Coupled scalar fields in the late Universe: The mechanical approach and the late cosmic acceleration

TL;DR

This paper explores how a coupled scalar field, behaving as a two-component perfect fluid, can drive late cosmic acceleration by being concentrated around galaxies and screening their gravitational effects.

Contribution

It demonstrates that a minimally coupled scalar field can become coupled at late times, acting as a two-component fluid that explains cosmic acceleration and galaxy screening effects.

Findings

Scalar field behaves as a network of frustrated cosmic strings and a cosmological constant.

Coupled scalar field can cause late cosmic acceleration.

Scalar field fluctuations are concentrated around galaxies.

Abstract

In this paper, we consider the Universe at the late stage of its evolution and deep inside the cell of uniformity. At these scales, we consider the Universe to be filled with dust-like matter in the form of discretely distributed galaxies, a minimally coupled scalar field and radiation as matter sources. We investigate such a Universe in the mechanical approach. This means that the peculiar velocities of the inhomogeneities (in the form of galaxies) as well as fluctuations of other perfect fluids are non-relativistic. Such fluids are designated as coupled because they are concentrated around inhomogeneities. In the present paper we investigate the conditions under which a scalar field can become coupled, and show that, at the background level, such coupled scalar field behaves as a two component perfect fluid: a network of frustrated cosmic strings with EoS parameter $w=-1/3$ and a cosmological constant. The potential of this scalar field is very flat at the present time. Hence, the coupled scalar field can provide the late cosmic acceleration. The fluctuations of the energy density and pressure of this field are concentrated around the galaxies screening their gravitational potentials. Therefore, such scalar fields can be regarded as coupled to the inhomogeneities.