Exploring material strengths of dust aggregates in planet formation by numerical simulations

December 8 (Fri) at 14:00 - 15:15, 2023

Misako Tatsuuma (Research Scientist, iTHEMS)

The planet formation process is the growth from sub-micrometer-sized cosmic dust grains to thousand-kilometer-sized planets. This growth process has broadly two phases: the growth from dust grains to kilometer-sized planetesimals, mainly driven by intermolecular forces like van der Waals forces and hydrogen bonds, and the subsequent growth from planetesimals to planets, governed by gravitational forces. However, the planetesimal formation process encounters various challenges, including fragmentation and bouncing resulting from collisions among dust aggregates. To gain insights into the planetesimal formation process and how to avoid these obstacles, I have been focused on measuring and formulating the material strengths of dust aggregates using grain simulations. In this talk, I will introduce my works on the material strengths of dust aggregates and their applications to kilometer-sized bodies in the solar system, such as comets and asteroids.

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

Event Official Language: English

Gravitational Lensing in Black Hole Spacetimes of the Plebanski-Demianski Class

December 6 (Wed) at 16:00 - 17:30, 2023

Torben Christian Frost (Postdoctoral Researcher, Kavli Institute for Astronomy and Astrophysics, Peking University, China)

Einstein's field equations allow various different black hole solutions. Among these solutions, the most famous are most likely the Schwarzschild and the Kerr spacetimes, which are both special cases of the so-called Plebanski-Demianski spacetime. Besides the Schwarzschild and Kerr spacetimes, the Plebanski-Demianski spacetime also includes other solutions as special cases, among them the C-metric and the NUT metric. They describe a linearly accelerating black hole and a black hole with gravitomagnetic charge, respectively. The question is now how we can determine if an astrophysical black hole can be described by one of these spacetimes. We will address this question using gravitational lensing for the three spacetimes with the most salient lensing features, namely the C-metric, the NUT metric, and the Kerr metric. For this purpose, we will first outline how to solve the equations of motion analytically using elementary and Jacobi's elliptic functions as well as Legendre's elliptic integrals. Then we will fix an observer in the domain of outer communication and relate the constants of motion of the lightlike geodesics to latitude-longitude coordinates on the observer's celestial sphere. We will use the analytic solutions to write down the lens equations, calculate the redshift, and the travel time. Finally, we will discuss and compare the results and comment on how we can use them to place constraints on the spin parameter, the acceleration parameter, and the gravitomagnetic charge of a black hole.

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

Event Official Language: English

Cosection localization via shifted symplectic geometry

December 6 (Wed) at 10:00 - 11:30, 2023

Young-Hoon Kiem (Professor, School of Mathematics, Korea Institute for Advanced Study (KIAS), Republic of Korea)

Modern enumerative invariants are defined as integrals of cohomology classes against virtual fundamental classes constructed by Li-Tian and Behrend-Fantechi. When the obstruction sheaf admits a cosection, the virtual fundamental class is localized to the zero locus of the cosection. When the cosection is furthermore enhanced to a (-1)-shifted closed 1-form, the zero locus admits a (-2)-shifted symplectic structure and thus we have another virtual fundamental class by the Oh-Thomas construction. An obvious question is whether these two virtual fundamental classes coincide or not. In this talk, we will see that (-1)-shifted closed 1-forms arise naturally as an analogue of the Lagrange multiplier method. Furthermore, a proof of the equality of the two virtual fundamental classes and its applications will be discussed. Based on a joint work with Hyeonjun Park.

Venue: Seminar Room #359

Event Official Language: English

Translating between evolutionary game theory and theoretical ecology

December 5 (Tue) at 16:30 - 17:30, 2023

Arne Traulsen (Director, Department for Theoretical Biology, Max Planck Institute for Evolutionary Biology, Germany)

Both theoretical ecology and evolutionary game theory describe the dynamics of interacting populations. More than 40 years ago, Hofbauer and Sigmund established a mathematical equivalence between the Lotka-Volterra equations and the replicator dynamics from evolutionary game theory. However, this equivalence has not been exploited by empiricists so far. One of the issues is dimensionality: An ecological interaction of two species corresponds to an evolutionary game between three types. Only when we focus on a special case with identical growth rates, it is possible to translate without this trick, leading to a more direct equivalence between the frameworks. Consequently, one has to be particularly careful how to classify interactions and how to assess dynamical outcomes. For example, a ‘Prisoner's Dilemma’ interaction where the `cooperators' have a higher intrinsic growth rate than `defectors' can result in stable coexistence of the two types and may ultimately not represent a social dilemma at all.

Venue: via Zoom

Event Official Language: English

Rotating discs on the Kerr black hole background

December 5 (Tue) at 15:00 - 16:30, 2023

David Kofroň (Postdoctoral Researcher, Institute of Theoretical Physics, Charles University, Czechia)

Analytical solution of a rotating black hole surrounded by accretion disc in full GR is, so far, unknown. The Ernst equation is nonlinear. In this talk, we will provide a framework in which the solutions of linearised Ernst equations can be obtained from the linear perturbations of Kerr black hole treated in the formalism of the Debye potentials. In this way, we recover all the metric perturbations in term of a single complex scalar function (which solves the Laplace equation).

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

Event Official Language: English

Gravity of Accretion Discs and Black Holes

December 5 (Tue) at 14:00 - 15:00, 2023

Petr Kotlařík (Ph.D. Student, Institute of Theoretical Physics, Charles University, Czechia)

The typical black hole solutions describe only isolated black holes. However, in astrophysics, such a condition is never strictly satisfied. As matter accretes onto the black hole, disc structures are often formed. In this talk, I will summarize our recent attempts to find the gravitational field of such a nonisolated black hole. We start from the simplest case of static and axially symmetric metric. Although it is a somewhat "rough" approximation in the astrophysical context, this idealization may already help us to understand some interesting implications of the disc's gravity. Moreover, with such a simplification, we can obtain exact analytical "superpositions" of the Schwarzchild black hole and a disc. When some rotation is present, dragging effects complicate the situation dramatically. Then, one typically has to resort to numerical relativity or some approximate methods, e.g., perturbations. In the talk, I also address the stationary case and demonstrate what we can do on the level of the direct metric perturbation.

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

Event Official Language: English

Breaking down the magnonic Wiedemann-Franz law in the hydrodynamic regime

December 4 (Mon) at 15:00 - 16:30, 2023

Ryotaro Sano (Ph.D. Student, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)

Quantum transport has attracted a profound growth of interest owing to its fundamental importance and many applications in condensed matter physics. Recent significant developments in experimental techniques have further boosted the study of quantum transport. Notably in ultraclean systems, strong interactions between quasi-particles drastically affect the transport properties, resulting in an emergent hydrodynamic behavior. Recent experiments on ultrapure ferromagnetic insulators have opened up new pathways for magnon hydrodynamics. Hydrodynamic magnon transport implies exhibiting extraordinary features and has a potential for innovative functionalities beyond the conventional non-interacting magnon picture. However, the direct observation of magnon fluids remains an open issue due to the lack of probes to access the time and length scales characteristics of this regime. In this work, we derive a set of coupled hydrodynamic equations for a magnon fluid and study the spin and thermal conductivities by focusing on the most dominant time scales [1]. As a hallmark of the hydrodynamic regime, we reveal that the ratio between the two conductivities shows a large deviation from the so-called magnonic WF law. We also identify an origin of the drastic breakdown of the magnonic WF law as the difference in relaxation processes between spin and heat currents, which is unique to the hydrodynamic regime. Therefore, our results will become key evidence for an emergent hydrodynamic magnon behavior and lead to the direct observation of magnon fluids.

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

Event Official Language: English

Evolution by gene and genome duplications

November 28 (Tue) at 16:00 - 17:00, 2023

Jeffrey Fawcett (Senior Research Scientist, iTHEMS)

Each organism typically has (tens of) thousands of genes in its genome that perform various molecular and cellular functions, but how did these genes originate? The answer for most genes is by the duplication of another gene. In fact, all the genes (the entire genome) can get duplicated simultaneously on some instances. Thus, gene and genome duplications are considered key driving forces of evolution and are one of the most important topics in molecular evolutionary biology. In this talk, I will introduce the background and basic concepts related to gene and genome duplications. The talk will be aimed at non-experts so non-biologists are also welcome to attend.

Venue: via Zoom

Event Official Language: English

Joint RIKEN/N3AS Workshop on Multi-Messenger Astrophysics

November 26 (Sun) at 9:00 - 19:00, 2023

RIKEN iTHEMS and NSF Physics Frontier Center N3AS will jointly organize a workshop on "Multi-Messenger Astrophysics" on Sunday, November 26, at the Hilton Waikoloa Village, in conjunction with the JPS/ APS DNP meeting. For the program and registration form, please visit the workshop website at the related links. The workshop is open to all and there is no registration fee. However, we ask those wishing to attend to register, to help us plan. The workshop will conclude with an early evening poster session that will combine science with an opportunity for JPS and APS participants to socialize. Organizers: Tetsuo Hatsuda（RIKEN iTHEMS) Wick Haxton (UC Berkaley, N3AS) Baha Balentekin (UW-Madison, N3AS)

Venue: Hilton Waikoloa Village, Waikoloa, Hawaii

Event Official Language: English

Early Formation of Dark Matter Halos

November 24 (Fri) at 14:00 - 15:15, 2023

Derek Beattie Inman (Research Scientist, iTHEMS)

Cosmological observations have led to an extremely precise understanding of the large-scale structure of the Universe. A common assumption is to extrapolate large-scale properties to smaller scales; however, whether this is correct or not is unknown and many well-motivated early Universe scenarios predict substantially different structure formation histories. In this seminar I will discuss two scenarios where nonlinear structures form much earlier than is typically assumed. In the first case, the initial fluctuations are enhanced on small scales leading to either primordial black holes clusters or WIMP minihalos right after matter-radiation equality. In the second, I will show that an additional attractive dark force leads to structure formation even in the radiation dominated Universe. I will furthermore discuss possible observations of such early structure formation including changes to the cosmic microwave background, dark matter annihilation, and when the first galaxies form.

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

Event Official Language: English

Volume complexity of de Sitter bubbles

November 22 (Wed) at 16:00 - 17:30, 2023

Nicolò Zenoni (Postdoctoral 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

X's Fast and Slow

November 21 (Tue) at 16:00 - 17:00, 2023

Thomas Hitchcock (Special Postdoctoral Researcher, iTHEMS)

Sex chromosomes have long been suggested to undergo distinct evolutionary dynamics to the rest of the genome. Their distinct ploidy and transmission genetics may result in differing strengths of selection, magnitude of drift, and mutation rates to the autosomes, particularly if there are sex differences. Consequently, a body of theory and empirical work has developed investigating such differences, and how they might manifest in the rates of change between populations, and the diversity observed within populations. I will briefly review the theoretical basis of these comparisons, how we can infer rates of evolutionary change from genetic data, and what we can learn from non-model systems, particularly focusing on fungus gnats and pea aphids.

Venue: via Zoom

Event Official Language: English

Bridging physics and society: A case study of collective memory dynamics by socio-econophysics approach

November 20 (Mon) at 15:00 - 16:30, 2023

Yukie Sano (Associate Professor, Institute of Systems and Information Engineering, University of Tsukuba)

The movements of individuals with free will are unpredictable, complex, and, needless to say, fundamentally distinct from the movements of matter. Furthermore, studying society, which forms collectives while engaging in intricate individual interactions, using mathematical models seems incredibly daunting. However, when analyzing empirical data, relatively simple mathematics often emerge in the distribution and dynamics of society at the level of collective behavior. Additionally, such mathematics often share commonalities with physical phenomena. With this background, research is progressing by applying ideas from physics to social-economic phenomena, a field known as socio-econophysics. In this presentation, I will introduce a mathematical model that addresses the decay of collective memory using access logs on the web as an example of research in socio-econophysics.

Venue: Okochi Hall / via Zoom

Event Official Language: English

The 24th MACS Colloquium

November 20 (Mon) at 14:45 - 18:00, 2023

Satoshi Horike (Professor, Department of Chemistry, Graduate School of Science, Kyoto University)
Yoshihiro Morishita (Team Leader, Laboratory for Developmental Morphogeometry, RIKEN Center for Biosystems Dynamics Research (BDR))

14:45-15:00 Teatime discussion 15:00-16:00 Talk by Dr. Satoshi Horike (Professor, Department of Chemistry, Graduate School of Science, Kyoto University) Title: Material science of new glasses of metal-molecular architectures Abstract: The three major materials - ceramics, metals and organic polymers - all form glass phases and underpin our daily lives and industry. Unlike crystals, glasses have a random structure and designing their molecular structures and properties is still a major challenge. In this talk, I will present research on new glasses made of metals and molecules. It will discuss how the glasses are designed and synthesised, as well as the unique glass structures and functions that result from the metal-molecule chemical bonds. 16:15-17:15 Talk by Prof. Yoshihiro Morishita (RIKEN Center for Biosystems Dynamics Research, Team leader) Title: Optimal coding design of positional information and an appropriate spacetime coordinate system to capture an archetype of organ morphogenetic dynamics Abstract: We are engaged in developmental biology research at the intersection of experimentation and theory, driven by a curiosity to understand the laws governing organogenesis. Ideally, we aim to mathematically represent these laws and find commonalities across species and organs. In this seminar, I will present research on a coding design of spatial information within developing tissues, as well as propose spacetime coordinates to directly and quantitatively compare the morphogenetic dynamics of homologous organs across species with different sizes and developmental rates. Furthermore, we will introduce results suggesting the existence of speceis-independent morphogenetic dynamics (a kind of archetype) achieved by applying these methods to biological data. 17:15-18:00 Discussion

Venue: Maskawa Hall, 1F, Maskawa Building for Education and Research

Event Official Language: Japanese

6th QGG Intensive Lectures: Introduction to thermalization in isolated quantum systems

November 20 (Mon) - 21 (Tue), 2023

Ryusuke Hamazaki (RIKEN Hakubi Team Leader, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team, RIKEN Cluster for Pioneering Research (CPR))

This is the sixth event by the Quantum Gravity Gatherings (QGG) Study Group at RIKEN iTHEMS. For this event we have invited Professor Ryusuke Hamazaki from RIKEN, to deliver pedagogical lectures on the thermalization in isolated quantum systems. This topic describes how a non-equilibrium quantum state relaxes to thermal equilibrium through a unitary time evolution. This theme has applications not only in statistical physics but also in many areas of physics. In particular, in the context of quantum gravity, understanding thermalization via a unitary time evolution is anticipated to be a key to resolving the information loss problem of black holes. In addition, the notion of typicality and thermalization is important in the AdS/CFT correspondence to understand black hole physics. We expect this event to provide insights to researchers in related fields. Ryusuke Hamazaki is a leading expert in this subject. It is our enormous pleasure to have the opportunity to learn from him the idea of thermalization and its applications related to realistic systems. This intensive lecture series is designed to be an interactive event. The intensive talk will be given in a face-to-face blackboard style (in English, no online streaming) to encourage informal and lively Q&A discussions. The program will also include short talk sessions, where participants can present a 5-minute talk on a topic of their choice, which could be about their own research, reviews of specific works, or future study interests.

Venue: #435-437, Main Research Building

Event Official Language: English

Introduction and prospects of topological recursion

November 17 (Fri) at 15:00 - 17:00, 2023

Osuga Kento (JSPS Research Fellow PD, Graduate School of Mathematical Sciences, The University of Tokyo)

Topological recursion is a universal recursive formalism that connects many branches in mathematical physics, such as enumerative geometry, algebraic geometry, integrable hierarchy, matrix models, 2d gravity, and more. In the first half of this talk, I will give a pedagogical overview of topological recursion and present simple examples from which we learn how topological recursion works. Then in the second half, I will present some ongoing research projects as well as a few future directions in topological recursion.

Venue: Seminar Room #359

Event Official Language: English

Geometry of special nilpotent orbits

November 15 (Wed) at 14:00 - 15:30, 2023

Baohua Fu (Professor, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, China)

Special nilpotent orbits play a key role in representation theory, but their geometry is little understood. I'll first report a joint work with Yongbin Ruan and Yaoxiong Wen proposing a mirror symmetry conjecture for special nilpotent orbits and then a joint work with Daniel Juteau, Paul Levy and Eric Sommers on the proof of sliced version of Lusztig's conjecture on special pieces.

Venue: via Zoom

Event Official Language: English

MCME SYMPOSIUM 2023

November 15 (Wed) - 16 (Thu), 2023

Kyosuke Adachi (Research Scientist, iTHEMS)
Hiroshi Kokubu (Executive Vice-President, Kyoto University)
Masaru Hongo (Assistant Professor, Department of Physics, Faculty of Science, Niigata University)

"MCME SYMPOSIUM 2023" will be held at the Musashino Center of Mathematical Engineering (MCME), Musashino University. This symposium is free of charge and open to everyone. Colleagues of iTHEMS will also be speaking. Below is an excerpt from the program. Wednesday, November 15, 11:20 - 12:30 Kyosuke Adachi Wednesday, November 15, 16:20 - 17:30 Hiroshi Kokubu Thursday, November 16, 10:00 - 11:10 Masaru Hongo Registration is open until November 14. Please register from the related links below. Organized by Musashino Center of Mathematical Engineering (MCME), Musashino University Co-organized by RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS)

Venue: Hybrid Format (Room 301, Building 5, Ariake Campus, Musashino University and Zoom)

iTHEMS Graph-theory workshop: from basics to applications

November 13 (Mon) - 14 (Tue), 2023

Momoko Hayamizu (Assistant Professor, Faculty of Science and Engineering, School of Fundamental Science and Engineering, Waseda University / PRESTO Researcher, Japan Science and Technology Agency (JST))
Seiya Negami (Professor Emeritus, Yokohama National University)
Takashi Okada (Program-Specific Associate Professor, Institute for Life And Medical Sciences, Kyoto University)
Tasuku Soma (Associate Professor, Department of Statistical Inference and Mathematics, The Institute of Statistical Mathematics / Associate Professor, Statistical Science Program, The Graduate University for Advanced Studies (SOKENDAI))
Shun'ichi Azuma (Professor, Garduate School of Informatics, Kyoto University)

We will hold a two-day workshop on graph theory, which covers a variety of topics including basic concepts and theorems in graph-theory; graph-embedding; phylogenetic tree construction; chemical reaction networks; and cactus network theory. The details are found in the website linked below.

Venue: #435-437, Main Research Building

Event Official Language: English

Vortex Reconnection in Classical and Quantum Fluids

November 10 (Fri) at 15:00 - 16:30, 2023

Yoshifumi Kimura (Professor, Graduate School of Mathematics, Nagoya University)

As a fundamental process in turbulence, vortex reconnection has been studied widely not only in classical fluids but also in quantum fluids. For the latter, the first real observation of vortex reconnection was rather recent, and since then active analyses have been continued. On the other hand, vortex reconnection in the former has a long history, and it is now studied intensively as a candidate for a solution to the problem of the regularity/Singularity of the Navier-Stokes equations, which is one of the seven millennium prize problems of the Clay Mathematical Institute. In this talk, after introducing the problem, we will present some results of analysis and simulations of the dynamical system which has been proposed to describe a vortex reconnection of two vortex rings located symmetrically on two tilted planes [1], [2], [3]. Then it will be shown that this dynamical system can be written in noncanonical Hamiltonian form with Hamiltonian, H, and a Casimir invariant, C in the limit of zero viscosity [4].

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

Event Official Language: English