Spectral dimension in graph models of causal quantum gravity
arXiv:1310.8109
Abstract
The phenomenon of scale dependent spectral dimension has attracted special interest in the quantum gravity community over the last eight years. It was first observed in computer simulations of the causal dynamical triangulation (CDT) approach to quantum gravity and refers to the reduction of the spectral dimension from 4 at classical scales to 2 at short distances. Thereafter several authors confirmed a similar result from different approaches to quantum gravity. Despite the contribution from different approaches, no analytical model was proposed to explain the numerical results as the continuum limit of CDT. In this thesis we introduce graph ensembles as toy models of CDT and show that both the continuum limit and a scale dependent spectral dimension can be defined rigorously. First we focus on a simple graph ensemble, the random comb. It does not have any dynamics from the gravity point of view, but serves as an instructive toy model to introduce the characteristic scale of the graph, study the continuum limit and define the scale dependent spectral dimension. Having defined the continuum limit, we study the reduction of the spectral dimension on more realistic toy models, the multigraph ensembles, which serve as a radial approximation of CDT. We focus on the (recurrent) multigraph approximation of the two-dimensional CDT whose ensemble measure is analytically controlled. The latter comes from the critical Galton-Watson process conditioned on non-extinction. Next we turn our attention to transient multigraph ensembles, corresponding to higher-dimensional CDT. Firstly we study their fractal properties and secondly calculate the scale dependent spectral dimension and compare it to computer simulations. We comment further on the relation between Horava-Lifshitz gravity, asymptotic safety, multifractional spacetimes and CDT-like models.
DPhil (Phd) thesis (2013), University of Oxford, 145 pages, 18 figures, new results/analysis on 2+1 model, comments are welcome
References in corpus (18)
- Quantum Gravity at a Lifshitz Point
- Spectral Dimension of the Universe in Quantum Gravity at a Lifshitz Point
- Fractal properties of quantum spacetime
- Second- and First-Order Phase Transitions in CDT
- Probing the quantum nature of spacetime by diffusion
- The Semiclassical Limit of Causal Dynamical Triangulations
- On the number of relevant operators in asymptotically safe gravity
- 2d CDT is 2d Horava-Lifshitz quantum gravity
- Quantum Gravity via Causal Dynamical Triangulations
- The spectral dimension of generic trees
- Condensation in nongeneric trees
- Diffusion in quantum geometry
- A bootstrap strategy for asymptotic safety
- Introduction to multifractional spacetimes
- Fractal Quantum Space-Time
- Biased random walks on combs
- Aspects of dynamical dimensional reduction in multigraph ensembles of CDT
- Spectral dimension flow on continuum random multigraph