Seminar
963 events
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Seminar
Clumpy Outflows from Super-Eddington Accreting Black Holes
April 10 (Fri) 14:00 - 15:15, 2026
Haojie Hu (JSPS Research Fellow, University of Tsukuba)
Recent advances in X-ray spectroscopic observation have enabled researchers to reveal distinct clumpy structures in the super-Eddington outflows from the supermassive black hole in PDS 456 (XRISM Collaboration 2025), initiating detailed investigation of fine-scale structures in accretion-driven outflows. In this talk, I will introduce our high-resolution, two-dimensional radiation-hydrodynamics simulations with time-varying and anisotropic initial and boundary conditions that reproduce clumpy outflows from super-Eddington accretion flows. The resulting clumpy outflows extend across a wide range of radial distances and polar angles, exhibiting typical properties such as a size of ~10 rg (where rg is the gravitational radius), a velocity of ~0.05–0.2 c (where c is the speed of light), and about five clumps along the line of sight. Although the velocities are slightly smaller, these characteristics reasonably resemble those obtained from the XRISM observation. The gas density of the clumps is on the order of 10^-13–10^-12 g cm^-3, and their optical depth for electron scattering is approximately 1–10. The clumpy winds accelerated by radiation force are considered to originate from the region within <300 rg.
Venue: #220, 2F, Main Research Building (Main Venue) / via Zoom
Event Official Language: English
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Seminar
A Hybrid Pseudo-spectral–PINN Approach to Black Hole Quasinormal Modes
April 3 (Fri) 14:00 - 15:15, 2026
Alexandre M. Pombo (PD, Institute of Physics of the Czech Academy of Sciences, Czechia)
Gravitational-wave detections by the LIGO-Virgo-KAGRA network have turned compact-object mergers into precision probes of strong gravity. The post-merger ringdown is particularly incisive: it is governed by quasinormal modes (QNMs), the damped oscillations that encode the remnant's structure and provide a fingerprint of the final object. While current detectors constrain the dominant mode, next-generation observatories will resolve multiple modes with high precision, placing stringent demands on the accuracy of theoretical predictions. Computing QNMs for rotating black holes is, however, a non-trivial task, as it requires solving highly coupled, complex-valued perturbation equations where standard methods struggle. In this talk, I present SpectralPINN, a hybrid solver combining Pseudo-spectral methods with Physics-Informed Neural Networks, validated at 10⁻⁵ relative accuracy. I will present results for Kerr and Kerr-Newman black holes, demonstrating the method's robustness and accuracy across parameter space, and discuss its potential for extension to more exotic compact objects relevant to next-generation detector science.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
A mathematical promenade in microscopic locomotion
April 2 (Thu) 13:00 - 14:00, 2026
Clément Moreau (CNRS Researcher, CNRS, France)
The microscopic world offers a fascinating diversity of locomotion strategies, relying primarily on the use of flagella and cilia. These slender structures, capable of complex periodic deformations, serve as a major source of inspiration for medical microrobotics. At this scale, fluid dynamics is governed by the predominance of viscosity over inertia. This low-Reynolds number regime imposes strict physical constraints, summarized by the famous « scallop theorem »: a reciprocal deformation cannot produce any net displacement. Mathematically, this is framed by the Stokes connection, which links changes in body shape to net movement in space. This presentation proposes a journey through the modeling principles of microscopic swimmers. We will see how to derive analytical solutions to the locomotion problem by simplifying degrees of freedom or by assuming small deformation amplitudes. I will then present the perspective of control theory to address the « controllability » property, i.e. the ability of a locomotor to reach any target position and shape. Finally, I will question a classic hypothesis in the field: the inextensibility of flagella. Although the literature often assumes these structures are rigid in the longitudinal direction, certain micro-organisms and artificial robots exhibit significant compression variations. I will present recent results, based on classical modeling tools, exploring the influence of compression-curvature coupling on locomotion efficiency at low Reynolds numbers.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
QFT as a set of ODEs
March 27 (Fri) 13:30 - 15:30, 2026
Qiao Jiaxin (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
Correlation functions of local operators in Quantum Field Theory (QFT) on hyperbolic space can be fully characterized by the set of QFT data. These are the scaling dimensions of boundary operators, the boundary Operator Product Expansion (OPE) coefficients and the Boundary Operator Expansion (BOE) coefficients that characterize how each bulk operator can be expanded in terms of boundary operators. For simplicity, we focus on two dimensional QFTs and derive a universal set of first order Ordinary Differential Equations (ODEs) that encode the variation of the QFT data under an infinitesimal change of a bulk relevant coupling. In principle, our ODEs can be used to follow a renormalization group flow starting from a solvable QFT into a strongly coupled phase and to the flat space limit.
Venue: via Zoom (Main Venue) / Seminar Room #359
Event Official Language: English
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SeminarQuantum States Over Time: From Foundations to Applications
March 24 (Tue) 15:30 - 17:00, 2026
Minjeong Song (Research Fellow, Centre for Quantum Technologies, National University of Singapore, Singapore)
In this talk, I will introduce quantum states over time (QSOT), a formalism for describing quantum systems over space-time. I will begin by reviewing how QSOT has emerged in the literature. While conventional density operator formalism has been effective across many areas of quantum information theory, QSOT was developed to meet more specialized research needs— most notably, as a key ingredient to develop a quantum version of Bayes’ theorem. I will end the first part of my talk by comparing various QSOT that have been proposed. In the second part, I will discuss the causal compatibility problem as an application of QSOT. I will focus on the temporal compatibility problem, which asks the following: from correlations in measurement outcomes alone, can two otherwise isolated parties establish whether such correlations are atemporal (i.e., temporally incompatible)? That is, can they rule out that they have been given the same system at two different times? I will first explain how characterizing measurement statistics in a causal agnostic scenario is equivalent to characterizing a specific type of QSOT, known as pseudo-density operators. I will then present our recent findings obtained by analyzing pseudo-density operators; In particular, we demonstrate that atemporality is distinct from entanglement, though they appear to be equivalent at first glance. Specifically, we show atemporality implies entanglement, but not vice versa, thus revealing that atemporality is a strictly stronger form of quantum correlations than entanglement. Nevertheless, we also find that sufficiently strong entanglement does imply atemporality.
Venue: #359, Seminar Room #359
Event Official Language: English
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Seminar
Mouse Limb Bud Skeletal Patterning Description and Modelling
March 19 (Thu) 13:00 - 14:00, 2026
Laura Aviñó Esteban (Ph.D. Candidate, European Molecular Biology Laboratory, Barcelona, Spain)
Understanding how complex organs reliably form during development remains a key question in biology. In this talk, I discuss how gene regulatory networks may generate skeletal patterns in the vertebrate limb, using Sox9 expression as a proxy, as it marks the earliest stages of cartilage formation. To address this, I developed new computational tools for reconstructing spatiotemporal gene expression and built models ranging from machine learning approaches to mechanistic frameworks. These analyses reveal that limb patterning cannot be explained by a single universal mechanism. Instead, different regions of the limb appear to use distinct regulatory strategies, uncovering an unexpected qualitative modularity in skeletal development. Together, these findings lead to a new hypothesis in which other systems, such as the vasculature may actively shape skeletal spacing in specific limb regions.
Venue: via Zoom / Seminar Room #359
Event Official Language: English
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Seminar Tomorrow
Critical Phenomena on the Bethe Lattice
March 18 (Wed) 16:00 - 18:00, 2026
Saswato Sen (Ph.D. Student, Okinawa Institute of Science and Technology Graduate University (OIST))
We investigate the critical behavior of a family of Z2-symmetric scalar field theories on the Bethe lattice (the tree limit of regular hyperbolic tessellations) using both the non-perturbative Functional Renormalization Group and perturbation theory. Due to the hyperbolic nature of Bethe lattices, the Laplacian lacks a zero mode and exhibits a spectral gap. We demonstrate that closing the spectral gap via a modified Laplacian leads to novel critical behavior governed by interacting fixed points. This stands in contrast to the nearest-neighbor Ising model, which exhibits a phase transition with mean-field critical exponents. We further comment on the possible reasons for such a deviation.
Venue: via Zoom / #359, Main Research Building
Event Official Language: English
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Seminar Tomorrow
Testing the quantum nature of gravity "ab absurdo"
March 18 (Wed) 14:00 - 16:00, 2026
Emanuele Panella (Postdoctoral Researcher, Tor Vergata University of Rome, Italy)
The quest for a quantum description of gravity has been long, diverse, and productive. Yet, despite decades of theoretical progress, there is still no direct experimental evidence for the quantum nature of spacetime. In this talk, I explore an alternative, indirect route to probing quantum gravity by assuming the fundamental classicality of the gravitational field and examining the resulting observational conflicts. In particular, I will discuss a key consistency condition—known as the decoherence–diffusion trade-off—that any theory of fundamentally classical gravity coupled to quantum matter must satisfy. By analysing a toy model of a linearised classical–quantum (CQ) gravity–matter system, I will explicitly show how this trade-off implies unavoidable, measurable effects, such as a fundamental stochastic gravitational-wave background, which cannot be eliminated by fine-tuning the model parameters.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Quantum modular form and quantum invariants
March 13 (Fri) 14:00 - 16:00, 2026
Yuya Murakami (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Quantum invariants are invariants of knots and 3-manifolds which relate deeply to mathematical physics and representation theory. In recent years, it has become increasingly clear that it is also deeply related to number theory, that is, quantum modularity for quantum invariants. This topic is interesting from a topological viewpoint since this is a refinement of establishing asymptotic expansions of quantum invariants, which is an important problem in quantum topology, and is interesting from a number-theores[tic viewpoint since this gives examples of quantum modular forms, which are mysterious objects in number theory. I obtained two linked results on topology and number theory: Establishing explicit asymptotic expansions of quantum invariants for negative definite plumbed 3-manifolds and establishing quantum modularity of false theta functions in full generality. In this talk, I will outline previous progress on quantum modularity for quantum invariants and my results.
Venue: via Zoom / Seminar Room #359, Seminar Room #359
Event Official Language: English
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Seminar
Chronotaxicity and Dynamic Stability: From Theory to Quantitative Measures
March 12 (Thu) 13:00 - 14:00, 2026
Aneta Stefanovska (Professor, Lancaster University, UK)
Living systems operate far from equilibrium under continuous time-varying forcing across multiple temporal and spatial scales. From neural and cardiovascular rhythms to microcirculatory dynamics and circadian cycles, physiological processes are inherently nonautonomous. Classical stability concepts based on autonomous attractors and stationary limit cycles are therefore insufficient to explain how such systems remain robust yet adaptable. In this talk, I will introduce chronotaxicity as a framework for nonautonomous oscillatory systems possessing time-dependent point attractors and contraction regions. Chronotaxic systems maintain stability under continuous forcing, providing a rigorous theoretical description of dynamic robustness. To illustrate the generality of this concept, I will show how chronotaxicity can be observed in a controlled physical experiment. I will then present a new order parameter based on angular velocity for quantifying phase dynamics in numerical simulations of coupled nonautonomous oscillators, along with the methods collected in the Multiscale Oscillatory Dynamics Analysis (MODA) toolbox for analysing time-dependent oscillatory behaviour. This approach provides a unified perspective on dynamic stability in complex systems, highlighting how living systems remain robust yet adaptable and suggesting quantitative signatures of dysfunction in health and disease. While the focus is on physiological and numerical models, it is broadly applicable to complex nonautonomous systems, underscoring its generality as a dynamical principle.
Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
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
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Seminar
How does the brain compute the value of odors and trigger adaptive behavior?
March 5 (Thu) 13:00 - 14:00, 2026
Hokto Kazama (Team Director, Laboratory for Circuit Mechanisms of Sensory Perception, RIKEN Center for Brain Science (CBS))
The world is filled with numerous odors that are impossible to experience all in our lifetime. Perhaps to cope with this situation, the brain is equipped with an ability to recognize whether an odor is attractive or aversive even from the first encounter and guide adaptive behavior. However, how information about the innate value of odors (attractiveness/aversiveness) is computed and transformed into appropriate behavioral outputs in the brain remains poorly understood. We are addressing this question in the olfactory circuit of fruit flies by combining behavioral analysis in virtual reality, comprehensive neuronal activity imaging, neuronal connectivity analysis, and computational modeling. In this talk, I will present our latest efforts to decipher how odor value is computed and how this information is transformed into motor-related signals in a tiny brain.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
The minimal absolute value of sums of fifth-roots of unity
March 5 (Thu) 11:00 - 12:00, 2026
Guillermo Núñez Ponasso (JSPS Research Fellow, Graduate School of Information Sciences, Tohoku University)
The minimal absolute value \sigma_{\ell}(n) of a weight-n sum of \ell-th roots of unity, for all n and a fixed \ell, is an interesting value in the study of maximal determinant matrices. In the cases where \ell=2,3,4, or 6, this minimal absolute value is either 0 or 1. Thus \ell=5 constitutes the smallest non-trivial case. In this talk I will discuss recent results in collaboration with Akihiro Munemasa, where we determined \sigma_5(n) for all n\geq 1. This problem turns out to be related to the Diophantine approximation of the golden ratio, and can be tackled using the theory of continued fractions.
Venue: via Zoom / SUURI-COOL (Kyushu)
Event Official Language: English
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Seminar
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
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Seminar
The career talk: From Quarks to Cinematic Sparks
February 27 (Fri) 15:00 - 16:30, 2026
Agnes Mocsy (Professor, Department of Mathematics and Science, Pratt Institute, USA)
While my career began in a linear way, it gradually opened into a non-traditional path through unexpected mergings, where theoretical nuclear physics, filmmaking, and creative public and academic engagement intertwined. I will share how scientific inquiry, artistic practice, and storytelling began shaping one another, opening new ways to explore complexity, emotion, and connection. Drawing on work from my physics research to cinema projects like Rare Connections, I will reflect on how curiosity and creative thinking move freely across science and art, deepening each and expanding how we understand the human experience. My aim is to offer a perspective on the possibilities that emerge when we allow our multitudes to meet and transform one another.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Scattering in de Sitter space
February 26 (Thu) 14:00 - 15:30, 2026
Jason Kristiano (Program-Specific Researcher, Yukawa Institute for Theoretical Physics, Kyoto University)
The analytic structure of scattering amplitudes provides a framework for mapping the fundamental properties of a high-energy (UV) theory onto non-perturbative constraints for low-energy (IR) effective field theories. While this structure is well understood in flat space, its extension to de Sitter space is hindered by the expanding background, which complicates the definition of asymptotic states and breaks time-translation symmetry. In this talk, I will outline a foundational approach to bridging this gap. I will demonstrate how the analytic properties of flat-space amplitudes are imprinted on their de Sitter counterparts. The ultimate goal of this program is to derive Swampland-type constraints for cosmological EFTs, ensuring they admit a consistent UV completion.
Venue: #345-347, Main Research Building, RIKEN Wako Campus (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Sex ratio theory for facultative parthenogens: from fortuitously optimal stick insects to the origin of haplodiploidy in Hymenoptera
February 26 (Thu) 13:00 - 14:00, 2026
Kora Klein (Visiting Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
First, I will present one of my PhD papers which focuses on sex ratios when females can reproduce both sexually and asexually. This paper features two models: an optimality model and an evolutionary invasion analysis. More generally, it shows my style of approaching evolutionary theory. After this, I will briefly discuss the work I plan to do in the next 6 months during my JSPS fellowship where I will focus on the evolution of more conventional X-chromosomes. Short Bio: I studied biology at the Free University in Berlin. My biology studies were mostly empirical but I attended several Bachelor courses for mathematicians (calculus and stochastics) and did my Master thesis in Zurich with Hanna Kokko, a theoretical evolutionary ecologist who studies a broad range of topics. In 2020, I then continued working in Hanna Kokko’s group for my PhD and moved with her from Zurich to Mainz in 2023. During my time with Hanna Kokko, worked on various topics centered around intraspecific diversity, including projects on sexual dimorphism, sex ratio theory, intralocus sexual conflict, and a female-limited color polymorphism in a butterfly. Since then, I started my first PostDoc in 2024 with Laura Ross in Edinburgh (UK) where I modelled how the unusual genetic systems of Scarid flies could have evolved, and have now started a 6 month JSPS fellowship with Ryosuke Iritani.
Venue: #435-437, Main Research Building
Event Official Language: English
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Seminar
Testing the quantum nature of gravity with optomechanical systems
February 26 (Thu) 10:00 - 12:00, 2026
Yuta Michimura (Assistant Professor, Department of Physics, Graduate School of Science, The University of Tokyo)
Quantum gravity remains one of the major challenges in modern physics. Even at the most fundamental level, there is no experimental confirmation of whether a mass placed in a spatial superposition generates a corresponding superposition of gravitational fields. In recent years, experiments aiming to create gravity-induced quantum entanglement have attracted significant attention as a way to probe the quantum nature of non-relativistic gravity. In particular, optomechanical systems, which exploit the interaction between light and mechanical oscillators, provide a promising platform for such studies. We are pursuing experiments at the milligram scale, which lies between the smallest mass scale at which classical gravity has been tested and the largest mass scale at which quantum states of mechanical oscillators have been realized [1]. In this seminar, I will discuss experimental approaches to testing the quantum nature of gravity using suspended and levitated mirrors. I will also discuss our recent proposal to use inverted oscillators to enhance gravity-induced entanglement exponentially [2].
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
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
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Seminar
Quantitative phylogenomics
February 24 (Tue) 13:00 - 14:00, 2026
Hector Banõs (Assistant Professor, Department of Mathematics, California State University, USA)
Benjamin Teo (Postdoc, Mathematical Analysis of Cellular Systems, University of Melbourne, Australia)This session features two speakers: Hector Banos, Assistant Professor of Mathematics at California State University, whose research focuses on phylogenetic inference and network models, and Benjamin Teo, a Postdoctoral Researcher at the University of Melbourne, working on probabilistic and computational methods for continuous trait evolution on phylogenetic networks. See below for details. 【Talk 1】 Speaker: Hector Banos Title: Bringing a Knife to a Gunfight: Pitfalls of Phylogenetic Inference under Model Misspecification Abstract: Phylogenetic networks provide a flexible framework for representing evolutionary histories that include hybridization, introgression, and other reticulate processes. However, inferring such networks remains computationally and statistically difficult. Many current methods often scale only to restricted classes of networks. Consequently, researchers frequently analyze their data using simpler models (most commonly phylogenetic trees) even when there is strong evidence that the underlying evolutionary history is more complex. In this talk, we examine the impact of model misspecification on phylogenetic inference, focusing on situations in which data are generated by a complex network but are analyzed using simpler tree or network models. I then show how this mismatch can influence the topology of inferred trees, as well as the structure of inferred networks. These results highlight the limitations and the practical consequences of using simplified models for phylogenetic inference. 【Talk2】 Speaker: Benjamin Teo Title: Adapting cluster graphs for inference of continuous trait evolution on phylogenetic networks Abstract: I consider a new approach ("loopy belief propagation") for fitting Gaussian models on a phylogenetic network to explain the data observed across present-day species for a continuous univariate or multivariate trait. We previously showed [1] that a trait evolution model coupled to a network can be readily cast as a probabilistic graphical model, so that the likelihood can be efficiently computed using a dynamic programming framework ("belief propagation") defined on an auxiliary graph ("cluster graph") that is tree-structured. Even so, maximum likelihood estimation can grow computationally prohibitive for large complex networks. Belief propagation can be applied more generally to non-tree ("loopy") cluster graphs to compute a factored energy approximation to the log-likelihood. "Loopy" belief propagation may provide a more practical trade-off between estimation accuracy and runtime. However, the influence of cluster graph structure on this trade-off is not precisely understood. We conduct a simulation study using our Julia package PhyloGaussianBeliefProp [2] to investigate how varying the maximum cluster size of a cluster graph affects this trade-off. We discuss recommended choices for maximum cluster size, and prove the equivalence of likelihood-based and factored-energy based estimates for the homogeneous Brownian motion trait model. The talk is based on our preprint [3]. I will introduce the key concepts from the ground up.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
963 events