Perfect fluids coupled to inhomogeneities in the late Universe

Abstract

We consider the Universe at the late stage of its evolution and deep inside the cell of uniformity. At such scales, the Universe is highly inhomogeneous and is filled with inhomogeneities in the form of galaxies and groups of galaxies. We also suggest that the Universe is filled with a perfect fluid, and its fluctuations have peculiar velocities of the same (nonrelativistic) order of magnitude as for the inhomogeneities. In this sense, the inhomogeneities (e.g., galaxies) and fluctuations of perfect fluids are coupled to each other. We clarify some important points of this approach and present a brief review of previous studies (e.g., the Chevallier-Polarski-Linder (CPL)model and a Chaplygin gas). We demonstrate that the perfect fluids which satisfy our approach are really coupled to galaxies, concentrating around them. The averaged (over the whole Universe) value of their fluctuations is equal to zero.