Scalar Dark Energy Models and Scalar-Tensor Gravity: Theoretical Explanations for the Accelerated Expansion of Present Universe

TL;DR

This paper reviews scalar field models and scalar-tensor theories as explanations for the Universe's accelerated expansion, addressing challenges like fine-tuning, coincidence, and Hubble tension through dynamical analysis and experimental constraints.

Contribution

It provides a comprehensive review of scalar field and scalar-tensor models, highlighting their potential to solve key cosmological problems and fit observational data.

Findings

Scalar field models can produce accelerating solutions as attractors.

Constraints on parameters are crucial for model viability.

Some models may address the Hubble tension.

Abstract

The reason for the present accelerated expansion of the Universe stands as one of the most profound questions in the realm of science, with deep connections to both cosmology and fundamental physics. From a cosmological point of view, physical models aimed at elucidating the observed expansion can be categorized into two major classes: dark energy and modified gravity. We review various major approaches that employ a single scalar field to account for the accelerating phase of our present Universe. Dynamical system analysis is employed in several important models to seek for cosmological solutions that exhibit an accelerating phase as an attractor. For scalar field models of dark energy, we consistently focus on addressing challenges related to the fine-tuning and coincidence problems in cosmology, as well as exploring potential solutions to them. For scalar-tensor theories and their generalizations, we emphasize the importance of constraints on theoretical parameters to ensure overall consistency with experimental tests. Models or theories that could potentially explain the Hubble tension are also emphasized throughout this review.