Liu, Yang
(2025)
Aspects of de Sitter Vacua in Theoretical Cosmology.
PhD thesis, University of Nottingham.
Abstract
The work presented in this thesis is based on research conducted at the Particle Theory Group, School of Physics and Astronomy, University of Nottingham, United Kingdom. In this thesis we explore de Sitter vacua in theoretical cosmology. The thesis has two parts. In Part I we consider different stringinspired models which are related to the cosmological constant problem. In Part II we study the de Sitter vacua solutions in one string cosmology model, namely, O(d, d) invariant cosmology.
In Part I we study three aspects of de Sitter vacua in string-inspired models. First of all, in chapter 2 we review the related concepts and tools we need in this part, namely, string landscape, swampland conjecture and bubble nucleation. In chapter 3, we consider a wide class of 4D effective field theories in which gravity is coupled to multiple four-forms and their dual scalar fields, with membrane sources charged under the corresponding three-form potentials. We propose a “probabilistic” argument to explain the small cosmological constant instead of the dominant anthropic principle. Chapter 3 is based on original work in collaboration with Antonio Padilla and Francisco G. Pedro [1]. In chapter 4, we explore in detail whether or not different models are compatible with the membrane weak gravity conjecture. We find that these models can be generalised to the DBI action. Chapter 4 is based on original work in collaboration with Antonio Padilla and Francisco G. Pedro [2]. In chapter 5, we generalise the results in chapter 3 and 4 to D-dimensions including the Gauss-Bonnet term. We find that the Euclidean action of the bounce, B, describing the decay of a de Sitter vacuum, is proportional to k^{−(D−2)}_{+} , which has a pole as k^2_{+} → 0 where k^2_{+} is the curvature of the parent vacuum. We find a new decay channel, describing up-tunneling from anti-de Sitter into de Sitter. Chapter 5 is based on original work done by the author [3].
In Part II we explore the de Sitter vacua solutions in O(d, d) invariant cosmology. We briefly review the basic concepts of O(d, d) invariant cosmology in chapter 6. Then in chapter 7 we perform a thorough analysis of de Sitter solutions in O(d, d) invariant cosmologies. We elucidate the nature of the instability in the string frame vacuum. For the Einstein frame, we find that the de Sitter solutions cannot be eternal. Then we extend our analysis to include Bianchi I universes. We show that we can extend the theory to the anisotropic case so that de-Sitter solutions can exist. All results in chapter 7 are presented in this paper [4], which is done in collaboration with Antonio Padilla, Paul Saffin and Robert Smith.
| Item Type: |
Thesis (University of Nottingham only)
(PhD)
|
| Supervisors: |
Antonio, Padilla
Paul, Saffin |
| Keywords: |
de Sitter vacua, theoretical cosmology |
| Subjects: |
Q Science > QB Astronomy |
| Faculties/Schools: |
UK Campuses > Faculty of Science > School of Physics and Astronomy |
| Item ID: |
81700 |
| Depositing User: |
Liu, Yang
|
| Date Deposited: |
28 Aug 2025 10:08 |
| Last Modified: |
28 Aug 2025 10:08 |
| URI: |
https://eprints.nottingham.ac.uk/id/eprint/81700 |
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