Introduction to Modified Gravity and Gravitational Alternative for Dark Energy

TL;DR

This paper reviews various modified gravity theories as alternatives to dark energy, highlighting their ability to explain cosmic acceleration, pass Solar System tests, and exhibit rich cosmological structures including late-time acceleration and transition from deceleration.

Contribution

It provides a comprehensive overview of modified gravity models like $f(R)$, $f(G)$, and string-inspired theories, emphasizing their potential to explain dark energy phenomena.

Findings

Some models pass Solar System tests

They can naturally describe late-time cosmic acceleration

They offer explanations for the coincidence problem

Abstract

We review various modified gravities considered as gravitational alternative for dark energy. Specifically, we consider the versions of $f(R)$, $f(G)$ or $f(R,G)$ gravity, model with non-linear gravitational coupling or string-inspired model with Gauss-Bonnet-dilaton coupling in the late universe where they lead to cosmic speed-up. It is shown that some of such theories may pass the Solar System tests. On the same time, it is demonstrated that they have quite rich cosmological structure: they may naturally describe the effective (cosmological constant, quintessence or phantom) late-time era with a possible transition from decceleration to acceleration thanks to gravitational terms which increase with scalar curvature decrease. The possibility to explain the coincidence problem as the manifestation of the universe expansion in such models is mentioned. The late (phantom or quintessence) universe filled with dark fluid with inhomogeneous equation of state (where inhomogeneous terms are originated from the modified gravity) is also described.