Quantum Fields and Strings

Euclidean wormholes are exotic types of gravitational solutions that we still don't understand completely. In the first part of the talk, I will analyze asymptotically AdS wormhole solutions from a gravitational point of view. By studying correlation functions of local and non-local operators, the universal properties that any putative holographic dual should exhibit, become manifest. In the second part, I will describe some concrete field theoretic models (both effective and microscopic) that share these properties.

TTbar and JTbar - deformed CFTs provide an interesting example of non-local, yet UV-complete two-dimensional QFTs that are entirely solvable. I will start by showing that both classes of theories possess Virasoro x Virasoro or Virasoro- Kac- Moody x Virasoro - Kac- Moody symmetry. For the case of JTbar, I will discuss the classical realization of these symmetries in terms of field-dependent coordinate transformations and show how the associated generators can be used to define an analogue of "primary" operators in this non-local theory, whose correlation functions are entirely fixed in terms of those of the undeformed CFT. In particular, two and three-point functions are simply given by the corresponding momentum-space correlator in the undeformed CFT, with all dimensions replaced by particular momentum-dependent conformal dimensions. Interestingly, scattering amplitudes off the near-horizon of extremal black holes are known to take a strikingly similar form.

Several puzzles and issues in quantum gravity are related to the question of what type of high-energy information is actually available to low-energy observers. In this talk I will try to argue that the best description available to a low-energy observer is probably of a statistical nature. Such a statistical description naturally connects to e.g. the eigenstate thermalization hypothesis, and wormholes and the apparent lack of factorization. I will describe this statistical picture and the evidence we have for it, and what we could possibly learn from it. Partially based on work in progress.

Zoom Link: https://pitp.zoom.us/j/96646337719?pwd=UVRKN3Y4Q05xNXk1VExOenYwSFZvZz09

In this talk, we will discuss the late time behavior of out of time ordered correlators (OTOC). First part of the talk, we argue the nonlinear behavior of OTOC for general chaotic system is controlled by a Dray t’Hooft like action. Second part of the talk, we will discuss the late time nonperturbative effects in OTOC and its relation with random unitary behavior.

A salient feature of black holes near extremality is the appearance of an AdS$_2$ throat in their near-horizon geometry. Depending on the underlying theory, these AdS$_2$ throats may be unstable to fragmentation, wherein a single throat is instead replaced by a tree-like structure of branched AdS$_2$ throats. For Einstein-Maxwell theory, the underlying reason behind this instability is the existence of multi-centered configuration in the moduli space of black hole solutions at fixed total charge. Given the success of the Schwarzian/SYK paradigm for understanding a single AdS$_2$ it is time to revisit the fragmentation story. To build up intuition, I will present a model, studied in the statistical mechanics literature, that shares many features with SYK, including exact solvability at large-N and an emergent conformal symmetry that gets weakly broken in the UV. The novel feature of this model is the appearance of a spin glass phase at O(1) temperatures, which I will try to relate to the fragmentation story.

In this talk I will review work on `decomposition,' a property of 2d theories with 1-form symmetries and, more generally, d-dim'l theories with (d-1)-form symmetries. Decomposition is the observation that such quantum field theories are equivalent to ('decompose into’)  disjoint unions of other QFTs, known in this context as "universes.” Examples include two-dimensional gauge theories and orbifolds with matter invariant under a subgroup of the gauge group. Decomposition explains and relates several physical properties of these theories -- for example, restrictions on allowed instantons arise as a "multiverse interference effect" between contributions from constituent universes. First worked out in 2006 as part of efforts to understand string propagation on stacks, decomposition has been the driver of a number of developments since. In the first half of this talk, I will review decomposition; in the second half, I will focus on the recent application to anomaly resolution of Wang-Wen-Witten in two-dimensional orbifolds.

The quantization of a non-linear sigma model in 1+1 dimensional space-time, whose target space is equipped with a non positive definite metric is an interesting open problem having potential applications to string theory and Condensed Matter Physics. Perhaps the simplest example where it can be studied is the 2D Lorentzian black hole sigma model that was introduced by Witten in '91. The purpose of this talk is to discuss the results of  the paper arXiv:2010.10613. There it was found that the scaling limit of a certain spin chain, that lies within the integrability class of an inhomogeneous six-vertex model, is closely related to the Lorentzian black hole sigma model and its Euclidean version.

Zoom Link: https://pitp.zoom.us/j/91519988720?pwd=RTdrT1NWc3V0SXZVTGxETVBlTm5YUT09

Fractons are relatively new types of quasiparticles which have recently been inspiring activity within several branches of physics. I will offer some motivations and perspectives from quantum information theory, condensed matter physics, and high energy physics, focusing mainly on my work in the latter two subjects. This talk is primarily based on https://arxiv.org/abs/2108.08322 and a paper to appear shortly with Dominic Williamson.

I will present a class of recently computed holographic correlators between half-BPS operators in a vast array of SCFTs with non-maximal superconformal symmetry in dimensions d=3,4,5,6. Via AdS/CFT, these four-point functions are dual to gluon scattering amplitudes in AdS. Exploiting the notion of MRV limit I will show that, at tree level, all such correlators are completely fixed by symmetries and consistency conditions. Our results encode a wealth of novel CFT data and exhibit various emergent structures, including Parisi-Sourlas supersymmetry, hidden conformal symmetry and color-kinematics duality. This talk will be based on https://arxiv.org/pdf/2103.15830.pdf.

We investigate interactions between branes of various dimensions, both charged and uncharged, in some non-supersymmetric string models. These include the USp(32) and U(32) orientifolds of the type IIB and type 0B strings, as well as the SO(16) x SO(16) projection of the exceptional heterotic string. The resulting ten-dimensional spectra are free of tachyons and the combinations of branes that they contain give rise to rich and varied dynamics. We focus on potentials that describe their mutual interactions, both in the probe regime and in the string-amplitude regime, finding qualitative agreement despite the absence of supersymmetry and confirming the Weak Gravity Conjecture for charged branes.

We discuss aspects of the possible transition between small black holes and highly excited fundamental strings. We focus on the connection between black holes and the self gravitating string solution of Horowitz and Polchinski. This solution is interesting because it has non-zero entropy at the classical level and it is natural to suspect that it might be continuously connected to the black hole. Surprisingly, we find a different behavior for heterotic and type II cases. For the type II case we find an obstruction to the idea that the two are connected as classical solutions of string theory, while no such obstruction exists for the heterotic case. We further provide a linear sigma model analysis that suggests a continuous connection for the heterotic case. We also describe a solution generating transformation that produces a charged version of the self gravitating string. This provides a fuzzball-like construction of near extremal configurations carrying fundamental string momentum and winding charges. We provide formulas which are exact in α′ relating the thermodynamic properties of the charged and the uncharged solutions.

Zoom Link: https://pitp.zoom.us/j/96056818482?pwd=cXdxSGRBTEI5N2d6Z21XbTF1Z1lLQT09

Recent studies revealed that wormhole geometries play a central role in understanding quantum gravity. After disorder-averaging over random couplings, Sachdev-Ye-Kitaev (SYK) model has a collective field description of wormhole saddles. A recent paper by Saad, Shenker, Stanford, and Yao studied the SYK model with fixed couplings and found that the wormhole saddles persist, but that new saddles called “half-wormholes” also appear in the path-integral. 

In this talk, we introduce a “partially disorder-averaged SYK model” and study how these half-wormholes emerge as we gradually fix the coupling constants. This model has a real parameter that smoothly interpolates between the ordinary totally disorder-averaged SYK model and the totally fixed-coupling model. For the large N effective description, in addition to the usual bi-local collective fields, we also introduce a new additional set of local collective fields. These local fields can be understood as the “half” of the bi-local collective fields, and they represent the half-wormholes in the totally fixed-coupling limit. We found that the large N saddles of these local fields vanish in the total-disorder-averaged limit, while they develop non-trivial profiles as we gradually fix the coupling constants. This illuminates how the half-wormhole saddles emerge in the SYK model with fixed couplings.