  Various classification schemes exist for homogeneous and isotropic (CP) world
models, which include pressureless matter (so-called dust) and Einstein's
cosmological constant Lambda. We here classify the solutions of more general
world models consisting of up to four non-interacting fluids, each with
pressure P, energy density epsilon and an equation of state P = (gamma - 1)
epsilon with 0 <= gamma <= 2.
  In addition to repulsive fluids with negative pressure and positive energy
density, which generalize the classical repulsive (positive) Lambda component,
we consider fluids with negative energy density as well. The latter generalize
a negative Lambda 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
`shifting-type' 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
Lambda fluid (with Lambda > 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.
