Gravitational waves in cosmological models with negative pressure

We study the tensor modes of perturbations of a two-fluid model in general relativity, where one of the fluids has the equation of state p = -rho/3 (a variable cosmological constant, a cosmic string fluid, a texture) or p = - rho (a cosmological constant), while the other is ordinary matter (radiatio n, stiff matter, incoherent matter). In case (a), with the stringlike fluid , the spatial section is compact but the scale factor evolves in a way simi lar to that in an open Universe with positive pressure, providing alternati ve solutions for the age and horizon problems. The study of gravitational w aves is extended to all values of the effective curvature k(eff) = k - 8/3 pi G rho(0s), that is, positive, negative or zero, k being the curvature of the spacelike section. In case (b), with a cosmological constant, we restr ict ourselves to a flat spatial section. The behaviour of gravitational wav es has, in each case, particular features, that can be reflected in the ani sotropy spectrum of the cosmic microwave background radiation. We also make some considerations on these exotic fluids as candidates for dark matter m odels.