The dark side of gravity: Modified theories of gravity

TL;DR

This paper reviews modified theories of gravity as alternative explanations for dark energy and dark matter, discussing their potential to address key cosmological and astrophysical challenges with recent observational evidence.

Contribution

It provides a comprehensive overview of various modified gravity models proposed to explain dark energy and dark matter phenomena in modern cosmology.

Findings

Modified gravity can potentially explain cosmic acceleration without dark energy.

Certain theories can account for galactic rotation curves without dark matter.

Observational data supports some modified gravity models as viable alternatives.

Abstract

Modern astrophysical and cosmological models are faced with two severe theoretical difficulties, that can be summarized as the dark energy and the dark matter problems. Relative to the former, it has been stated that cosmology has entered a 'golden age', in which high-precision observational data have confirmed with startling evidence that the Universe is undergoing a phase of accelerated expansion. Several candidates, responsible for this expansion, have been proposed in the literature, in particular, dark energy models and modified gravity, amongst others. One is liable to ask: What is the so-called 'dark energy' that is driving the acceleration of the universe? Is it a vacuum energy or a dynamical field (''quintessence'')? Or is the acceleration due to infra-red modifications of Einstein's theory of General Relativity? In the context of dark matter, two observations, namely, the behavior of the galactic rotation curves and the mass discrepancy in galactic clusters, suggest the existence of a (non or weakly interacting) form of dark matter at galactic and extra-galactic scales. It has also been proposed that modified gravity can explain the galactic dynamics without the need of introducing dark matter. We briefly review some of the modified theories of gravity that address these two intriguing and exciting problems facing modern physics.