Abstract
Gravitational wave physics has experienced a significant advancement, particularly with the GW150914 detection [2] and numerous subsequent compact binary events [5, 10, 11]. First predicted by Einstein in 1916, the recent detection of gravitational waves by LIGO and Virgo has opened a new window to the Universe, offering insights into black holes and numerous astrophysical phenomena, and challenging our understanding of stellar evolution and binary populations [2, 4, 12, 1].Moreover, gravitational waves hold the exciting potential to illuminate the nature of dark matter. Gravitational interactions are essential for probing dark matter’s fundamental properties, such as mass, spin, and self-interactions, especially as weakly interacting massive particles (WIMPs) remain elusive in direct detection experiments. This has led to a resurgence in interest in other dark matter candidates, notably low-mass (m ≤ eV) bosonic particles like QCD axions, axion-like particles (ALPs), and "dark photons" [28, 90, 155, 220, 132, 30, 183, 235, 173, 232]. Gravitational wave physics also connects with other fields, particularly particle/high-energy physics and the exploration of the dark sector [65, 35]. A key aspect of this connection is the interaction of fundamental fields with compact objects through superradiance [58] and the formation of distinct compact objects like boson stars [233]. Gravitational wave observations now systematically search for boson star populations, heavily relying on accurate gravitational wave signal predictions, a topic central to our work using numerical relativity techniques [37].
This thesis primarily focuses on the collision behavior of binary boson stars using numerical relativity, exploring their initial conditions, the afterglow phenomenon, and the regularities of gravitational wave energy emission. Specifically, the structure of this thesis is as follows.
Chapter 1 offers a concise overview of the fundamental concepts of General Relativity, along with the basic theory related to gravitational waves.
Chapter 2 explores the fundamental principles of numerical relativity, including its theoretical foundations and mathematical tools. In particular, it offers a detailed discussion of the evolution methods in numerical relativity, as well as the approaches for computing
initial data and selecting gauges.
Chapter 3 is dedicated to the discussion of boson stars and also briefly introduces relevant background information.
Chapter 4 focuses on two main areas. The first section examines numerical simulations of boson-star head-on collisions, particularly the quality of binary initial data from single-star spacetime superposition. We find that using only superposition for boosted boson-star spacetimes leads to significant unphysical results. This issue can be addressed by modifying the initial data as suggested in [110] for oscillaton collisions. Our focus is on massive complex scalar field boson star models with a 6th-order-polynomial potential, but we believe this issue is common in various exotic compact systems, meriting further research [113]. The second section explores the lasting gravitational wave signature after a boson star binary coalescence. We fully use numerical relativity to simulate the post-merger phase and observe the extended gravitational afterglow. Recent advancements in binary
initial data have been applied, reducing false initial scalar field excitations and including a metric for angular momentum to track the total momentum of the spatial volume, including curvature effects. A key finding is the persistence of the afterglow beyond the spin-down timescale, emitting a unique gravitational wave signal that could distinguish it from other astrophysical phenomena [77].
Chapter 5 examines the behavior of binary solitons in head-on collisions. We examine a two-dimensional hypersurface of the parameter space spanned by σ and the boson star compactness, controlled through the central scalar-field amplitude. A novel superposition method for initial conditions, as discussed in Chapter 4, was employed. Additionally, this research utilized a more efficient two-dimensional code, which significantly enhanced the speed of the evolution process compared to the previous three-dimensional approach. Our results reveal that the patterns of gravitational wave energy exhibit complex structures, which are closely related to the stability of the solitons.
| Date of Award | 1 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Eugene Lim (Supervisor) |