iTHEMS Theoretical Physics Seminar

Loop expansion in polymer field theory: application to phase separation

July 30 (Thu) 16:00 - 17:00, 2026

Kiyoharu Kawana (Research Fellow, Korea Institute for Advanced Study (KIAS), Republic of Korea)

Liquid-liquid phase separation underlies phenomena ranging from protein condensate formation to the phase coexistence of synthetic polymers. In this talk, we develop a field theoretic loop expansion in homopolymer systems by identifying the inverse polymer density ρ^{-1} as the Planck constant ℏ in quantum field theory. The 1-loop approximation is known as the random phase approximation (RPA) and has been extensively applied to many (hetero)polymer systems. We calculate the leading-order (2-loop) and next-to-leading-order (3-loop) corrections to the RPA free energy, denoted as RPA+ and RPA++, respectively. Testing the binodal predicted by the RPA+ against molecular dynamics simulations of bead-spring chains with Gaussian pair interactions, we find that the RPA+ qualitatively improves the dilute-phase coexistence density over the RPA, while the critical point error remains comparable to that of the RPA. Our results establish the loop expansion as a systematic route for refining the RPA-based binodal predictions for polymer phase separation. This talk is based on arXiv: 2605.01261.

Venue: via Zoom

Event Official Language: English

Unraveling the very early universe with black holes, boson stars, and cannibal stars

July 24 (Fri) 14:00 - 16:00, 2026

Takeshi Kobayashi (Associate Professor, International School for Advanced Studies (SISSA), Italy)

According to the standard picture of cosmology, the rich structure of our universe began to form roughly 50,000 years after the big bang. In this talk I will explore the possibility that cosmic structures could also have formed in the extremely early universe, within a fraction of a second after inflation. I will show how this early structure formation can give rise to compact objects, including exotic stars and primordial black holes. These relics provide powerful probes of the first instants of cosmic history, especially the reheating epoch, and may even act as seeds for cosmological phase transitions. Note: This seminar is jointly organized by the iTHEMS-phys Study Group and the iTHEMS-ABBL Joint Astro Study Group.

Venue: Seminar Room #359 / via Zoom

Event Official Language: English

The Virasoro TQFT approach to 3D gravity and the sum over topologies (QuIG Seminar)

June 19 (Fri) 13:30 - 16:00, 2026

Mengyang Zhang (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

In the first part of this talk, I will review the construction of Virasoro TQFT from the Chern–Simons formulation of pure AdS_3 gravity and its application to the statistics of two-dimensional holographic CFT data. I will then discuss its extension to three-dimensional de Sitter gravity and its relation to the double-scaled SYK model. In the second part, I will address the issue of topological invariance in Virasoro TQFT. Despite being “topological,” its partition function is not well-defined on arbitrary three-manifolds, distinguishing it from conventional Reshetikhin–Turaev–Witten TQFTs. I will explain how far the standard proofs of topological invariance can be generalized to this framework. Finally, I will comment on the role of the sum over topologies in the 3D gravitational path integral.

Venue: via Zoom / #359, Main Research Building

Event Official Language: English

Which Cosmological EFTs Survive the UV? A first step from quantum consistency to late-time cosmology

June 15 (Mon) 10:00 - 11:30, 2026

Carlos Pastor-Marcos (Ph.D. Student, ITP, Heidelberg University, Germany)

EFTs for cosmology are one of our best tools to describe possible departures from GR in the Universe we observe. However, not every low-energy theory can arise from a consistent quantum theory at high energies. In this talk, I will discuss how this question can be addressed using asymptotic safety (AS), and how UV consistency can constrain the space of viable modified-gravity EFTs. Instead of treating all EFT parameters as equally possible, we can ask which regions of theory space are connected to a well-defined fixed point in the UV. This provides the first ingredients of a UV-to-IR strategy, restricting the allowed low-energy theories and indicating how quantum-gravity information may reach cosmology. I will first give a pedagogical introduction to AS and the functional RG, focusing on the physical picture rather than technical details. I will then apply the framework to generalized Proca theories, a class of vector–tensor modified-gravity EFTs with relevant cosmological applications, to illustrate how this analysis is performed in practice and how it can constrain viable IR theories. I will close by discussing how UV completion can become a practical guide for cosmology, translating quantum-consistency conditions into phenomenological signatures, from late-time modified gravity to early-universe observables, strong-gravity tests and GW probes.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Noncritical Conformal Gravity and 4D Liouville Theory

June 12 (Fri) 15:00 - 16:30, 2026

Nobuyoshi Ohta (Visiting Professor, Nambu Yoichiro Institute of Theoretical and Experimental Physics (NITEP), Osaka Metropolitan University)

We study the quantum aspects of the conformal gravity in four dimensions, specifically addressing a known discrepancy in beta functions between general quadratic curvature theories and conformal gravity, which corresponds to two scalar degrees of freedom. We demonstrate that this mismatch is resolved by carefully introducing gauge-fixing and ghost terms via the BRST symmetry, which effectively adds the two scalar modes. Drawing lessons from two-dimensional quantum gravity and Liouville theory, we proceed to integrate the four-dimensional trace anomaly to derive a consistent Liouville action, which is given by a free-field action for the conformal mode with a consistent conformal anomaly. We give the condition that the BRST transformation is anomaly free. Finally I would like to talk about some application of this theory.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Quantum Improved Black Holes in Asymptotically Safe Gravity

June 11 (Thu) 15:00 - 16:30, 2026

Chiang-Mei Chen (Professor, Department of Physics, National Central University, Taiwan)

In this talk, I will explore quantum-improved black hole solutions within the framework of asymptotic safety. In this approach, the Newton coupling becomes scale-dependent, necessitating a meaningful identification between the energy scale and a corresponding physical (length) scale to derive observable consequences for black hole spacetimes. I will argue that the requirement of consistency with the first law of black hole thermodynamics provides a physically motivated criterion for this scale-setting, particularly near the event horizon. Applying this principle, we propose a specific identification scheme that leads to a regularized geometry capable of resolving the ring singularity of Kerr black holes.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

reflection positivity in de Sitter space

June 5 (Fri) 10:30 - 11:30, 2026

Yuki Suzuki (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Bootstrapping Cosmological Correlators

May 28 (Thu) 16:00 - 18:00, 2026

Mang Hei Gordon Lee (Post-Doctoral fellow, Leung Center for Cosmology and Particle Astrophysics, National Taiwan University, Taiwan)

Currently there are hundreds of models describing inflation, a period of accelerated expansion in our universe. Each model lead to different imprints in cosmological observables, and for the purpose of testing the idea of inflation itself, it is essential to understand which predictions are model independent. This lead to the idea of cosmological bootstrap, a set of constraints from physical principles and symmetries alone. In this talk I will give an overview on the cosmological bootstrap program. I will first explain how locality, unitarity and symmetry can constrain the kinematics of cosmological correlators. I will then talk about some recent progress on constructing positivity bounds on cosmology, which places constraints on the interactions of fields in inflation.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Entanglement entropy and conformal bounds for five-dimensional CFTs

May 21 (Thu) 15:00 - 16:00, 2026

Javier Moreno (Project Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)

Abstract: The entanglement entropy of spatial regions in odd-dimensional conformal field theories contains a universal constant contribution that encodes important information about the theory. This quantity can be defined in a robust way using mutual information between slightly deformed versions of a given region. In three-dimensional conformal field theories, it is known that this quantity is always positive and bounded from below by the value corresponding to a spherical region. There is also strong evidence that, for any region, its normalized value is maximized by the free scalar theory. In this work, we show that the situation changes significantly in five dimensions. Although the spherical region remains a local minimum under small shape deformations, more general regions can lead to values that become arbitrarily large in magnitude, with either sign. This implies that, in five-dimensional conformal field theories, the quantity is not bounded from above or below. We also demonstrate that the analogous maximization property observed in three dimensions does not hold in five dimensions when considering general regions. Despite this, we find that existing evidence is consistent with a weaker statement: for small deformations of a spherical region, the normalized quantity remains bounded above by the free scalar result across all five-dimensional conformal field theories. This leads to a new conjectured universal bound relating two key physical quantities—the coefficient governing stress-tensor correlations and the sphere free energy—which appears to hold for all currently known examples.

Venue: via Zoom / Seminar Room #359, Seminar Room #359

Event Official Language: English

Positivity constraints for the gravitational path integral

May 21 (Thu) 10:00 - 11:50, 2026

Gabriele Di Ubaldo (Postdoctoral Researcher, RIKEN-Berkeley Center, Division of Global Collaborations and Research Talent Development, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

For a quantum theory of gravity to have a well-defined Hilbert space, the inner product between different states of open and closed universes must be positive semi-definite. Positivity however is not manifest in the low-energy effective theory and in fact imposes nontrivial constraints on the theory. Working in the Gravitational Path Integral (GPI) approach, we derive the general set of positivity constraints on the closed and open universe Hilbert spaces. In the case of AdS gravity, open universe positivity in principle follows from CFT unitarity, however the holographic description of closed universes remains unclear. Strikingly, we exhibit positivity of closed universes across many theories and prove that open positivity implies closed positivity, showing that the CFT 'knows' about the closed universe hilbert space. We then analyze positivity constraints on gravitational theories coupled to axions. We present a method to compute off-shell axion wormholes in AdS and flat space which we use to show that positivity is violated if the axion shift symmetry is exact. In low-energy EFTs where these wormholes are perturbatively stable, to restore positivity the wormhole must have a non-perturbative instability due to instantons that breaks the shift symmetry. Positivity then leads to a proof of a sharp version of the Axion Weak Gravity Conjecture A-WGC, including precise numerical constants. For the QCD axion this provides a bound on the axion decay constant which has phenomenological and experimental consequences for axion searches. In string theory, positivity gives a bound on the coupling between the axion and the dilaton in the low energy effective action.

Venue: via Zoom

Event Official Language: English

Causality Constraints on Black Hole Thermodynamics in Nonlinear Electrodynamics

March 6 (Fri) 15:30 - 17:00, 2026

Kaho Yoshimura (Ph.D. Student, Graduate School of Arts and Sciences, The University of Tokyo)

Black holes exhibit thermodynamic properties and provide an important window into the quantum aspects of gravity. In this context, nonlinear electrodynamics (NLED) offers a useful framework for constructing and analyzing charged black-hole solutions beyond Maxwell theory. Requiring causality - namely, excluding superluminal signal propagation - imposes nontrivial constraints on the allowed form of the NLED Lagrangian. In this talk, we focus on two quantities: the charge-to-mass ratio and the entropy density (entropy-to-mass squared ratio). The charge-to-mass ratio is expected to obey a monotonic behavior consistent with the Weak Gravity Conjecture, while the entropy density is also anticipated to be monotonic, reflecting the expectation that higher-energy effective theories contain more degrees of freedom. We show that these monotonic behaviors follow directly from the causality constraints on the NLED sector.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Non-perturbative geodesic length in JT gravity and universal time evolution of holographic complexity

March 2 (Mon) 16:00 - 17:00, 2026

Shono Shibuya (Ph.D. Student, Nagoya University)

The interplay between black hole interior dynamics and quantum chaos provides a crucial framework for probing quantum effects in quantum gravity. According to the holographic "Complexity=Volume" proposal, we investigated non-perturbative generating function of geodesic length in Jackiw-Teitelboim (JT) gravity to uncover universal signatures of quantum chaos and quantum complexity. We observed that the generating function interpolates between two major probes of quantum chaos - spectral form factor and complexity - highlighting its utility as a probe of chaotic spectrum in quantum gravity. Generalizing the result to general chaotic systems, we demonstrated that time evolution of the complexity is universally governed by a certain pole structure of observables, suggesting a validity of wide class of observables as a probe of quantum chaos in quantum gravity.

Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Noninvertible symmetry protected topological phases on lattice

February 25 (Wed) 10:30 - 11:30, 2026

Weiguang Cao (PD, Centre for Quantum Mathematics, University of Southern Denmark, Denmark)

The recent discovery of noninvertible symmetries—a radical extension of conventional symmetry—has challenged long-standing paradigms in condensed matter physics and quantum information and opened new territory in both theory and technology. Unlike ordinary symmetries, which can be inverted, these symmetries behave like projections (one-way operations) yet still strongly constrain quantum dynamics and enable new classes of phases and phase transitions. However, their role in organizing and stabilizing novel quantum phases remains poorly understood. One important example is a symmetry protected topological (SPT) phase, characterized by nontrivial edge modes and potential applications in quantum information. In this talk, I will discuss the classification of noninvertible symmetry-protected topological (NISPT) phases in both closed and open quantum systems using a duality-based method, and present concrete lattice realizations. These lattice models provide controlled playgrounds in which the physics of noninvertible symmetry can be explored numerically and, potentially, experimentally.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Tunneling with physics-informed renormalisation group flows in the anharmonic oscillator

February 20 (Fri) 10:00 - 11:00, 2026

Friederike Ihssen (Postdoctoral Fellow, Physics, Ruhr University Bochum, Germany)

The resolution of strongly correlated physical systems is computationally hard, but can be simplified enormously by a formulation in terms of suitable dynamical degrees of freedom. Within the functional renormalisation group framework, physics-informed renormalisation group flows (PIRG flows) [1] implement scale-dependent coordinate transformations that can be used to devise optimal expansion schemes around such degrees of freedom. Recently, we have applied PIRG flows to the anharmonic oscillator, with an emphasis on the weak coupling regime with its instanton-dominated tunnelling processes [2]. We show that the instanton physics behind the exponential decay of the energy gap is already covered in the first order of the derivative expansion of the PIRG. The crucial new ingredients in the present analysis are the use of the ground state expansion within PIRG flows, as well as precision numerics based on Galerkin methods. Our result a_inst = 1.910(2) for the decay constant is in quantitative agreement with the analytic one, a_inst = 1.886 with a deviation of 1%. This illustrates very impressively the capacity of the PIRG for fully capturing non-perturbative physics already in relatively simple approximations.

Venue: via Zoom / #359, Main Research Building

Event Official Language: English

Classical Spinning Black Hole Scattering from Quantum Amplitudes

January 15 (Thu) 14:00 - 15:30, 2026

Dogan Akpinar (Ph.D. Student, Higgs Centre for Theoretical Physics, School of Physics and Astronomy, University of Edinburgh, UK)

Scattering amplitudes have recently become a powerful tool for extracting classical observables in two-body gravitational dynamics, with direct relevance for current and future gravitational-wave experiments. In this talk, I will review how quantum scattering amplitudes can be used to obtain classical black hole scattering observables. A key focus will be the inclusion of spin effects, modelled by treating black holes as point particles in fixed-spin representations. This approach introduces a subtle ambiguity in the separation between classical and quantum information, which we resolve using our spin interpolation method. Leveraging this, we obtain, for the first time, the classical two-loop amplitude accurate to quartic order in spin, from which we extract physical observables such as linear and angular impulses using covariant Dirac brackets. Remarkably, the resulting amplitude obeys a spin-shift symmetry, remaining invariant under a shift of the black hole spin by the momentum transfer in the scattering process. Motivated by this structure, we examine the conserved quantities governing scattering and show that—at least asymptotically—the probe dynamics remain integrable through quartic order in spin. Under this asymptotic integrability, together with the spin-shift symmetry, we demonstrate that the quartic-in-spin radial action is fully determined by the aligned-spin sector. Taken together, these results advance our understanding of spinning black hole scattering and illuminate new structural features of Kerr dynamics.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Cosmological correlators beyond the de-Sitter invariance

December 24 (Wed) 15:00 - 17:00, 2025

Zhu Yuhang (Postdoctoral Researcher, Particle Theory and Cosmology Group, Center for Theoretical Physics of the Universe (CTPU), Institute for Basic Science (IBS), Republic of Korea)

Cosmological correlators serve as powerful probes of the physics that governed the Universe in its earliest moments. Yet analytic calculations of correlators involving massive spinning fields are highly challenging. Recent progress in the cosmological bootstrap program has greatly deepened our understanding of these correlators. In this talk, we will show how to extend the bootstrap program beyond exact de Sitter invariance by studying two types of symmetry breaking: explicit scale-invariance breaking and boost breaking. We will present the boundary differential equations that characterise correlators in these settings and highlight the rich phenomenology that emerges. Finally, we will also show recent developments in approximation methods, based on the combination of exact WKB and saddle-point method, which provide a precise, efficient, and physically transparent way to capture and classify the non-analytic features of correlators.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

The Functional Renormalisation Group: From the physics of strongly correlated systems to generative models

December 5 (Fri) 10:30 - 12:00, 2025

Jan Martin Pawlowski (Professor, Institute for Theoretical Physics, University of Heidelberg, Germany)

In the past decades, the functional renormalisation group (fRG) has matured into a comprehensive approach to strongly-correlated (non-perturbative) systems, covering quantitatively both universal and non-universal phenomena. The fRG also constitutes an ideal approach for unravelling structural aspects of quantum field theories. This is not only interesting for studies in mathematical physics, but also guides systematic diagrammatic expansion schemes. It is also used to set up novel statistical (lattice) approaches to non-perturbative phenomena. In the present talk I survey these advances and illustrate the progress with selected examples ranging from ultracold atoms, QCD and quantum gravity to novel generative architectures for lattice simulations and beyond.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Towards the prediction of clusters of primordial black holes

November 7 (Fri) 16:00 - 17:30, 2025

Danilo Artigas (JSPS Postdoctoral Research Fellow, Department of Physics Ⅱ, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)

Primordial black holes (PBHs) are a major candidate for dark matter, expected to form from the collapse of large density fluctuations generated during inflation. Their abundance is highly sensitive to non-linear effects, some of which can be described through the δN formalism. This approach models the universe as a set of locally homogeneous patches evolving independently throughout inflation. However, accounting for the spatial correlations between these patches is crucial to predicting the spatial distribution of PBHs and the formation of clusters. In this talk, after reviewing the δN formalism, I will show how to include spatial correlations within this framework. As an illustration, I will discuss the ultra-slow-roll model and compute the curvature perturbation ζ — necessary to determine PBH formation — and its spatial correlations at the end of inflation. In the future, this could enable the prediction of PBH binaries and clusters, which may leave observable imprints such as gravitational waves.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Topological Field Theory Coupled to Parameter Spaces

October 15 (Wed) 13:30 - 15:00, 2025

Takamasa Ando (Ph.D. Student, Yukawa Institute for Theoretical Physics, Kyoto University)

Topological quantum field theories (TQFTs) describe the IR fixed points of wide classes of gapped theories and are useful for studying many-body quantum phases of matter. In this talk, I will talk about TQFTs coupled to parameter spaces. I first explain the motivation for studying such TQFTs with parameter spaces from two perspectives: generalizing the description of the partition function with background gauge fields, and generalizing to invariants of many-body gapped phases over parameter spaces, known as the Berry phase. Then I will explain how these two are related by showing two physically motivated maps that connect them. The construction of these maps provides physical evidence for the Cobordism Hypothesis. I also discuss other related topics, such as the bulk-boundary correspondence. The talk is based on my ongoing work with Ryan Thorngren (UCLA).

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English