Perfect fluids coupled to inhomogeneities in the late Universe

TL;DR

This paper investigates the late Universe where inhomogeneities like galaxies coexist with a coupled perfect fluid, analyzing their interactions and fluctuations, and reviewing models such as CPL and Chaplygin gas.

Contribution

It introduces a framework for coupling perfect fluids with inhomogeneities in the late Universe and clarifies their fluctuation behavior, extending previous models.

Findings

Perfect fluids are coupled to galaxy inhomogeneities.

Fluctuations of the fluids average to zero across the Universe.

Galaxies concentrate around fluid fluctuations.

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 the groups of galaxies. We also suggest that the Universe is filled with a perfect fluid, and its fluctuations have peculiar velocities of the same (non-relativistic) order of magnitude as for the inhomogeneities. In this sense, the inhomogeneities (e.g. galaxies) and fluctuations of perfect fluids are coupled with each other. We clarify some important points of this approach and present a brief review of previous studies (e.g. CPL model and Chaplygin gas). We demonstrate that considered 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.