# String/Gravity Theory Seminar Spring 2021

Organizers: Netta Engelhardt, Chris Akers, Patrick Jefferson, Washington Taylor**Held every** **Wednesday**, 2-3pm EST.

#### Seminars

February 24**Roberto Emparan**

Title: **Quantum BTZ black hole**

Abstract: The study of quantum effects on black holes including their gravitational backreaction is an important but notoriously hard problem. I will begin by reviewing how the framework of braneworld holography allows to solve it for strongly-coupled quantum conformal fields. Then I will describe a holographic construction of quantum rotating BTZ black holes (quBTZ) using an exact dual four-dimensional bulk solution. Besides yielding the quantum-corrected geometry and the renormalized stress tensor of quBTZ, we use it to show that the quantum black hole entropy, which includes the entanglement of the fields outside the horizon, rather non-trivially satisfies the first law of thermodynamics, while the Bekenstein-Hawking-Wald entropy does not.

March 3**Kenneth Intriligator**, UCSD

Title: **RG flows, anomalies, and 2-group global symmetries in 4d and 6d**

Abstract: 4d Abelian gauge theories, and 6d non-Abelian gauge theories, can have 2-form global symmetries. Mixed anomalies, involving the gauge fields and ordinary global chiral symmetries, can be reinterpreted as a quantum deformation (rather than violation) of the global symmetries into a 2-group symmetry. I will review this, and applications to constraining RG flows, in both 4d and 6d examples. In the 4d context, simple close cousins of QED exhibit 2-group global symmetries. In 6d, there are little string theory examples with 2-group global symmetries. 2-group global symmetries obstruct conformal symmetries, so they are IR free. For 6d interacting (super)conformal theories, absence of 2-group symmetry requires vanishing mixed anomalies. This understanding allows us to establish a previously conjectured algorithm for computing ’t Hooft anomalies in 6d SCFTs. We then apply this to prove that the a-type Weyl anomaly of all 6d SCFTs with a tensor branch satisfies a>0, as expected for the (conjectured) 6d a-theorem.

March 10**Simeon Hellerman**, IPMU Tokyo

Title: **Quantum Information Theory of the Gravitational Anomaly**

Abstract: I will show that the notion of quantum entanglement is not defined for gravitationally anomalous two-dimensional theories, because they do not admit a local tensor factorization of the Hilbert space into local Hilbert spaces. Qualitatively, the modular flow cannot act consistently and unitarily in a finite region, if there are different numbers of states with a given energy traveling in the two opposite directions. I will make this precise by decomposing it into two observations: First, a two-dimensional conformal field theory admits a consistent quantization on a space with boundary only if it is not anomalous. Second, a local tensor factorization always leads to a definition of consistent, unitary, energy-preserving boundary condition. As a corollary I will establish a generalization of the Nielsen–Ninomiya theorem to all two-dimensional unitary local quantum field theories: No continuum quantum field theory in two dimensions can admit a lattice regulator unless its gravitational anomaly vanishes. I will advocate that these points be used to reinterpret the gravitational anomaly quantum-information-theoretically, as a fundamental obstruction to the localization of quantum information.

March 17**Dieter Luest**, LMU Munich

Title: **The swampland at a large number of space-time dimensions**

Abstract: In this talk we discuss some aspects of swampland constraints – especially the swampland distance conjecture – in a large number of space-time dimensions D.

We analyze Kaluza-Klein (KK) states at large D and find that some KK spectra possess an interesting dependence on D. On the basis of these observations we propose a new large dimension conjecture. We apply it to KK states of compactifications to anti-de Sitter backgrounds where it predicts an upper bound on the dimension of space-time as a function of its characteristic radius.

March 24**Yasunori Lee**, IPMU

Title: **Some comments on 6d global gauge anomalies**

Abstract: Given a G gauge theory, there can be global gauge transformations under which the partition function is not invariant. In 6d, relevant cases are G = SU(2), SU(3), and G2, and the old computations utilizing homotopy groups affirmed that the anomalous phases can indeed arise in all three cases. On the other hand, from the modern point of view utilizing bordism groups, there should not be such global gauge anomalies in the first place. In this talk, I will describe how this apparent conflict is resolved by carefully examining the cancellation of perturbative gauge anomalies via 6d Green-Schwarz mechanism.

March 31 at 12:30pm* (note change in time for this week only)**Edgar Shaghoulian**

title TBA

April 7**Jorrit Kruthoff**, Stanford

Title: **Classical aspects of black hole interiors**

Abstract: We will discuss the geometry behind the horizon of various asymptotically AdS black holes when the boundary CFT is deformed by a scalar operator. The dynamics of classical GR in the region inside the black hole turns out to be rather intricate, with even fractal like behavior emerging in some situations. We will see that, generically, in the presence of a scalar deformation, Cauchy horizons are replaced by a spacelike singularity. This motivates a notion of ‘holographic’ cosmic censorship. This talk is based on 2004.01192, 2006.10056 and 2008.12786.

April 14**Emily Nardoni**, UCLA

Title: From SU(N) Seiberg-Witten Theory to Adjoint QCD

Abstract: Standard lore suggests that four-dimensional SU(N) gauge theory with 2 massless adjoint Weyl fermions (“adjoint QCD”) flows to a phase with confinement and chiral symmetry breaking. In this talk, we will test and present new evidence for this lore. Our strategy involves realizing adjoint QCD in the deep IR of an RG flow descending from SU(N) Seiberg-Witten theory, deformed by a soft supersymmetry-breaking mass for its adjoint scalars. A crucial role in the analysis is played by a dual Lagrangian that originates from the multi-monopole points of Seiberg-Witten theory, and which can be used to explore the phase diagram as a function of the supersymmetry-breaking mass. The semi-classical phases of this dual Lagrangian suggest that the softly broken SU(N) theory traverses a sequence of phases, separated by first-order transitions, that interpolate between the Coulomb phase of Seiberg-Witten theory and the confining, chiral symmetry breaking phase expected for adjoint QCD.

April 21 at 11:30 am**Suvrat Raju**, ICTS

Title TBA

April 28**Alejandra Castro**

Title TBA

May 5**Xiaoliang Qi**, Stanford

Title: Entanglement island, miracle operators and the firewall

Abstract: In this work, we obtain some general results on information retrieval from the black hole interior, based on the recent progress on quantum extremal surface formula and entanglement island. We study an AdS black hole coupled to a bath with generic dynamics, and ask whether it is possible to retrieve information about a small perturbation in the interior from the bath system. We prove that as long as the quantum extremal surface formula applies to the original system, any subsystem of the bath that is only classically correlated with the remainder of the system will never contain information about the interior, because it has a trivial entanglement island. This leads to an apparent contradiction since it is possible to retrieve the information from the Hawking radiation that is coupled with the black hole quantumly, which implies that we should be able to retrieve the same information from an ancilla that is coupled with the radiation classically and carries the particular measurement. By contradiction, this paradox tells us that the measurement operators that can retrieve information about the interior must have the special property that it makes the quantum extremal surface formula of the original system invalid.

We explicitly construct such operators using replica trick. Using these operators we obtain new understanding to the firewall paradox. On one hand, it is possible to carry a measurement to an interior qubit by acting on the radiation. On the other hand, in an entanglement checking setup, one can check a Hawking mode is entangled with earlier radiation, and then jump in to find that the same mode is also entangled with its interior partner. This result that appears to violate monogamy of entanglement is made possible because of “additional copies of space-time geometry” carried by the entanglement checking operator itself.

May 12**Dan Freed**, UT Austin

Title TBA

May 19**Sergio Hernandez**–

**Cuenca**, UCSB

Title TBA