Density Perturbations in an Universe Dominated by the Chaplygin Gas

We study the fate of density perturbations in an Universe dominate by the Chaplygin gas, which exhibit negative pressure. We show that it is possible to obtain the value for the density contrast observed in large scale structure of the Universe by fixing a free parameter in the equation of state of this gas. The negative character of pressure must be significant only very recently. PACS number(s): 98.80.Bp, 98.65.Dx The nature of ”missing mass” that must dominate the Universe today is one of the most intriguing problem in modern physics. This problem appears at various levels, beginning with the rotation curve of spiral galaxies [1], and extending to the total mass of the Universe as it is deduced through the first acoustic peak of the spectrum of the anisotropy of cosmic background radiation [2]. A fraction of this missing mass has been called dark energy since it does cluster, remaining a smooth component in the Universe. This problem has acquired a most dramatic face through the results coming from the observation of supernova type Ia, which indicate that the Universe today is in an accelerated expansion [3, 4]. Hence, the inflationary paradigma, first conceived for the primordial Universe, can be applicable to our actual Universe. In this way, the cosmological constant problem was revived: it is possible that a cosmological constant may be the dominant component of matter in the Universe. However, other types of matter with negative pressure have been considered as candidate of the so-called dark energy of the Universe: fluids of topological defects (domain walls, strings), quintessence (a self-interacting scalar field minimally coupled to gravity), etc. Recently, a different matter component with negative pressure has been presented as possible candidate for this dark energy: the Chaplygin gas [5]. The Chaplygin gas has an equation of state of the type