Being genuine propagating degrees of freedom of the spacetime metric, gravitational waves (GWs) serve as an independent "eye" through which we can probe the evolution history of our universe. They are complementary to electromagnetic observables such as cosmic microwave background (CMB) and can act as direct messengers from the earliest stage of the universe, where conventional probes lose access. In particular, a stochastic background of GWs is widely regarded as a smoking gun of cosmic inflation.
In this talk, I introduce the basic theoretical framework for GWs produced in the primordial universe and discuss how they arise from vacuum fluctuations of the metric. I also outline additional production mechanisms sourced by matter fields in the early universe and contrast their characteristic observational signatures with those of vacuum tensor modes. The emphasis of my talk will be on physical intuition and analytic derivations, with the aim of making the subject accessible to non-specialists in the astrophysics community.

This seminar is jointly organized with the RIKEN Center for Sustainable Resource Science (CSRS).

Social evolution in aphids is tightly linked to the formation of galls on their host plants. Galls provide efficient colony defense and nutritionally rich feeding sites such that colony members need not forage outside, leading to high intra-group relatedness. Typically, social aphids form a gall on their primary host plant, after which winged morphs disperse to secondary host plants and establish a free-living, open colony. Remarkably, sterile soldier castes have independently evolved twice in these open colonies, where individuals live on plant surfaces without modifying their structure. These aphids raise intriguing questions about the prerequisites for eusocial evolution and the mechanisms by which two distinct social systems are maintained within a single genome.

In this talk, I will first provide an overview of the life cycle and the diversity of altruistic behaviors in gall-forming aphids, and then present our studies of the evolution of a sterile soldier caste in aphids inhabiting open colonies. From a developmental perspective, we tested the hypothesis that the sterile soldiers evolved through the co-option of pre-existing soldier phenotypes in a gall, based on similarity in morphology, transcriptome and behavior. From an ecological perspective, we investigated the kin structure and altruistic behavior of young nymphs in the open colonies of pre-eusocial species, and demonstrate that young aphids exhibit altruism by yielding feeding sites to older kin. Together, we propose that the open colonies of social aphids provide an ideal model system for studying the evolution of altruism.

Recent developments in quantum computers and related theoretical/technical advancements have brought attention to "new computational methods in quantum field theory" in the fields of high energy/nuclear physics.

Main targets of this school are graduate students and postdocs. This school provides opportunities to discuss recent research trends and their applications through lectures by experts and presentations by participants.

Lecturers:
Junichi Haruna (University of Osaka) "Introduction to Quantum Error Correction (tentative)"
Yoshimasa Hidaka (Yukawa Institute for Theoretical Physics/RIKEN iTHEMS) “Introduction to Hamiltonian Lattice Gauge Theory (tentative)”
Tokiro Numasawa (University of Tokyo) "Open Majorana system (tentative)"

The series of DEEP-IN meetings (Jan 26–30, 2026) are joint with UTokyo Institute for Physics of Intelligence (iπ), which is a multi-day scientific program bringing together researchers to explore quantum simulations, machine learning physics, and applications in particle and nuclear physics.

The tentative schedule is,
UTokyo-iπ Session (Venue: #512, Faculty of Science Bldg.1, School of Science, UTokyo)

Day 1: Jan 26 (Mon)
14:30–16:00 Onset of Bjorken flow in a quantum many-body simulation of the massive Schwinger model, Shuzhe Shi
Day 2: Jan 27 (Tue)
14:30–16:00 Physics of Diffusion Models, Gert Aarts
16:00–17:30 Discovering Symmetry from Energy-Based Diffusion Models, Jinyang Li

RIKEN-iTHEMS Session (Venue: Seminar Room #359, Main Research Building)

Day 3: Jan 28 (Wed)
14:30–16:00 Understanding Galactic Dark Matter with Generative Models, Sung Hak Lim
16:00–18:00 Free Discussion ML Physics-1
Day 4: Jan 29 (Thu)
10:00–11:30 Quantum Simulations of HEP and Beyond, Xingyu Guo
14:30–16:00 Physics of Machine Learning, Gert Aarts
16:00–17:30 Storage capacity of perceptron with variable selection, Yingying Xu

Day 5: Jan 30 (Fri)
11:00–14:00: Free Discussion ML Physics-2

This fifth workshop will bring together researchers exploring the effective field theory (EFT) framework in diverse cosmological contexts. Topics will include EFT formulations of interacting dark matter and dark energy, open EFTs for gravity, and multi-field inflationary dynamics. By highlighting recent progress and open questions, the workshop seeks to bridge insights from the early and late universe through the unifying language of EFT. In addition to the invited talks, the workshop will feature a panel discussion designed to promote interaction between the speakers and participants.

One of the key goals of this event is to foster collaboration among researchers working in neighboring fields, and to encourage participation from young and early-career researchers who are interested in, but may not yet have worked on, these themes. The workshop welcomes a broad audience with an interest in theoretical cosmology, gravitation, and quantum field theory.

The workshops are organised by the Cosmology Study Group at RIKEN iTHEMS.

Understanding how to gauge-fix open quantum field theories is essential for building consistent open frameworks for cosmology and gravity, where gauge symmetry must coexist with dissipation and noise and decoherence. I will present our recent work developing explicit top-down constructions of open effective field theories (EFTs) for gauge degrees of freedom, with particular emphasis on the role of gauge fixing. We implement BRST quantisation on the Schwinger-Keldysh contour and show that the in-in boundary conditions reduce the doubled global BRST symmetry to a single diagonal copy. This diagonal BRST symmetry is nevertheless sufficient to guarantee that the influence functional remains gauge invariant under two independent gauge transformations, retarded and advanced, independently of the choice of initial state, the presence of symmetry-breaking phases, and whether the gauge theory is Abelian or non-Abelian. We further clarify how this is compatible with the decoupling limit, in which the global advanced symmetry is generically broken by the state. I will conclude by outlining bottom-up implications, and how these principles provide a systematic route to causal, gauge-invariant open EFTs suitable for cosmological and gravitational applications.

Mathematical Application Research Team invites Riccardo Muolo fom Division of Fundamental Mathematical Science to this meeting. You are welcome to join the meeting.

Title: Dynamics beyond nodes: a topological framework for oscillatory dynamics on higher-order networks

Abstract: In recent years, increasing attention has been given to dynamical processes taking place on higher-order networks, where interactions are not limited to links, but may involve also higher-dimensional simplices [1]. While classical network models assume that state variables live on nodes and interact through links, many real systems — including brain, climate, and transportation systems — cannot be fully described within this node-centric perspective [2]. In this seminar, I will introduce the framework of higher-order networks and the concept of topological signals, namely, dynamical variables defined on simplices of higher dimensions. I will briefly present the basic tools required for this setting, including elementary notions of discrete calculus, discrete topology and geometric algebra, which serve as the mathematical foundation for modeling dynamical processes beyond the node-based paradigm.
Next, I will discuss models of oscillatory dynamics extended to this framework. First, I will present the topological Kuramoto model [3], in which phases are not restricted to nodes but may also be associated with links, and where the coupling arises from the combinatorial structure of the simplicial complex. Then, I will introduce the discrete Hodge Laplacian and the Dirac-Bianconi operator [4], the former generalizing diffusive interactions to the higher-order setting, while the latter provides cross-talk between signals defined on simplices of different dimensions. Finally, I will introduce the notion of Dirac-Bianconi driven oscillators, where the dynamics of node- and link-signals coexist, interact and may give rise to collective oscillatory behaviors [5].

The electromagnetic transients accompanying neutron-star mergers (NSMs), called kilonovae, are powered by the radioactive decay of freshly synthesized heavy elements. As such they should contain rich information about the ejected matter and the properties of the extremely dense meta-stable neutron-star remnant formed right after the collision. However, extracting such information from observed kilonova light curves and spectra remains a challenging endeavor, which requires sophisticated models of various hydrodynamic processes and neutrino transport effects, detailed knowledge of nuclear and atomic physics, as well as complex radiative transfer calculations. In this talk I will report recent efforts from our "HeavyMetal" collaboration aimed at deciphering kilonovae.

14:45-15:00 Teatime discussion

[15:00-16:00 The 31th MACS Colloquium]
Talk by Dr. Yujiro Eto (Associate Professor, Center for Science Adventure and Collaborative Research Advancement (SACRA), Graduate School of Science, Kyoto University)

[16:10-18:30 2025 MACS Achievement Report Meeting]
16:10-17:10 Flash Talks to report results
17:10-18:00 Poster Session by SG participating students

In this talk, I will discuss the problem of inferring an evolutionary tree from DNA sequence data. The main focus will be on the sample complexity of this problem---i.e., the question of how much data is required to achieve high probability of correct inference. After introducing a standard stochastic model of gene and DNA evolution, I will highlight some surprising features of DNA sequence data that complicate inference. Finally, I will present an impossibility result which takes the form of an information-theoretic lower bound on the minimum amount of data needed for accurate inference when genes exhibit variation in mutation rates. No prior knowledge of phylogenetics or information theory is assumed. Based on joint work with Sebastien Roch.

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].

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.

This joint workshop will bring together physicists and mathematicians who work with asymptotics and perturbation theory techniques. This includes theorists in cosmology, high energy physics, quantum gravity, solar physics, astrophysics.

Workshop overview
Over three days, there will be approximately 15 invited (1 hour slot) or contributed (20-30 min slot) talks about:
Fundamental asymptotics and perturbation theory techniques used in theoretical physics. Various applications of asymptotics and perturbation theory techniques in (wave transport or oscillation related) astrophysics and cosmology eigenvalue problems.

The workshop will also feature hands-on Mathematica and Python tutorials introducing:
Practical use of WKB methods in applied mathematics for any “Schrodinger-like” wave equations, Resummation methods in high energy theory, Deriving normal modes in stars, and their application to tidal evolution in binary star or planet systems, Eigenvalue problems in core collapse supernova theory.

The two fundamental questions—“What is quantum?” and “What is spacetime?”—are deeply intertwined. On one hand, the formulation and interpretation of quantum theory depend both implicitly and explicitly on our conceptions of time and space. On the other hand, we believe that fully taking into account the quantum character of nature will force us to revise our understanding of spacetime. These two conceptual problems lie at the heart of the unsolved challenge of how to quantize classical spacetime, and conversely, how (semi-) classical descriptions of spacetime emerge from quantum theory. Furthermore, if the entire matter-spacetime system is a kind of quantum many-body system, thermodynamics—which governs its statistical behaviors—should play a key role in elucidating these problems.

This workshop will discuss the question “How can quantum theory and spacetime be understood in a consistent manner?” from a fundamental and broad perspective. To tackle this challenge, we gather researchers in foundations of quantum theory, quantum gravity, and related fields from around the world, providing a "space and time" to share various ideas with open minds and engage in lively discussions. By exploring new concepts and principles, we hope to uncover directions to guide quantum theory over the next 100 years.

This workshop covers…

Foundations of quantum theory
Quantum gravity and emergence of spacetime
Formulation of semi-classical gravity
Experimental aspects of fundamental properties in nature and quantum gravity
Foundations of quantum many-body systems and thermodynamics
Other related topics are welcome.

We welcome short talk presentations and poster presentations.

This event is a workshop jointly organized by KEK Theory Center and RIKEN iTHEMS.

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.