Quantum Gravity Gatherings
9 events
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Seminar
Stability of nonsingular black holes
March 27 (Thu) at 15:00 - 16:30, 2025
Shinji Tsujikawa (Professor, Graduate School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University)
We show that nonsingular black holes (BHs) realized in nonlinear electrodynamics are always prone to Laplacian instability around the center because of a negative squared sound speed in the angular direction. This is the case for both electric and magnetic BHs, where the instability of one of the vector-field perturbations leads to enhancing a dynamical gravitational perturbation in the even-parity sector. Thus, the background regular metric is no longer maintained in a steady state. We also generalize our analysis to the case in which a scalar field is present besides the U(1) gauge field and find no explicit examples of linearly stable nonsingular BHs. Our results suggest that the construction of regular BHs without instabilities is generally challenging within the scheme of classical field theories.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Asymptotically flat black hole spacetimes with multiple injections
March 14 (Fri) at 15:30 - 17:00, 2025
Yuta Saito (Ph.D. Student, Graduate School of Science and Technology, Nihon University)
In quantum gravity, Hawking radiation presents several fundamental problems. One of the problems is the black hole (BH) information paradox, in which the entanglement entropy (EE), which quantifies quantum entanglement, exceeds its upper bound. In the absence of the paradox, EE follows the Page curve. Recent progress has been made in resolving this paradox using the island formula, a method for computing EE that successfully reproduces the expected Page curve. In this approach, a portion of the black hole interior is treated as part of the radiation region. Meanwhile, an alternative scenario has been proposed where multiple collapsing shells prevent the formation of a well-defined event horizon [1]. In this case, radiation is emitted throughout the collapse process, shifting dynamically the Schwarzschild radius inward, and a surface structure is formed just outside. This leads to a distinction between the conventional event horizon and the surface, introducing an intermediate region between the Schwarzschild radius and the surface. Interestingly, this model also suggests that part of the black hole interior effectively belongs to the radiation region, drawing a possible parallel to the island formula. In this talk, we explore spacetimes with multiple energy injections in asymptotically flat two-dimensional black hole backgrounds and analyze the entanglement entropy in such scenarios. Since considering backreaction in gravitational collapse in two dimensions is difficult, we instead construct a spacetime solution with multiple energy injections and analyze EE within this background. The main focus of this talk is to derive the spacetime and examine its properties. Additionally, we perform EE calculations in parallel with previous studies [2], which consider the case of α single injection, and confirm that the behavior of EE depends on the interval between energy injections.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
7th QGG Intensive Lectures: Emergence of space-time in matrix models
December 17 (Tue) - 19 (Thu), 2024
Asato Tsuchiya (Professor, Shizuoka University)
Emergence of space-time is a key concept in matrix models as a nonperturbative formulation of string theory. In this lecture, starting with a brief introduction to nonperturbative effects in string theory, I will review various aspects of emergence of space-time in matrix models. The topics I discuss include dynamical triangulation, double scaling limit, eigenvalue instanton, large-N reduction, T-duality for D-brane effective theories (orbifolding), noncommutative geometry and covariant derivative interpretation. Finally, I will introduce the type IIB matrix model. (This is the 7th Intensive Lectures by Quantum Gravity Gatherings in iTHEMS. ) Program December 17 10.15~10.30 Registration and Coffee 10.30~12.00 Lecture 1 12.00~13.30 Lunch 13.30~15.00 Lecture 2 15.00~16.00 Coffee break 16.00~17.00 Lecture 3 17.30~19.30 Banquet December 18 10.15~11.45 Lecture 4 11.45~13.30 Lunch 13.30~15.00 Lecture 5 15.00~16.00 Coffee break 16.00~17.00 Lecture 6 December 19 10.15~11.45 Lecture 7 11.45~13.30 Lunch 13.30~15.00 Lecture 8 15.00~16.00 Coffee break 16.00~17.00 Lecture 9
Venue: #435-437, 4F, Main Research Building
Event Official Language: English
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Seminar
Global Thermodynamics for Heat Conduction Systems
December 2 (Mon) at 14:00 - 15:30, 2024
Naoko Nakagawa (Professor, Ibaraki University)
Non-equilibrium phenomena are typically addressed through continuum descriptions based on local equilibrium and linear response theory, such as hydrodynamics. While effective, these approaches often overlook global characteristics. We propose Global Thermodynamics as a minimal-variable framework to describe weak non-equilibrium systems, focusing on two-phase coexistence under weak heat flux. By introducing a unique global temperature and extending entropy to non-equilibrium systems with a non-additive term, the framework predicts phenomena like metastable state stabilization—beyond the scope of traditional heat conduction equations. This talk will outline the framework, its key predictions, and validation efforts through numerical simulations and experiments. (This is a joint seminar with Informatin Theory Study Group.)
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Lectures on Black Holes, Holography and Quantum Gravity
July 29 (Mon) - August 1 (Thu), 2024
Yasunori Nomura (Director, Berkeley Center for Theoretical Physics, University of California, Berkeley, USA)
The information problem of black holes has evolved modern physics and led to the holographic principle, considered the fundamental principle of quantum gravity. Through a series of four lectures (blackboard style), including naive questions from the audience and lively discussions, I will introduce these fundamental ideas as well as the current state of the art and problems in cutting-edge research. Lecture 1: July 29 (Mon) 13:30~15:00 (Lecture 2: July 30 (Tue) 13:30~15:00 was canceled) Lecture 3: July 31 (Wed) 13:30~15:00 Lecture 4: Aug 1 (Thu) 13:30~15:00 (+ A possible lecture )
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Black hole graviton and quantum gravity
May 16 (Thu) at 15:00 - 16:30, 2024
Yusuke Kimura (Research Scientist, Analytical quantum complexity RIKEN Hakubi Research Team, RIKEN Center for Quantum Computing (RQC))
Drawing from a thought experiment that we conduct, we propose that a virtual graviton gives rise to a black hole geometry when its momentum surpasses a certain threshold value on the Planck scale. This hypothesis implies that the propagator of a virtual graviton, that possesses momentum surpassing this threshold, vanishes. Consequently, a Feynman diagram containing this type of graviton propagator does not add to the overall amplitude. This mechanism suggests the feasibility of formulating an ultraviolet-finite four-dimensional quantum gravitational theory. The elementary particles including the gravitons are treated as point particles in this formulation.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Coarse-graining black holes out of equilibrium with boundary observables on time slice
April 1 (Mon) at 16:00 - 17:30, 2024
Daichi Takeda (Ph.D. Student, Theoretical Particle Physics Group, Kyoto University)
In black hole thermodynamics, defining coarse-grained entropy for dynamical black holes has long been a challenge, and various proposals, such as generalized entropy, have been explored. Guided by the AdS/CFT, we introduce a new definition of coarse-grained entropy for a dynamical black hole in Lorentzian Einstein gravity. On each time slice, this entropy is defined as the horizon area of an auxiliary Euclidean black hole that shares the same mass, (angular) momenta, and asymptotic normalizable matter modes with the original Lorentzian solution. The entropy is shown to satisfy a generalized first law within Einstein theory and, through holography, the second law as well. This second law corresponds to the positivity of the relative entropy in the CFT. Furthermore, by applying this thermodynamics to several Vaidya models in AdS and flat spacetime, we discover a connection between the second law and the null energy condition.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Volume complexity of de Sitter bubbles
November 22 (Wed) at 16:00 - 17:30, 2023
Nicolò Zenoni (Project Researcher, Department of Physics, Osaka University)
Holographic complexity is conjectured to probe the evolution of spacetime. For black holes in anti-de Sitter (AdS) spacetime the growth rate of complexity approaches a constant value at late times, while in de Sitter (dS) spacetime it diverges at a finite critical time. In this talk, we consider geometries interpolating between AdS and dS. In particular, we discuss the evolution of volume complexity in a class of three-dimensional asymptotically anti-de Sitter geometries including dynamical bubbles of de Sitter in their interior. According to the dynamics of the bubble, complexity qualitatively behaves either as in the AdS black hole or as in the dS case. We conclude with some remarks about the possible dual interpretation.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Quasi-local holography in 3d quantum gravity
August 4 (Fri) at 14:00 - 15:30, 2023
Etera Livine (Research Director CNRS, Ecole Normale Supérieure de Lyon, France)
Since the idea appeared in black hole physics, the concept of holography has become a guiding principle for quantum gravity. It is the notion that the dynamics of the geometry of a region of space-time can be entirely encoded in a theory living on its boundary. Although such holographic dualities have been well-developed in an asymptotical context, it remains a challenge to realize it exactly at finite distances. I will draw a possible route in 3d quantum gravity, by showing a duality between the Ponzano-Regge path integral for 3d quantum gravity as a topological field theory and the 2d (inhomogeneous) Ising model. This leads to an intriguing geometrical interpretation of the Ising critical couplings and opens the door to a possibly rich interplay between 3d quantum gravity and 2d condensed matter built out of holographic dualities.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
9 events