Antony Speranza (UIUC): Generalized entropy for general subregions in quantum gravity | 9/6/2023
I will describe a construction of algebras of observables associated with local subregions in quantum gravity in the small G_N limit. This algebra consists of operators dressed to a semiclassical observer degree of freedom which serves as an anchor defining the subregion. I will argue that properly implementing the gravitational constraints on this algebra results in a type II von Neumann algebra, which possesses a well-defined notion of entropy. Up to a state-independent constant, this entropy agrees with the UV-finite generalized entropy of the subregion, consisting of a Bekenstein-Hawking area term and a bulk entropy term. This gives an algebraic explanation for the finiteness of the generalized entropy, and provides a number of tools for investigating aspects of semiclassical gravitational entropy, including the generalized second law, the quantum focusing conjecture, and the quantum extremal surface prescription in holography.
Alex Millar (Fermilab): Physical Signatures of Fermion-Coupled Axions | 9/13/2023
While there is an abundance of experiments searching for axion dark matter (DM) via its electromagnetic coupling, there are fewer utilizing its derivative coupling to electrons and nucleons. This direct coupling generates dynamical effects through the fermion spin, and therefore spin-polarized targets are a naturally useful target. We find that spin-polarized or magnetized analogs of layered dielectric haloscopes can be powerful probes at both radio frequencies, with sensitivity to currently unexplored parameter space, and optical frequencies, with sensitivity comparable to current astrophysical bounds.
Bruno Balthazar (UChicago): Geometry of Conformal Manifolds and the Inversion Formula | 9/20/2023
Families of conformal field theories are naturally endowed with a Riemannian geometry which is locally encoded by correlation functions of exactly marginal operators. We show that the curvature of such conformal manifolds can be computed using Euclidean and Lorentzian inversion formulae, which combine the operator content of the conformal field theory into an analytic function. Analogously, operators of fixed dimension define bundles over the conformal manifold whose curvatures can also be computed using inversion formulae. These results relate curvatures to integrated four-point correlation functions which are sensitive only to the behavior of the theory at separated points. We apply these inversion formulae to derive convergent sum rules expressing the curvature in terms of the spectrum of local operators and their three-point function coefficients. We further show that the curvature can smoothly diverge only if a conserved current appears in the spectrum, or if the theory develops a continuum. We verify our results explicitly in 2d examples.
Jan Albert (YITP, Stonybrook): Bootstrapping large-N confining gauge theories | 9/27/2023
Obtaining the low-energy EFT of a given large-N confining gauge theory is in general a very difficult problem. Instead, one can proceed by carving out the space of allowed EFTs using the constraints on scattering amplitudes that follow e.g. from unitarity and crossing symmetry. In this talk I will review how to do this in the context of pion physics, with large-N QCD as our target. I will discuss what bounds this imposes on the chiral Lagrangian, and what theories saturate the bounds. I will end by discussing how a mixed system of pions and photons allows us to input symmetries and anomalies into the bootstrap, paving the way for bootstrapping large N QCD.
Alexander Blum (MPI-Berlin): Heisenberg's Path to Quantum Mechanics | 10/03/2023
In the summer of 1925, Heisenberg wrote the paper Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen, which laid the foundations of quantum mechanics. For a long time, this paper was considered to be inscrutable. This talk will show how one can make sense both of Heisenberg's formal manipulations and of his philosophical rhetoric, in particular by studying the letters he wrote in months leading up to his breakthrough. A particular emphasis will be placed on how different the theory that Heisenberg originally aimed to construct was from modern quantum mechanics
Jordan Cotler (Harvard): Non-perturbative de Sitter quantum gravity in low dimensions | 10/04/2023
Little is known about non-perturbative quantum gravity in de Sitter spacetimes. As a useful low-dimensional model, we consider de Sitter Jackiw-Teitelboim (dS JT) gravity and solve it non-perturbatively in the genus expansion. This amounts to the first non-perturbatively solvable model of de Sitter cosmology. We find that dS JT gravity has an effective string coupling which is pure imaginary, rendering the S-matrix genus expansion Borel resummable. We further establish that dS JT gravity is dual to a formal matrix integral with a negative number of degrees of freedom. More broadly, our analysis unveils new ingredients in the de Sitter holographic dictionary, which may be applicable in more general contexts.
Hofie Hannisdottir (IAS): What can be measured asymptotically? | 10/25/2023
We consider asymptotic observables in quantum field theories in which the S-matrix makes sense. We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time ordering is relaxed. These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes. We explain how to calculate them in different ways: by relating them to amplitudes and products of amplitudes and by using a generalization of the LSZ reduction formula. Finally, we discuss how to relate them to one another through new versions of crossing symmetry. As an application, we discuss one-loop contributions to gravitational radiation in the post-Minkowski expansion, emphasizing the role of classical cut contributions and highlighting the infrared physics of in-in observables.
Sangmin Choi (Ecole Polytechnique, CPHT): Loop-corrected soft photon theorems and large gauge transformations | 11/01/2023
In the last few years, a remarkable link has been established between the soft theorems and asymptotic symmetries of quantum field theories: soft theorems are Ward identities of the asymptotic symmetry generators. In quantum electrodynamics, Weinberg's soft photon theorem is nothing but the Ward identity of a gauge transformation whose parameter is non-trivial at infinity. Likewise, Low's tree-level subleading soft photon theorem is the Ward identity of a gauge transformation whose parameter diverges linearly at infinity. More recently, it has been shown that Low's theorem receives loop corrections that are logarithmic in soft photon energy. Then, it is natural to ask whether such corrections are associated with some asymptotic symmetry of the S-matrix. There have been proposals for conserved charges whose Ward identities yield the loop-corrected soft theorems, but a clear symmetry interpretation remains elusive. We explore this question in the context of scalar QED, in hopes of shedding light on the connection between asymptotic symmetries and loop-corrected soft theorems.
Sergio Hernandez-Cuenca (MIT): Near-Extremal Black Hole Entropies from Replica Matrices | 11/08/2023
A generic pathology one encounters when computing the thermal entropy of a black hole is that it becomes negatively divergent as the temperature goes to zero, and only those whose extremal limit preserve some supersymmetry yield a sensible low-temperature entropy. The physics relevant to these phenomena are all captured by Jackiw-Teitelboim theories of gravity, which have been rather explicitly shown to be dual to various matrix ensembles. The issues and features mentioned above can all be precisely understood from this perspective: traditional gravitational calculations are computing annealed quantities, which give inherently wrong approximations near extremality.
We use the matrix integral formulation to show how quenched quantities do in fact behave sensibly and yield non-negative entropies at all temperatures. By using a suitable replica trick, this is done for a completely general matrix ensemble, thus settling the question for any black hole whose near-extremal spectrum is captured by such ensembles. Crucially, this result only requires working perturbatively to leading order in the size of the matrices, which hints at the possibility of an analogous semiclassical gravitational computation where one just needs to account for wormhole contributions appropriately (and not for doubly non-perturbative effects in 1/G).
Matthew Heydeman (Harvard): Probing Quantum Black Hole Microstates | 11/15/2023
It is now widely believed that black holes should be described by ordinary (though complicated) quantum systems. This can be made precise for supersymmetric (BPS) black holes in Anti de-Sitter space, where the AdS/CFT correspondence may be used to reliably count black hole microstates. We will review this proposal for 4d superconformal field theories dual to AdS5 black holes and explain the challenges in characterizing these microstates directly in terms of gravitational variables. Surprisingly, a gravitational path integral calculation predicts certain universal features of the spectrum, including a large number of exactly degenerate states and a "mass gap" between the BPS and non-BPS states.
If BPS black holes are described by ordinary quantum systems, we should be able to act with operators which probe the microstates. We find one such probe is a certain generalization of the supersymmetric Wilson line in 4d N=4 SYM; holographically dual to a D-brane which wraps the horizon, and further demonstrate a matching of these descriptions when the spacetime description is valid. In addition to detecting the familiar deconfinement transition in conformal gauge theories, this provides an example of a system in which a black hole interacts with other degrees of freedom but has an exact microscopic description.
Hongwan Liu (UChicago/Fermilab): Exotic energy injection in the early Universe | 11/29/2023
The production of electromagnetically interacting particles in the early Universe is a generic prediction of many extensions of both the standard models of particle physics and cosmology. In this talk, I will give an overview of recent progress in understanding how injected particles deposit their energy into regular matter, and highlight some novel signatures of new physics that are well within current and near-future experimental reach.
