Dark energy: EFTs and supergravity

TL;DR

This thesis explores the cosmological implications of string compactifications, focusing on effective field theories, dark energy models, and the challenges of realizing quintessence within type IIB string theory, ultimately questioning its viability compared to a cosmological constant.

Contribution

It provides a detailed analysis of dark energy models derived from string theory, highlighting the difficulties in constructing viable quintessence models and the potential for de Sitter solutions.

Findings

Quintessence models require non-supersymmetric Minkowski vacua at leading order.

Quantum fluctuations impose fine-tuning issues for slow-rolling scalar fields.

Quintessence faces significant challenges compared to a cosmological constant.

Abstract

The subject of this thesis is cosmological implications of string compactifications understood in a broad sense. In the first half of the thesis, we will begin by reviewing the four-dimensional description of the tree-level perturbative type IIB action. We will then review a number of open questions in cosmology and their relevance with regards to the remainder of the thesis. We will first explore some of these cosmological questions from the perspective of effective field theories motivated by supergravity. From the naturalness of dark energy and how to obtain a naturally light dark energy field in terms of the clockwork mechanism and the Dvali-Kaloper-Sorbo four-form mixing. We also discuss the coincidence problem for dynamical models of dark energy consistent with a quintessence field slowly rolling down a potential slope, of the type one would expect from the asymptotics of moduli space. In the second half of the thesis, we introduce the effects of perturbative and non-perturbative corrections to the tree-level type IIB action. We then focus on obtaining a viable model of quintessence from the type IIB effective field theory. However, we are able to show that such a model must have a non-supersymmetric Minkowski vacuum at leading order. When we consider the effects of quantum fluctuations during the early Universe, we see that such models must have extremely fine-tuned initial conditions to describe a slow-rolling scalar field at present times. We conclude that quintessence faces more challenges than a true cosmological constant, to the point that quintessence is very unattractive for model building modulo a ruling out of the cosmological constant by observations. Following this line of reasoning, we consider whether other perturbative corrections can generate de Sitter solutions in an appropriate setting.