Future of the universe in modified gravitational theories: Approaching to the finite-time future singularity

TL;DR

This paper explores the evolution of dark energy-dominated universes in various modified gravity theories, analyzing conditions for finite-time future singularities and proposing scenarios to resolve or modify these singularities.

Contribution

It provides a unified framework for analyzing different modified gravities and constructs examples of singularity-producing models, also proposing methods to avoid or alter these singularities.

Findings

Modified gravity theories can produce accelerating universes ending in finite-time singularities.

Certain modifications of gravitational action can prevent or alter the nature of future singularities.

Non-minimal gravitational couplings can generate, remove, or modify finite-time future singularities.

Abstract

We investigate the future evolution of the dark energy universe in modified gravities including $F(R)$ gravity, string-inspired scalar-Gauss-Bonnet and modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these theories may be presented in universal form by using the effective ideal fluid with an inhomogeneous EoS without specifying its explicit form. We construct several examples of the modified gravity which produces accelerating cosmologies ending at the finite-time future singularity of all four known types by applying the reconstruction program. Some scenarios to resolve the finite-time future singularity are presented. Among these scenarios, the most natural one is related with additional modification of the gravitational action in the early universe. In addition, late-time cosmology in the non-minimal Maxwell-Einstein theory is considered. We investigate the forms of the non-minimal gravitational coupling which generates the finite-time future singularities and the general conditions for this coupling in order that the finite-time future singularities cannot emerge. Furthermore, it is shown that the non-minimal gravitational coupling can remove the finite-time future singularities or make the singularity stronger (or weaker) in modified gravity.