## String Theory Seminar Series

NOTE: String Theory Seminars are presently held via Zoom, not on campus.

Except where otherwise noted, seminars are held on Tuesdays at 11 a.m. in Abelson 307 (directions to Abelson). The length of the talk depends on the mode of presentation: 60 minutes for a computer talk and 80 minutes for a blackboard talk.

### Spring 2020 Seminars

January 21, 2020

Replica wormholes and the black hole information paradox

March 24, 2020

Entanglement entropy, mutual information, and state decomposition for an arithmetic perspective

Abstract: I will present some work in progress on understanding information theoretic quantities in holography from a p-adic point of view. The talk will be informal.

March 31, 2020

The UV structure of scalar field theories

Abstract: I have recently proposed a rigorous procedure for establishing the correspondence between lattice and continuum theories of fermions. This procedure led to a definition of current algebras, infrared dualities, and operator product expansions in a finite, completely well defined setting. The crucial insight was that the UV behavior of fermion field theories must be defined using two different UV cutoffs, a lattice spacing and a "string scale" that controls how smoothly the fields vary in space. In this talk I will briefly review this past work, and then I will describe how the analogous procedure is carried out for bosonic quantum theories. This case -- still a work in progress -- is considerably subtler, and I will show that even the ordinary free scalar field theories require a hierarchy of at least five scales to endow them with a proper UV completion.

April 7, 2020

The UV structure of scalar field theories, part 2

Abstract: I have recently proposed a rigorous procedure for establishing the correspondence between lattice and continuum theories of fermions. This procedure led to a definition of current algebras, infrared dualities, and operator product expansions in a finite, completely well defined setting. The crucial insight was that the UV behavior of fermion field theories must be defined using two different UV cutoffs, a lattice spacing and a "string scale" that controls how smoothly the fields vary in space. In this talk I will briefly review this past work, and then I will describe how the analogous procedure is carried out for bosonic quantum theories. This case -- still a work in progress -- is considerably subtler, and I will show that even the ordinary free scalar field theories require a hierarchy of at least five scales to endow them with a proper UV completion.

April 14, 2020

April 21, 2020

Quantum corrections to entanglement entropy after local quenches in AdS_3/CFT_2

Abstract: We compute the leading 1/N corrections to entanglement entropy after a local quench created by a light primary operator of dimension Δ, both in the language of large-c CFTs and in the gravitational dual. The bulk picture of the quench corresponds to a one-particle excited state of a scalar field theory coupled to gravity. The state is fully time-dependent: the particle is initially created at (or close to) the boundary of AdS and then falls into the deep IR, backreacting the geometry as its wavefunction evolves in time. According to the FLM prescription, the entanglement entropy at O(1) receives two contributions: one due to the change in the area in the backreacted geometry, and another one due to bulk entanglement entropy. We compute both contributions and find an exact match with the CFT calculation, providing a non-trivial check of the FLM formula in a dynamic setting. We comment on the implications of our results in the context of black holes and the information paradox.

April 28, 2020

Dark Sectors, Naturalness, and the Higgs

Abstract: Modern developments in quantum gravity, especially string theory, suggest that the Standard Model (SM) degrees of freedom are not unique; that a typical low energy effective theory should include a large assortment of hidden sector degrees of freedom. It is therefore puzzling that cosmological constraints from BBN and CMB reveal that the early universe was almost entirely dominated by the SM, when the inflaton could have decayed into many sectors. Furthermore, the SM taken seriously to high scales, possesses an instability that would be catastrophic during inflation, as I will quantify in detail, and yet no new physics has been seen to correct this. In this talk, I put forth an explanation for all of these puzzles: the hidden sectors are in fact entirely natural with O(1) input parameters; this means all unprotected masses are pushed up to high scales and project out of the spectrum, while only massless (or protected) degrees of freedom remain, and so the inflaton can only reheat these sectors through higher dimension operators. On the other hand, the SM possesses a special feature: it includes a light Higgs, presumably for life to exist, and hence it allows a renormalizable coupling to the inflaton, which allows rapid decay into the SM. I then show that this naturally (i) removes the instability in the Higgs potential both during and after inflation, (ii) explains why the SM is dominant in the early universe, (iii) allows dark matter to form in hidden sector/s through subsequent strong dynamics, which I describe in detail, (iv) allows for high reheating and baryogenesis, and (v) accounts for why there so far has been no direct detection of dark matter or new physics beyond the SM.

### Fall 2019 Seminars

September 6, 2019

Quantum Gravity in a Finite Box

September 10, 2019

All-Loop Singularities of Scattering Amplitudes in Massless Planar Theories

September 17, 2019

Gauge Theories With Nonstandard Gauge Constraints

Abstract: Pure Yang-Mills theory obeys a local constraint given by the Gauss law div E = 0. Alternative gauge constraints are known to exist: the most notable example is provided by Chern-Simons theory. However, detailed explorations of theories with nonstandard gauge constraints in the UV (e.g. on a lattice) have not been performed until very recently, when their importance was affirmed on several different fronts. In this talk I will discuss two new classes of nonstandard gauge theories. The first class involves theories in arbitrary dimension that can be shown to dualize to theories of ordinary fermions (these are higher-dimensional exact bosonization dualities). The second class involves theories of fractons, pioneered by condensed matter and quantum information communities, which are some of the most interesting examples of gapped theories not described by any kind of conventional topological order.

September 24, 2019

D-instantons and the non-perturbative completion of c=1 string theory

Abstract: I will discuss the effect of ZZ instantons in c=1 string theory, which leads to a new proposal for the non-perturbative completion of the duality between c=1 string and the matrix quantum mechanics.

October 1, 2019

Anomalies and Bounds on Charged Operators

Abstract: We study the implications of ’t Hooft anomaly (i.e. obstruction to gauging) on conformal field theory, focusing on the case when the global symmetry is Z2. Using the modular bootstrap, universal bounds on (1+1)-dimensional bosonic conformal field theories with an internal Z2 global symmetry are derived. The bootstrap bounds depend dramatically on the ’t Hooft anomaly. In particular, there is a universal upper bound on the lightest Z2 odd operator if the symmetry is anomalous, but there is no bound if the symmetry is non-anomalous. We comment on the implication to the Weak Gravity Conjecture in AdS3.

October 8, 2019

Hamiltonian fluid dynamics and the quasilinear approximation

October 15, 2019

Different perspectives on AdS3/CFT2 holography

Abstract: I will discuss AdS3/CFT2 holography from the two perspectives of integrability and worldsheet CFT and comment on their relations and recent results. In particular, I will show how to match null-vector constraints on correlation functions and explain how some structure constants can be explicitly derived and matched on the two sides of the duality. This amounts to one of the very first holographic matches of non-protected AdS3/CFT2 correlators.

October 22, 2019

Bit threads and holographic entropy inequalities

Abstract: I will discuss the ongoing effort to understand holographic entropy inequalities in the language of bit threads. I will start with the proof of the MMI inequality, focusing on the proof method. I will then explain why this method does not immediately extend to higher inequalities, and I will describe the current state of our efforts to develop new methods to prove those inequalities.

November 5, 2019

A canonical purification for the entanglement wedge cross-section

Abstract: I will discuss a new proposal for the CFT dual of the entanglement wedge cross-section. Our results will be compared to the original entanglement of purification conjecture.

November 12, 2019

The Schottky Anomaly of de Sitter Black Holes

Abstract: Black holes with Λ > 0 (SdS) have fascinating properties that are distinct from the asymptotically flat or AdS cases, starting with the fact that there are two horizons in the spacetime, one black hole and one cosmological. The two horizons have different temperatures and the total gravitational entropy is the sum of the horizon areas. As a result, both the mass M and entropy S are bounded between minimum and maximum values. Intriguingly, there is an extremum in the specific heat δM/δT_{b} as well as in the curve δS/δT_{b}, which resemble the Schottky anomaly of a two level system in statistical mechanics. In this talk we investigate classical and quantum mechanical features of SdS thermodynamics that make it resemble the physics of a paramagnet. We start by showing that the Schottky behavior is to be expected for classical fluctuations based on the first laws for SdS black holes. Second, we present calculations of black hole and cosmological particle production in SdS and find that the quantum fluctuations share the behavior of classical ones.

November 19, 2019

Constraining higher order gravities with subregion duality

Abstract: In higher derivative theories, gravity can travel slower or faster than light. With this feature in mind, I will revisit the construction of the causal and entanglement wedges in this type of theories, and argue that they must be constructed using the fastest mode instead of null rays. I will explain how the property of causal wedge inclusion, i.e., the fact that the causal wedge must be contained in the entanglement wedge, can be used to obtain strong constraints on the higher derivative gravity couplings. The results are similar to the bounds previously obtained by Camanho et. al. based on high energy graviton scattering. I will present a systematic analysis in Gauss-Bonnet gravity to illustrate our findings.

December 3, 2019

Explanation for why the early universe was dominated by the standard model and stable

Abstract: Modern developments in quantum gravity, especially string theory, suggest that the Standard Model (SM) degrees of freedom are not unique; that a typical low energy effective theory should include a large assortment of hidden sector degrees of freedom. It is therefore puzzling that cosmological constraints from BBN and CMB reveal that the early universe was almost entirely dominated by the SM, when the inflaton could have decayed into many sectors. Furthermore, the SM taken seriously to high scales, possesses an instability that would be catastrophic during inflation, as I will quantify in detail, and yet no new physics has been seen to correct this. In this talk, I put forth an explanation for all of these puzzles: the hidden sectors are in fact entirely natural with O(1) input parameters; this means all unprotected masses are pushed up to high scales and project out of the spectrum, while only massless (or protected) degrees of freedom remain, and so the inflaton can only reheat these sectors through higher dimension operators. On the other hand, the SM possesses a special feature: it includes a light Higgs, presumably for life to exist, and hence it allows a renormalizable coupling to the inflaton, which allows rapid decay into the SM. I then show that this naturally (i) removes the instability in the Higgs potential both during and after inflation, (ii) explains why the SM is dominant in the early universe, (iii) allows dark matter to form in hidden sector/s through subsequent strong dynamics, which I describe in detail, (iv) allows for high reheating and baryogenesis, and (v) accounts for why there so far has been no direct detection of dark matter or new physics beyond the SM.

December 10, 2019

Void formation in operator growth, entanglement, and unitarity

Abstract: The structure of the Heisenberg evolution of operators plays a key role in explaining diverse processes in quantum many-body systems. We discuss a new universal feature of operator evolution: an operator can develop a void during its evolution, where its nontrivial parts become separated by a region of identity operators. Such processes are present in both integrable and chaotic systems, and are required by unitarity. We show that void formation has important implications for unitarity of entanglement growth and generation of mutual information and multipartite entanglement. As an application, we argue that operators which make up the density operator of a black hole can “jump” outside the black hole after the Page time, providing the underlying physical mechanism for a recent semi-classical prescription for the resolution of a black hole information loss puzzle. We study explicitly the probability distributions of void formation in a number of unitary circuit models, and conjecture that in a quantum chaotic system the distribution is given by the one we find in random unitary circuits, which we refer to as the random void distribution. We also show that the random void distribution leads to the same pattern of entanglement growth for multiple intervals as in (1 + 1)-dimensional holographic CFTs after a global quench, which implies that it leads to maximal entanglement growth, and suggests that it underlies the time-evolution of holographic systems.