Johanna Erdmenger (U Wurzburg): Discrete Holography | 9/3/2022
I will review recent progress towards establishing a holographic duality for discrete spaces involving a regular tiling of hyperbolic space. In particular, I will present a recent example where an aperiodic XXZ spin chain is obtained naturally by extrapolating the bulk tiling to its boundary. The properties of this model are studied using RG techniques, which provide a tensor network construction for its ground state. We calculate and compare the entanglement entropy both at the boundary and in the bulk. I will comment on the next steps towards obtaining a full dynamical duality of discrete systems. In particular, this may be useful for going beyond the large N limit on the gravity side.
Keisuke Harigaya (U Chicago): Parity symmetry breaking scale and Standard Model parameters | 9/30/2022
The strong CP problem can be solved by parity symmetry. We first review two classes of models: the ones with the minimal fermion content and the ones with the minimal Higgs content. We then focus on the latter class of models and show that the parity symmetry breaking scale is predicted to be the energy scale at which the standard model Higgs quartic co
Robert Wald (UChicago): The Memory Effect and Infrared Divergences in Quantum Field Theory and Quantum Gravity | 10/6/2022
The "memory effect" refers to the fact that at order 1/r, a massless field generically will not return to the same value at asymptotically late retarded times as it had at asymptotically early retarded times. There is nothing singular about states with memory in quantum field theory, but they do not lie in the standard Fock space and infrared divergences will arise as artifacts of trying to represent states with memory in the standard Fock space. As a practical matter, if one is interested only in quantities directly relevant to collider physics, one can deal with infrared divergences by well defined procedures for obtaining "inclusive quantities," but this is clearly unsatisfactory from a fundamental viewpoint on scattering theory. For QED with massive charged particles, Faddeev and Kulish gave a construction of "in" and "out" Hilbert spaces that incorporates memory (via the "dressing" of the charged particles) and thereby provides a well defined scattering theory. However, we show that this construction fails in massless QED (because the required dressing is highly singular) and fails in (nonlinear) quantum gravity (because, in essence, the dressing would require its own further dressing and there is no self-consistent way of accomplishing this). We believe that if one wishes to treat scattering at a fundamental level in quantum gravity—as well as in massless QED and Yang-Mills theory—it is necessary to approach it from an algebraic viewpoint on the “in” and “out” states, wherein one does not attempt to “shoehorn” these states into some pre-chosen “in” and “out” Hilbert spaces.
Veronika Hubeny (UC Davis): Covariant prescriptions for holographic entanglement | 10/21/2022
In the holographic setting of gauge/gravity duality, there has been a growing expectation that the emergence of the bulk gravitational spacetime is rooted in the entanglement structure of the dual gauge theory. While the deep connection remains mysterious, an early hint came from the holographic entanglement entropy prescription, which expresses the boundary entanglement entropy in terms of simple geometric constructs in the bulk. This talk will review the recent reformulations of this prescription presented in [2208.10507] in terms of several new (and fully covariant) formulas, including minimax, max U-flow, and min V-flow (the latter covariantizing the bit thread formulation of holographic entanglement entropy by Freedman and Headrick).
Peter Adshead (Illinois): Gravitational waves and the propagation of fermions in gauge-field inflation | 10/28/2022
Degree-scale B-mode polarization of the cosmic microwave background sourced by primordial gravitational waves is the so-called smoking-gun signature of inflation. The detection of these primordial CMB B-modes is often described as would-be evidence that inflation occurred at or near the scale of grand unification, that the inflaton rolled over a super-Planckian distance in field space, and that gravity is quantized. In this talk, I describe a class of inflationary models that serve as a counter-example to these claims. Models involving classical non-Abelian gauge fields can produce observable gravitational waves at energy scales far below the scale of grand unification without the quantization of gravity, or super-Planckian field excursions. I show how the gauge field background violates parity, leading to the helical polarization of the gravitational wave background; a signature of this class of models. I also discuss the propagation and backreaction of fermions on the background field configurations, and some subtleties associated with renormalization of fermions in axion backgrounds.
Hong Liu (MIT): Subalgebra-subregion duality: emergence of space and time in holography | 11/11/2022
In holographic duality, a higher dimensional quantum gravity system emerges from a lower dimensional conformal field theory (CFT) with a large number of degrees of freedom. In this talk, I will introduce a framework to describe how geometric notions in the gravity system, such as spacetime subregions, different notions of times, and causal structure, emerge from the dual CFT.
Lance Dixon (slac): An Antipodal Duality Between Amplitudes and Form Factors | 11/18/2022
Scattering amplitudes are where quantum field theory directly meets collider experiments. An excellent model for scattering in QCD is provided by N=4 super-Yang-Mills theory, particularly in the planar limit of a large number of colors, where the theory becomes integrable. The first nontrivial amplitude in this theory is for 6 gluons. It can be computed to 7 loops using a bootstrap based on the rigidity of the function space of multiple polylogarithms, together with a few other conditions. One can also bootstrap a particular form factor, for the chiral stress-tensor operator to produce 3 gluons, through 8 loops. This form factor is the N=4 analog of the LHC process, gluon gluon --> Higgs + gluon. Remarkably, the two sets of results are related by a mysterious “antipodal” duality, which exchanges the role of branch cuts and derivatives. I will describe how bootstrapping works and what we know about this new duality.
Andreas Karch (UTexas): A top-down dictionary for double holography | 12/2/2022
Holographic interpretations of Randall-Sundrum (RS) branes provide a laboratory to explore the way quantum information evolves in field theories coupled to gravity. Despite this importance, the holographic interpretation of RS branes in terms of a theory of gravity coupled to a CFT is rather ad-hoc. In this talk, we use top-down constructions of RS branes in order to work out a precise dictionary for this "intermediate" holographic prescription, resolving serious causality problems of the naive picture often used in the literature while preserving many of the successes of the RS construction.
