Classification of multifluid CP world models

Various classification schemes exist for homogeneous and isotropic (CP) world models, which include pressureless matter (so-called dust) and Einstein’s cosmological constant Λ. We here classify the solutions of more general world models consisting of up to four noninteracting fluids, each with pressure Pj, energy density ǫj and an equation of state Pj = (γj − 1)ǫj with 0 ≤ γj ≤ 2. In addition to repulsive fluids with negative pressure and positive energy density, which generalize the classical repulsive (positive) Λ component, we consider fluids with negative energy density as well. The latter generalize a negative Λ component. This renders possible new types of models that do not occur among the classical classifications of world models. Singularity-free periodic solutions as well as further ‘hill-type’, ‘hollow-type’ and ‘shiftingtype’ models are feasible. However, if one only allows for three components (dust, radiation and one repulsive component) in a spatially flat universe the repulsive classical Λ fluid (with Λ > 0) tends to yield the smoothest fits of the Supernova Ia data from Perlmutter et al. (1999). Adopting the SN Ia constraints, exotic negative energy density components can be fittingly included only if the universe consists of four or more fluids.