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A fluid of domain walls may have an effective equation of state p w = − 2 3 ρ w . This equation of state is qualitatively in agreement with the supernova type Ia observations. We exploit a cosmological model where the matter content is given by a dust fluid and a domain wall fluid. The process of formation of galaxies is essentially preserved. On the other hand, the behaviour of the density contrast in the ordinary fluid is highly altered when domain walls begin to dominate the matter content of the Universe. This domain wall phase occurs at relative recent era, and its possible consequences are discussed, specially concerning the Sachs-Wolfe effect.One of the most surprising observational results in cosmology in the end of this century is due to the use of the Supernova type Ia as standard candles in the evaluation of luminosity distance as function of the redshift z. The two groups consecrated to this program [1,2] arrived at the same conclusion: the Universe is now in an accelerated phase. The inflationary paradigm, initially restricted to the very early Universe, was transferred to the Universe today with strinking consequences, for example, for the age of the Universe and many other cosmological parameters [3,4]. The most accepted results [5] indicate that the value of the decelerating parameter today is given by q = −ä Hence, the Universe today should be dominated by a fluid with negative pressure such that the strong energy condition is violated.One of the main issues related to this observational results, is the nature of this negative pressure fluid. The position of the first acoustic peak in the spectrum of the anisotropy of cosmic microwave background radiation is related to the total density of the Universe. In spite of the fact that there is not until now doubtless observational results indicating where precisely this first acoustic peak is located, the recent data coming from BOOMERANG and MAXIMA projects indicate that the density of the Universe is near the critical density [6,7]. Hence, it can be assumed that the Universe is spatially or nearly spatially flat. On the other hand, the clustered mass is responsable for 0.3 ∼ 0.4 of the critical density. Consequently, from this data it is possible to conclude that 0.7 ∼ 0.6 of the total matter of the Universe is a smooth component which is generally called dark energy.A fluid of negative pressure violating the strong energy condition does not cluster at large scale. In particular, a cosmological constant, which can be represented by an equation of state p = −ρ, remains perfectly smooth, since its density fluctuations are exactly zero. However, the above mentioned results for the deceleration parameter suggest a fluid different from the cosmological constant. A very popular model to describe this dark energy is the so-called "quintessence", a scalar field with an appropriate potential term such that the effective equation of state evolves from a typical radiation equation of state (p = ρ 3 ) to a negative equation of state [8][9][10][11]. But, it is d...

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 = -/3 (a variable cosmological constant, a cosmic string fluid, a texture) or p = - (a cosmological constant), while the other is ordinary matter (radiation, stiff matter, incoherent matter). In case (a), with the stringlike fluid, the spatial section is compact but the scale factor evolves in a way similar to that in an open Universe with positive pressure, providing alternative solutions for the age and horizon problems. The study of gravitational waves is extended to all values of the effective curvature keff = k-(8/3) G0s, that is, positive, negative or zero, k being the curvature of the spacelike section. In case (b), with a cosmological constant, we restrict ourselves to a flat spatial section. The behaviour of gravitational waves has, in each case, particular features, that can be reflected in the anisotropy spectrum of the cosmic microwave background radiation. We also make some considerations on these exotic fluids as candidates for dark matter models.