December 2 (Tue.)
- Sang-Jin Sin
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- O-Kab Kwon
Title : Asymptotic Weyl Symmetry and Its Anomaly in a Curved Spacetime
Abstract : We explore an unusual symmetry in a field theory on a specific curved spacetime in (1+1) dimensions, which has an interesting interpretation as an approximate asymptotic Weyl symmetry. Unlike the conventional Weyl symmetry, the boundary term plays a crucial role, and as a result the well-known anomaly emerges, but in an extended form that includes the boundary contribution. After converting a two-dimensional field theory on curved spacetime to an inhomogeneous field theory, we obtain the energy-momentum tensor and show the existence of an Unruh-like effect and the friction in the bubble wall expansion at the zero temperature.
- Bum-Hoon Lee
Title : Early Dark Energy Models and Cosmological Tensions
Abstract : Recent observations, such as H0 and cosmic birefringence, challenge the Standard cosmological model of ΛCDM. We introduce the Ultralight Axion-like model of Early Dark Energy (EDE). With the potential of n=3, we work with the observations of the EB mode of the cosmic microwave background (CMB) radiation and local expansion rate measurements. Our results show that the shape of the CMB EB angular power spectrum is sensitive to the background cosmological parameters. Our result is the first to show that the axion-like EDE model with n=3 can provide a unified explanation for the observed cosmic birefringence and the Hubble tension. In addition to the above observation based on the Planck data and Supernova, we analyse the recent data from ACT and SPIDER in relation to the Planck. We get constraints on Cosmic Birefringence from SPIDER, Planck, and ACT observations in various combinations.
- SeungJun Jeon
Title : Inhomogeneous Field Theory toward Experimental Verification or Falsification
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December 3 (Wed.)
- Gautam Satishchandran
Title : Bulk Locality and the Asymptotic Behavior of Massless Fields in Quantum Field Theory and Quantum Gravity
Abstract : Often in the description of isolated systems, it is convenient to describe the local properties and behavior of the system in terms of scattering from past to future infinity (e.g., the merger of black holes or the scattering of particles at the LHC) . Classically, this is an extremely good approximation since the behavior of the system at large but finite times is essentially the same as the behavior at infinite times. Thus, there is a simple description of local scattering experiments or phenomena in terms of data at infinity. In this talk, I will show that this is not the case if the theory coupled to any massless fields. The key difference is that the asymptotic behavior of these quantum theories is dominated by soft radiation in the far past and future. This radiation is highly entangled with incoming/outgoing bodies and contains all of the localization and bulk information of the scattering. I will show that, in the absence of this radiation, an ordinary Fock electron with no incoming radiation does not localize at all in the bulk of the spacetime. I will explain why scattering calculations (which do not accompany incoming bodies with incoming soft radiation) can give reliable answers for certain observables and comment on the implications for other theories with massless fields such as QED with massless charged particles, (unconfined) Yang-Mills theories and quantum gravity. In total, these results illustrate a surprising connection between the IR behavior of quantum theories and locality in the bulk.
- Sangmin Choi
Title : Log soft graviton theorem from BMS superrotation symmetry
Abstract : In four spacetime dimensions, the soft graviton theorem exhibits a term that arises exclusively from one-loop diagrams and is logarithmic in the soft graviton energy. In this talk, we demonstrate that this term, known as the log soft graviton theorem, can be derived as the Ward identity of BMS superrotation symmetry, once long-range interactions are properly taken into account. We carry out a systematic analysis to all orders in the coupling and confirm that the log soft theorem is indeed one-loop exact.
- Chanyong Park
Title : Reconstructing the dual gravity from entanglement entropy
Abstract : In this work, we study how to reconstruct the dual gravity theory from the QFT data, especially entanglement entropy, by applying the rule-based method (holography).We first attempt to derive a 3-dimensional black hole geometry from the entanglement entropy of a 2-dimensional thermal system composed of two distinct types of matter. Using the reconstructed black hole geometry, we determine various thermodynamic properties of the thermal system. We further investigate how to reconstruct the dual gravity theory of a given entanglement entropy when a relevant operator deforms a UV CFT. After finding the dual deformed geometry, we reconstruct the dual gravity theory, which allows such a deformed geometry.
- Hyeong-Chan Kim
Title : Stationary axisymmetric systems in general relativity
Abstract : We present a general equation for stationary, axisymmetric spacetimes in general relativity by decomposing the metric functions into a tractable form, using a generalized version of the Newman-Janis algorithm as a means of separating variables. We then solve this equation after proposing a general ansatz for the metric component $\Sigma(r,\theta) \equiv g_{\theta\theta}$. In particular, we impose the axisymmetric condition that the off-diagonal stress-energy tensor component vanishes, $T_{\hat{1}\hat{2}} = 0$, which leads to a nonlinear, higher-order differential equation for four functions of the two variables $r$ and $\theta$. We reduce this equation to a set consisting of two ordinary equations and one Riccati-type differential equation. A wide variety of metrics can be obtained from this result.To illustrate the usefulness of the method, we explicitly construct two distinct classes of solutions: one is a generalization of the Kerr-NUT spacetimes incorporating a wormhole structure, and the other is a new rotating solution that can be interpreted as a black hole without static limit.
- Robert Saskowski
Title : Higher-derivative Kerr-Sen black holes
Abstract : Heterotic supergravity reduced on a torus exhibits a hidden O(d+p, d) symmetry that can be used to generate solutions. I will show how this can be used to generate four-derivative corrections to the Kerr-Sen solution in four dimensions by applying an O(2, 1) boost to the Kerr solution. In the process, we will find a new consistent four-derivative truncation of heterotic supergravity that is equivalent to the gauge field couplings of the Bergshoeff-de Roo action. While the Kerr-Sen solution has multipole moments matching those of the Kerr(-Newman) solution at the two-derivative level, the four-derivative corrections break this degeneracy, even for the most general four-derivative Einstein-Maxwell action, and open the possibility of future experimental tests.
- Sojeong Cheong
Title : Investigation of Einstein rings formed by dust shells with quantum hair
Abstract : In this talk, we study how the metric of a compact object receives quantum corrections that depend on its internal structure modeled by dust shells. Using an effective field theory approach, we derive a quantum-corrected metric for uniformly spaced shells with equal surface mass density and examine the dependence of quantum corrections on the deflection angle in gravitational lensing. In particular, we investigate quantum-corrected astrophysical observables such as the Einstein ring and image magnification. Compared to the classical scenario, the deflection angle and the corresponding Einstein angle differ by a term that depends explicitly on the number of dust shells, which plays the role of quantum hair. Specifically, the quantum correction to the observables diminishes as the number of dust shells increases, yet a finite deviation from the classical result remains even in the continuum limit of dust shells. These findings indicate that the internal structures of compact objects can be distinguished by quantum hair through their lensing observables.
- Theodoros Nakas
Title : Theoretical filters for shift-symmetric Horndeski gravities
Abstract : We investigate the structure of nontrivial maximally symmetric vacua and compact-object solutions in shift-symmetric scalar-tensor theories. Focusing on Horndeski gravity, we derive consistency conditions directly from the field equations to identify the subclasses that admit Minkowski and de Sitter vacua with a nontrivial scalar field. In doing so, we obtain a filtering mechanism that operates independently of observational data. In this context, we introduce the notion of stealth vacua, where the scalar field remains active without altering the vacuum. Building upon these frameworks, we construct exact solutions in beyond-Horndeski gravity by applying a linear disformal transformation to the regularized Einstein-Gauss-Bonnet black hole. This procedure yields solitonic spacetimes with scalar hair as well as black holes carrying primary scalar hair, demonstrating how disformal maps can qualitatively modify solution properties. Our results provide both a principled criterion for selecting viable Horndeski models and a framework for exploring rich solution spaces in beyond-Horndeski gravity.
- Hong Guo
Title : Spontaneous Scalarization and its Application to the Holographic QCD Theory
Abstract : I would like to briefly introduce the phenomenon of spontaneous scalarization of black holes, which provides a new mechanism to evade the no-hair theorem. Within a simplified Einstein–Maxwell–Dilaton (EMD) framework, we have investigated the scalarization of Reissner–Nordström black holes. In this model, two types of hairy black hole solutions can be identified: one induced by the scalar potential and another arising from nonminimal coupling. I will show that the improved EMD model, which indicates a first-order QCD phase transition, can be understood in terms of the transition between these two classes of hairy black hole states.
- Gong-Show
1. Stephen Angus
2. Gitae Kim
3. Masroor Pookkillath
4. Daniele Bielli
December 4 (Thur.)
- Robert B. Mann
Title : Doubly Holographic Quantum Black Holes
Abstract : Quantum black holes are exact solutions to a semi-classical gravitational theory that incorporates all orders of quantum field backreaction on spacetime. These black holes have a doubly holographic thermodynamic description relating bulk to brane and brane to boundary. If uncharged, they exhibit re-entrant phase transitions and critical exponents that differ from standard mean field theory. The presence of charge and rotation eliminates these phenomena, with the critical exponents the same as in mean-field theory. An energy scale (or mass gap) $M_{\rm gap}$ can be computed to identify regimes where quantum fluctuations of spacetime geometry are expected to become significant. Their gyromagnetic ratio is a function of the backreaction, and approaches a constant in the limit backreaction effects are large. This class of black holes respects quantum versions of the Penrose and reverse isoperimetric inequalities. Taken as a whole, such black holes provide an interesting new test bed for confronting gravitation with quantum physics.
- Akihiro Ishibashi
Title : Semiclassical Einstein equations and instability from holography
Abstract : We first discuss how to formulate the semiclassical problem in the context of holography. Then, applying the holographic semiclassical Einstein equations, we study the dynamical instability of Minkowski, de Sitter and AdS spacetimes arising from backreaction of the quantum stress-energy tensor. This talk is based on a series of papers 2412.18764 [hep-th] 2312.10311 [hep-th] 2301.12170 [hep-th], and ongoing work.
- Stepano Scopel
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- Dong-Han Yeom
Title : Information loss paradox from canonical quantum gravity
Abstract : In this presentation, I will study the information loss paradox of black holes from the point of view of canonical quantum gravity. If we believe that the Wheeler-DeWitt (WDW) equation carries all information, there must be no loss of information; however, there is no time in the WDW equation, and hence, it is not possible to define the unitary 'time' evolution. Therefore, we need to introduce time using the clock at infinity. However, this is insufficient, because we need not only a time but also a 3-hypersurface, where there might be an infinite number of ways to introduce it. For a given time variable, what is the consistent way to extend the 3-hypersurface that carries the notion of classicality? In this regard, we introduce the concept of coherent states; we will further argue that we can assign a coherent state along the 3-hypersurface only if the slice covers the region outside the horizon. This observation provides important intuitions to the information loss paradox.
- Yun Soo Myung
Title : Gauss-Bonnet scalarization for qOS-black holes
Abstract : We obtain quantum parameter-mass induced spontaneous scalarization of quantum Oppenheimer-Snyder (qOS)-black hole in the Einstein-Gauss-Bonnet-scalar (EGBS) theory with the unknown qOS action. It is turned out that two Davies points of heat capacity are identified with two critical onset mass and quantum parameter, showing a close connection between thermodynamics of bald black hole and spontaneous scalarization. Furthermore, we study onset scalarization of the extremal qOS-black hole in the EGBS theory. To obtain the tachyonic scalar cloud which is a seed to generate the single branch of scalarized extremal qOS-black holes, we consider the near-horizon geometry of the Bertotti-Bobinson (BR) spacetime. In this case, it is shown that the appearance of a large scalar cloud at the horizon is a new feature to represent onset scalarization of extremal qOS-black holes for tachyon with negative mass. However, it is not related to the Aretakis instability of a propagating scalar with standard mass around the BR spacetime, showing polynomial instability of the ingoing time. The Aretakis instability is connected to the scalar cloud with standard mass.
- Wonwoo Lee
Title : Geometric realization of the concept of 'charge without charge'
Abstract : We present and investigate charged wormhole solutions to the Einstein-Maxwell equations, which are supported by anisotropic matter fields. Our aim is to establish the physical plausibility of these wormholes being traversable. To this end, we examine the flare-out condition and evaluate tidal forces in order to confirm their traversability. Furthermore, we present charged wormhole solutions in the presence of a cosmological constant, thereby extending the 'charge without charge' concept naturally.
- Hocheol Lee
Title : Violation of the Bound on Chaos in Charged and Rotating AdS Black Holes
Abstract : We investigate the bound on chaos for a charged probe particle in charged, rotating, AdS black hole backgrounds, particularly in the Kerr-Sen-AdS and dilatonic Reissner-Nordstrom-AdS spacetimes. By analyzing the particle’s radial dynamics near the local extremum of the effective potential, we compute the frame dependence of Lyapunov exponent and examine the potential violations of the bound on chaos proposed by Maldacena, Shenker, and Stanford.
- Stephen Angus
Title : O(D,D) string cosmology from double field theory
Abstract : The low-energy limit of string theory contains additional gravitational degrees of freedom that are not present in general relativity. Together with the metric, these fields are naturally embedded in the O(D,D)-symmetric framework of double field theory (DFT). First I will explain how the O(D,D) symmetry uniquely prescribes the interactions between the extended gravitational sector and other matter, leading to novel features beyond conventional string cosmology. Then I will present some examples of analytic solutions, including candidate models for bouncing cosmologies. Finally, I will discuss ongoing work on the implications of a dilaton-photon coupling for luminosity distance measurements.
- Special Session 1,2 : “Prof. Robert B. Mann”
- Paul McGrath :
- Robert B. Mann :
December 5 (Fri.)
- JeongHyuck Park
Title : Next Einstein Equation: Doubled Spacetime
Abstract : Double Field Theory (DFT) has emerged as a comprehensive framework for gravity, presenting a testable and robust alternative to General Relativity (GR), rooted in the O(D,D)symmetry principle of string theory. These lecture notes aim to provide an accessible introduction to DFT, structured in a manner similar to traditional GR courses. Key topics include doubled-yet-gauged coordinates, Riemannian versus non-Riemannian parametrisations of fundamental fields, covariant derivatives, curvatures, and the O(D,D)-symmetric augmentation of the Einstein field equation, identified as the unified field equation for the closed string massless sector. By offering a novel perspective, DFT addresses unresolved questions in GR and enables the exploration of diverse physical phenomena, paving the way for significant future research. This talk is based on a recent review article: https://arxiv.org/abs/2505.10163.
- Hideki Maeda
Title : Energy Conditions: Fundamentals and Applications
Abstract : Energy conditions are a set of inequalities imposed on the energy-momentum tensor that ensure the corresponding matter field is physically reasonable. In general relativity, some of the standard energy conditions are imposed as fundamental assumptions in the proofs of various strong theorems. For instance, the dominant energy condition is assumed for the positive mass theorem, and the weak energy condition is necessary for the black-hole area theorem, which is considered the second law of black-hole thermodynamics. Additionally, the null convergence condition—a geometric result used in Penrose's singularity theorem—is equivalent to the null energy condition when general relativity is assumed. Although fundamental matter fields, such as the electromagnetic field or a scalar field, are generally expected to satisfy the energy conditions, this is not an obvious or guaranteed outcome. This talk will present some basic results on the standard energy conditions and examine their relevance to the study of black holes.
- Seung Hoon Oh
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- Jae-Young Kim
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- Youngbok Bae
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- Kuan-Nan Lin
Title : Birthmark of wormhole entanglement in the CMB sky
Abstract : How was the Universe born and why is there only one direction of time in our universe? One way to solve both puzzles at one stroke is to posit that our universe was pair-created with a twin, whose time arrow is opposite to ours. If so, then the twins must naturally be quantum entangled. In Euclidean quantum cosmology, this implies the existence of a Euclidean wormhole bridging the twin universes. In this talk, I will show how the notion of entanglement emerges from Euclidean wavefunction by imposing a simple harmonic oscillator (SHO) boundary condition, and how the entanglement enhances the cosmic microwave background (CMB) power spectrum on large scales. Lastly, I will discuss the possibility of inflation as an emergent consequence of quantum entanglement.
