$.