## Program

Click on the name/title for the abstract, slides, and video

## Plenary Talks

[show/hide all abstracts]

Andreas Blommaert | (SISSA & INFN) | Lorentzian wormholes |
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I will discuss semiclassical Lorentzian wormhole geometries in
asymptotically AdS in arbitrary dimensions. I claim that counting such
Lorentzian wormholes is gauge-equivalent to path integrating over
(mostly) Euclidean smooth spacetimes. I will provide evidence for this
by reproducing (semiclassical features of) the full genus expansion of
the spectral form factor and of the late-time two point function,
matching precise predictions which follow from the chaotic nature of
quantum black holes. I will furthermore discuss an all-genus
calculation of the Stanford-Yang firewall probability and show that
the full answer plateaus at 1/2 for late times, and I will present the
Lorentzian wormhole geometries responsible for this behavior.
[slides] [video] | ||

Pierre Heidmann | (Johns Hopkins) | Geometric Resolution of Schwarzschild Horizon |

I will present a new technique to construct non-extremal solitons in gravity with compact dimensions. They correspond to smooth horizonless geometries induced by topology in spacetime and supported by electromagnetic flux, which characterize coherent states in quantum gravity. I will present various types of solutions contained in the ansatz such as topological stars and bubble bag ends. Then I will focus on neutral topological solitons that are indistinguishable from Schwarzschild black holes with a scalar field, called Schwarzschild scalarwall, but replace the horizon at r=2M with a smooth bubbling geometry. This is the first example of such a geometric transition in string theory for astrophysically relevant black holes. I will conclude by discussing some of their gravitational signatures. [slides] [video] | ||

Daniel Kabat | (CUNY) | A prescription for entanglement in field theory and gravity |

Entanglement entropy is often obtained using a replica trick. In QFT this is problematic since subregion density matrices are not well-defined. Equivalently the Hilbert space of a QFT is not a tensor product over subregions. We present a Euclidean prescription for defining entanglement entropy which does not rely on tensor factorization. We apply it to planar and spherical entangling surfaces in a variety of simple (conformal) field theories and show that it resolves several puzzles. In particular for gauge theories we argue that it computes the "extractable entanglement entropy" associated with Bell pairs while it avoids counting the classical correlations required by Gauss' law. We show that the prescription resolves a related paradox for non-minimal scalars and we comment on the dual prescription in the bulk. [slides] [video] | ||

David Kutasov | (Chicago) | On black holes in string theory |

I will describe some new features of black holes, that appear when they are embedded in string theory. [slides] [video] | ||

Hong Liu | (MIT) | Emergence of space and time in holography |

In holographic duality, a higher dimensional quantum gravity system is
equivalent to 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. [slides] [video] | ||

Emil Martinec | (Chicago) | String spectrum and dynamics in AdS_3 backgrounds |

Worldsheet methods can be used to describe string theory in particular AdS3 conical defect geometries, providing valuable information about the AdS/CFT duality map. We will describe this construction and its connection to the microstate geometries program. If time permits, we will also describe recent work on stringy decay channels for black holes, and their implications for black hole structure and the information puzzle. [slides] [video] | ||

Rodolfo Russo | (QMUL) | Black hole signatures in holographic correlators |

In holographic CFTs it is interesting to study operators whose
dimension scales as the central charge when the latter is taken to be
large. As an example of such operators, I consider multi-particle
states formed by a large number of BPS single-particle constituents.
In the context of the AdS3/CFT2 duality, there is a class of such
heavy operators whose gravitational dual is known and is described by
a family of regular solutions that is smoothly connected to the BTZ
black hole. The quadratic fluctuations around these geometries capture
the heavy-light (HHLL) four point correlators. I will show how to
derive explicit expressions for such holographic correlators (by using
recent progress on the Heun connection problem) and comment on
similarities and differences between these correlators and similar
quantities calculated in the case of the BTZ black hole. [slides] [video] | ||

Kotaro Tamaoka | (Nihon U) | Some candidates of atypical black hole microstates |

Recent progress in the information loss problem suggests that
atypical states play an important role after the Page time. In this talk,
we discuss some classes of atypical states (primary states and METTS) and
their interpretation in light of AdS/CFT correspondence. These states exist
in approximate or strictly sufficient numbers in describing thermal
ensembles dual to the black hole. In particular, we will discuss the
properties of the latter (METTS) in CFT in detail. [slides] [video] | ||

Nicholas Warner | (CEA Saclay & USC) | Black-Hole Microstructure in String Theory |

[slides (keynore | pdf)] [video] |

## Short Talks

[show/hide all abstracts] [video of all short talks]

Takanori Anegawa | (Osaka U) | Shockwave and Complexity in dS_3 |
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I would like to talk about complexity calculations in the context of recent developments in de sitter spacetime. In particular, I will review the bulk geometry when a shockwave is inserted and consider the qualitative behavior of complexity in that case. [slides] | ||

Iosif Bena | (CEA Saclay) | The amazing super-maze |

The entropy of the three-charge NS5-F1-P black hole in Type IIA string theory comes from the breaking of N1 F1 strings into N1N5 little strings, which become independent momentum carriers. In M theory, the little strings correspond to strips of M2 brane that connect pairs of parallel M5 branes separated along the M-theory direction. We show that if one takes into account the backreaction of the M-theory little strings on the M5 branes one obtains a maze-like structure, to which one can add momentum waves. We also show that adding momentum waves to the little strings gives rise to a momentum-carrying brane configuration -- a super-maze -- which locally preserves 16 supercharges. We therefore expect the backreaction of the super-maze to give rise to a new class of horizonless black-hole microstate solutions, which preserve the rotational symmetry of the black-hole horizon and carry \sqrt(5/6) of its entropy. [arXiv:2211.14326] [slides (keynote | pdf)] | ||

Dongsheng Ge | (Osaka U) | Entanglement and geometry from subalgebras of the Virasoro algebra |

In this work we study families of generalised coherent states constructed from SL(2,R)
subalgebras of the Virasoro algebra in two-dimensional conformal field theories. We
derive the energy density and entanglement entropy and discuss their equivalence with
analogous quantities computed in locally excited states. Moreover, we analyze their dual,
holographic geometries and reproduce entanglement entropies from the Ryu-Takayanagi
prescription. Finally, we outline possible applications of this universal class of states to
operator growth and inhomogeneous quenches.
[arXiv:2211.03630] [slides] | ||

Masanori Hanada | (Surrey U) | Holographic geometry from matrix degrees of freedom |

In 1999, Polchinski suggested a puzzling feature of large-N gauge theories that (seems to) forbid a naive way of encoding holographic emergent geometry into matrix degrees of freedom. We resolve this puzzle and explain how geometry is encoded into matrix degrees of freedom. As an application, we discuss the small Schwarzschild black hole in AdS5*S5.
[arXiv:2102.08982, 2204.06472] [slides] | ||

Jonathan Harper | (YITP Kyoto) | Timelike entanglement entropy |

We define a new complex-valued measure of information called the timelike entanglement entropy (EE) which in the boundary theory can be viewed as a Wick rotation that changes a spacelike boundary subregion to a timelike one. An explicit definition of the timelike EE in 2d field theories is provided followed by numerical computations which agree with the analytic continuation of the replica method for CFTs. We argue that timelike EE should be correctly interpreted as another measure previously considered, the pseudo entropy, which is the von Neumann entropy of a reduced transition matrix. Our results strongly imply that the imaginary part of the pseudo entropy describes an emergent time which generalizes the notion of an emergent space from quantum entanglement. For holographic systems we define the timelike EE as the total complex valued area of a particular stationary combination of both space and timelike extremal surfaces which are homologous to the boundary region. For the examples considered we find explicit matching of our optimization procedure and the careful implementation of the Wick rotation in the boundary CFT. We also make progress on higher dimensional generalizations and relations to holographic pseudo entropy in de Sitter space.
[arXiv:2302.11695] [slides] | ||

Masamichi Miyaji | (Nagoya U) | Random State and Hawking Radiation |

We study the ensemble average of entropy fluctuation of Hawking radiation in the west coast model of black hole evaporation. We explicitly evaluate the fluctuation of entanglement entropy of the Hawking radiation and find that it is given by $e^{-S}$ with order one coefficient, ensuring the error of the gravitational replica trick is exponentially small. We also find similar suppression in the random pure state, which is significantly small compared to the previously known upper bound from Levy's lemma. This result indicates that the fluctuating part of the subsystem entropy of the typical state is given by the inverse of complement Hilbert space dimension, thus predicting an explicit form of the subsystem ETH for entropy. We also explain how the random state fails to approximate the Hawking radiation. [slides] | ||

Sunil Kumar Sake | (Osaka U) | Sparse RMT and Sparse SYK |

SYK model can be thought of as a sparse random matrix, with only a fraction
of the full random matrix elements taken to be non-zero. To explore the
universality of the properties of the SYK model, we study numerically the
properties of the sparse random matrices of the standard ensemble, namely
GUE/GOE. We study the behaviour of the system as the randomness is changed,
by varying the type and number of coupling constants and the variance of
their probability distributions. We analyse various features including the
density of states and the Spectral form factor, as well as features tied to
chaos like level statistics, Out-of-time ordered correlators and operator
growth. We also analyze a particular sparse version of SYK model obtained
by setting some of the couplings of the SYK model to vanish. [slides] | ||

Ryo Suzuki | (Southeast U, China) | Tensionless limit of AdS3/CFT2 |

Despite impressive advances in the AdS3/CFT2 correspondence, the setup involving Ramond-Ramond backgrounds, which is related to the D1-D5 system of branes, remained relatively poorly understood. We use the Mirror TBA equations to study the spectrum of pure Ramond-Ramond AdS3× S3× T4 strings. We find that the leading-order contribution to the anomalous dimensions at small tension contain massless worldsheet excitations, i.e. to the T4 bosons and their superpartners. [arXiv:2303.02120] [slides] | ||

Masaki Tezuka | (Kyoto U) | Hayden-Preskill Recovery in Sachdev-Ye-Kitaev type models and spin chains |

We investigate the Hayden-Preskill information recovery for various time-independent Hamiltonians, including Sachdev-Ye-Kitaev (SYK) models and chaotic spin chains. Despite the fact that the Hayden-Preskill recovery highly relies on the assumption that is never satisfied in Hamiltonian systems, we show that information recovery is possible in certain, but not all, chaotic models: original and simplified SYK models achieve the Hayden-Preskill recovery at any temperature, but conventional chaotic models, such as Ising and Heisenberg spin chains with random magnetic fields, do not. We also show that a variant of the SYK model, known as SYK_{4+2}, shows two transitions in terms of the Hayden-Preskill recovery, which are induced by the drastic changes of information-theoretic properties of the system. This highlights the information recovery as an operational probe for the manifestation of novel complex quantum dynamics.
[arXiv:2303.02010] [slides] | ||

Nicolò Zenoni | (Osaka U) | Evolution of holographic complexity in (hybrid) dS |

Holographic complexity can act as a probe of spacetime evolution. In asymptotically Anti-de Sitter (AdS) spacetime, at late times complexity grows at a constant rate, as the Einstein-Rosen bridge beyond a black hole event horizon. In de Sitter (dS) spacetime, instead, complexity manifests a hyperfast growth, due to the inflationary expansion of spacetime beyond the cosmological horizon. In this seminar, we argue that for two-dimensional dS in Jackiw-Teitelboim gravity, the dilaton plays a crucial role for holographic complexity to behave as in higher dimensional dS. We then present the evolution of complexity in a class of three-dimensional hybrid geometries representing a dynamical dS bubble into AdS spacetime. We argue that according to the dynamics of the bubble, complexity behaves qualitatively either as in the AdS black hole or as in the dS case.
[arXiv: 2302.03584 [hep-th]] [slides] |

### Short talks program

13:00 | Dongsheng Ge |
---|---|

13:15 | Masanori Hanada |

13:30 | Jonathan Harper |

13:45 | Masamichi Miyaji |

14:00 | Sunil Kumar Sake |

(Break) | |

14:45 | Ryo Suzuki |

15:00 | Masaki Tezuka |

15:15 | Nicolò Zenoni |

15:30 | Takanori Anegawa |

15:45 | Iosif Bena |