Theoretical Models of Dark Energy

TL;DR

This paper reviews observational evidence and various theoretical models of dark energy, including cosmological constant, modified gravity, and inhomogeneous models, discussing their advantages, problems, and observational constraints.

Contribution

It provides a comprehensive comparison of dark energy models, highlighting recent developments, challenges, and potential solutions in explaining the Universe's accelerated expansion.

Findings

Cosmological constant faces fine-tuning and coincidence problems.

Modified gravity models can explain acceleration without dark energy.

Observational constraints help differentiate between models.

Abstract

Mounting observational data confirm that about 73% of the energy density consists of dark energy which is responsible for the current accelerated expansion of the Universe. We present observational evidences and dark energy projects. We then review various theoretical ideas that have been proposed to explain the origin of dark energy; they contain the cosmological constant, modified matter models, modified gravity models and the inhomogeneous model. The cosmological constant suffers from two major problems: one regarding fine-tuning and the other regarding coincidence. To solve them there arose modified matter models such as quintessence, k-essence, coupled dark energy, and unified dark energy. We compare those models by presenting attractive aspects, new rising problems and possible solutions. Furthermore we review modified gravity models that lead to late-time accelerated expansion without invoking a new form of dark energy; they contain f(R) gravity and the Dvali-Gabadadze-Porrati model. We also discuss observational constraints on those models and on future modified gravity theories. Finally we review the inhomogeneous Lemaitre-Tolman-Bondi model that drops an assumption of the spatial homogeneity of the Universe. We also present basics of cosmology and scalar field theory, which are useful especially for students and novices to understand dark energy models.