An effective description of dark energy: from theory to phenomenology

TL;DR

This paper explores the effective theory of dark energy within scalar-tensor theories, analyzing stability, matter coupling, and observational signatures, aiming to test deviations from the standard ${ m f ext{Lambda Cold Dark Matter}}$ model.

Contribution

It provides a comprehensive analysis of the effective theory of dark energy, including stability conditions, matter interactions, and potential observational effects, extending the understanding of scalar-tensor modifications to gravity.

Findings

Most general stable theories are Horndeski and beyond-Horndeski.

Future surveys can constrain dark matter-dark energy interactions.

Kinetic mixing can lead to observable weakening of gravity at large scales.

Abstract

In the last decades, a cosmological model that fits observations through a vast range of scales emerged. It goes under the name of ${\Lambda}$CDM. However, there are still challenging questions that remain unanswered by this model, such as what causes the observed accelerated expansion of the universe, and many alternatives have been proposed. This thesis concerns an approach to test such models known as "Effective Theory of Dark Energy" . It applies to all models where general relativity is modified by adding a single scalar degree of freedom, called "scalar-tensor theories". In Chapter 1 I summarise the most general class of such theories currently known, called "Degenerate higher-Order Scalar-Tensor" (DHOST) theories. In Chapter 2, I introduce the effective theory of dark energy. The inclusion of a general coupling between matter and the gravitational sector is the subject of Chapter 3. Chapter 4 analyses in details the stability of different classes of theories. Notably, I show that the most general class of theories free from instabilities reduces to the so-called Horndeski and beyond-Horndeski theories, up to a non minimal coupling to matter. Another goal of the thesis is to study the observable effects of deviations from ${\Lambda}$CDM. In Chapter 5, I consider the possibility of an interaction between dark matter and dark energy and I analyse the constraining power of future surveys on the free parameters of the theory. Chapter 6 focuses on the observational effects of theories where a kinetic mixing between matter and the scalar field exists. This gives a peculiar and potentially observable effect, namely the weakening of gravity at large scale structure scales.