Self-tuning and the cosmological constant problem
Publication Date
July 26, 2023
Creator
Abstract
The cosmological constant problem is one of the biggest theoretical hurdles of the modern age. It comes out of two great theories in physics: General Relativity (GR) and Quantum Field Theory (QFT). QFT predicts the existence of vacuum energy and GR predicts that it will gravitate like a cosmological constant, Λ_{vac}. Through observations we find that the Universe is undergoing an accelerated expansion which can be sourced by a constant term, Λ_{obs}, whose value is set by observations. Therefore it would be tempting to compare Λobs with Λ_{vac}, but through this we find that the predicted value of the vacuum energy is far, far
greater than that of observation. In fact, at a lower estimate it represents a fine-tuning of∼ 10^{36} orders of magnitude. However, this is not the full extent of the problem. Even if we accept a fine-tuning in Λ_{vac}, its value is unstable to higher order perturbations, leading to
repeated fine-tunings and re-tunings. This is known as radiative instability of the vacuum, and it is the true source of the cosmological constant problem.
This thesis chooses to focus on self-tuning as a method for alleviating this problem. Self-tuning refers to the practice of modifying GR by adding extra fields which act to force Λ_{vac} ∼ Λ_{obs}, removing the need for fine-tuning. In this thesis we review a variety of self-tuning mechanisms
to allow the reader to get a basic idea of the different approaches adopted.
The bulk of the thesis focuses on self-tuning with a massive scalar-tensor theory on an Anti-de Sitter (AdS) background, the idea for which originated by examining a range of allowed modifications to GR and placing some self-tuning conditions upon them. Here, we construct an explicit model and analyse the resultant field equations to check whether it can or cannot self-tune. We then perform a numerical analysis on the resultant cosmological equations to understand the dynamics of the system; focusing specifically on whether our model can self-tune regardless of initial conditions. Finally, we conduct a rudimentary analysis on the stability of this model to further understand whether we can consistently self-tune without fine-tuning.
Overall this work serves as an initial point of exploration in self-tuning on an AdS background. As we later discuss, there are many exciting future directions this model can take beyond this thesis.
Item Type
ethesis
Thesis Type
PhD
Supervisors
Subjects (LC)
Associated Schools / Departments
School of Physics and Astronomy
eprints ID
71895
UoN Repository URI
Except where otherwise noted, this item's license is described as
File(s)![Thumbnail Image]()
Name
Thesis_Corrected_Final.pdf
Type
Full-text
Description
Examined. Final corrected thesis
Size
1.34 MB
Format
Adobe PDF
Checksum (MD5)
605e3e45a002699b8205856441ded686