TL;DR
This paper reviews modified theories of gravity as potential solutions to the dark energy and dark matter problems, focusing on their foundations and implications for explaining the universe's accelerated expansion.
Contribution
It provides an analysis of various modified gravity models and explores their foundational aspects in addressing key cosmological challenges.
Findings
Modified gravity can explain cosmic acceleration without dark energy.
Different theories introduce new degrees of freedom in gravity.
Foundational issues are crucial for developing consistent models.
Abstract
Modern astrophysical and cosmological models are plagued with two severe theoretical difficulties, namely, the dark energy and the dark matter problems. Relative to the former, high-precision observational data have confirmed with startling evidence that the Universe is undergoing a phase of accelerated expansion. This phase, one of the most important and challenging current problems in cosmology, represents a new imbalance in the governing gravitational equations. Several candidates, responsible for this expansion, have been proposed in the literature, in particular, dark energy models and modified gravity, amongst others. Outstanding questions are related to the nature of this so-called "dark energy" that is driving the acceleration of the universe, and whether it is due to the vacuum energy or a dynamical field. On the other hand, the late-time cosmic acceleration may be due to…
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