Citation
Remmen, Grant Newton (2017) Defining Gravity: Effective Field Theory, Entanglement, and Cosmology. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z90R9MD1. http://resolver.caltech.edu/CaltechTHESIS:04072017131612641
Abstract
Many of the most exciting open problems in highenergy physics are related to the behavior and ultimate nature of gravity and spacetime. In this dissertation, several categories of new results in quantum and classical gravity are presented, with applications to our understanding of both quantum field theory and cosmology.
A fundamental open question in quantum field theory is related to ultraviolet completion: Which lowenergy effective field theories can be consistently combined with quantum gravity? A celebrated example of the swampland programthe investigation of this questionis the weak gravity conjecture, which mandates, for a U(1) gauge field coupled consistently to gravity, the existence of a state with chargetomass ratio greater than unity. In this thesis, we demonstrate the tension between the weak gravity conjecture and the naturalness principle in quantum field theory, generalize the weak gravity conjecture to multiple gauge fields, and exhibit a model in which the weak gravity conjecture solves the standard model hierarchy problem. Next, we demonstrate that gravitational effective field theories can be constrained by infrared physics principles alone, namely, analyticity, unitarity, and causality. In particular, we derive bounds related to the weak gravity conjecture by placing such infrared constraints on higherdimension operators in a photongraviton effective theory. We furthermore place bounds on highercurvature corrections to the Einstein equations, first using analyticity of graviton scattering amplitudes and later using unitarity of an arbitrary treelevel completion, as well as constrain the couplings in models of massive gravity. Completing our treatment of perturbative quantum gravity, outside of the swampland program, we also reformulate graviton perturbation theory itself, finding a field redefinition and gaugefixing of the EinsteinHilbert action that drastically simplifies the Feynman diagram expansion. Furthermore, our reformulation also exhibits a hidden symmetry of general relativity that corresponds to the double copy relations equating gravity amplitudes to sums of squares of gluon amplitudes in YangMills theory, a surprising correspondence that yields insights into the structure of quantum field theories.
Moving beyond perturbation theory into nonperturbative questions in quantum gravity, we consider the deep relation between spacetime geometry and properties of the quantum state. In the context of holography and the antide Sitter/conformal field theory correspondence, we test the proposed ER=EPR correspondence equating quantum entanglement with wormholes in spacetime. In particular, we demonstrate that the nocloning theorem in quantum mechanics and the nogo theorem for topology change of spacetime are dual under the ER=EPR correspondence. Furthermore, we prove that the presence of a wormhole is not an observable in quantum gravity, rescuing ER=EPR from potential violation of linearity of quantum mechanics. Excitingly, we also prove a new area theorem within classical general relativity for arbitrary dynamics of two collections of wormholes and black holes; this area theorem is the ER=EPR analogue of entanglement conservation. We next turn our attention to the emergence of spacetime itself, placing consistency conditions on the proposed correspondence between antide Sitter space and the Multiscale Entanglement Renormalization Ansatz, a special tensor network that constitutes a computational tool for finding the ground state of certain quantum systems. Further examining the role of quantum entanglement entropy in the emergence of general relativity, we ask whether there is a consistent microscopic formulation of the entropy in theories of entropic gravity; we find that our results weaken equationofstate proposals for entropic gravity while strengthening those more akin to holography, guiding future investigation of theories of emergent gravity.
Finally, we examine the consequences of the Hamiltonian constraint in classical gravity for the early universe. The Hamiltonian constraint allows for the Liouville measure on the phase space of cosmological parameters for homogeneous, isotropic universes to be converted into a probability distribution on trajectories, or equivalently, on initial conditions. However, this measure diverges on the set of spacetimes that are spatially flat, like the observable universe. In this thesis, we derive the unique, classical, Hamiltonianconserved measure for the subset of flat universes. This result allows for distinction between different models of cosmic inflation with similar observable predictions; for example, we find that the measure favors models of largescale inflation, as such potentials more naturally produce the number of efolds necessary to match cosmological observations.
Item Type:  Thesis (Dissertation (Ph.D.))  

Subject Keywords:  general relativity; particle physics; quantum gravity; weak gravity conjecture; effective field theory; swampland program; scattering amplitudes; black holes; graviton; unitarity; analyticity; causality; highercurvature operators; string theory; massive gravity; double copy; ER=EPR conjecture; AdS/CFT correspondence; holography; qubits; entanglement; wormholes; area theorem; tensor networks; multiscale entanglement renormalization ansatz; entropy; entropic gravity; cosmology; attractor; inflation; scalar field; measure; efolds  
Degree Grantor:  California Institute of Technology  
Division:  Physics, Mathematics and Astronomy  
Major Option:  Physics  
Awards:  John Stager Stemple Memorial Prize in Physics, 2015  
Thesis Availability:  Public (worldwide access)  
Research Advisor(s): 
 
Group:  Caltech Theory  
Thesis Committee: 
 
Defense Date:  17 May 2017  
NonCaltech Author Email:  gremmen (AT) theory.caltech.edu  
Funders: 
 
Record Number:  CaltechTHESIS:04072017131612641  
Persistent URL:  http://resolver.caltech.edu/CaltechTHESIS:04072017131612641  
DOI:  10.7907/Z90R9MD1  
Related URLs: 
 
ORCID: 
 
Default Usage Policy:  No commercial reproduction, distribution, display or performance rights in this work are provided.  
ID Code:  10132  
Collection:  CaltechTHESIS  
Deposited By:  Grant Remmen  
Deposited On:  30 May 2017 23:21  
Last Modified:  12 Dec 2018 22:44 
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