IBS CTPU-CGA 2025 Summer School and Workshop for High Energy Physics and Cosmology in Korea

28 Jul 2025, 12:00 → 1 Aug 2025, 23:30 Asia/Seoul

Jeonju, Best Western Hotel

The purpose of this school and workshop is to encourage IBS CTPU-CGA and Korean High Energy Physics and Cosmology Community to exchange ideas, to promote collaboration, and to discuss recent developments in the fields through review talks, lectures, and general talks.

The list of invited speakers is

Sebastian Bahamonde (IPMU/IBS CTPU-CGA)
Ho Seong Hwang (SNU)
Ji-Hoon Kim (SNU)
Keun Young Kim (GIST)
Sangmin Lee (SNU)
Atsuhisa Ota (Chongqing U)
David Parkinson (KASI)
Hyun Seok Yang (GIST)
Piljin Yi (KIAS)
Junggi Yoon (Kyunghee U)

and many more postdocs and students.

 

Organizing Committee

Mohammad Ali Gorji (IBS CTPU-CGA)
Dong-Won Jung (IBS CTPU-CGA)
Masahide Yamaguchi (IBS CTPU-CGA)

 

 

 

Monday, 28 July

17:00 → 18:00 Introduction to CTPU-CGA and brief self-introductions to break the ice: chaired by Masahide Yamaguchi

18:00 → 20:00 Welcome dinner

Tuesday, 29 July

10:00 → 11:00 35+25: chaired by Sangmin Lee

10:00 Lie, Noether, Kosmann, and Diffeomorphism Anomalies Revisited

Speaker: Prof. Piljin Yi (KIAS)

10:35 Energy-Momentum via Lie and Kosmann

Speaker: Taeyeon Kim (SNU/KIAS)

11:00 → 11:30 break

11:30 → 12:30 35+25: chaired by Ho Seong Hwang

11:30 Cosmology from measurements of the matter-radiation-equality scale

Speaker: Prof. David Parkinson (KASI)

12:05 Delensing CMB B-modes with Rubin LSST survey

Speaker: Chandra Shekhar Saraf (KASI)

12:30 → 14:30 lunch (bibimbap)

14:30 → 16:20 35+75: chaired by Mohammad Ali Gorji

14:30 Emergent Spacetime from Large N Model

Speaker: Junggi Yoon (Kyunghee U)

15:05 Krylov Complexity in Quantum Field Theories and Cosmology

Speaker: Vinay Malvimat (Kyunghee U)

15:30 Inferring properties of dark energy from the large scale structure of the universe

The structure of galaxies we see in the sky today was seeded in the very early universe. The evolution from initial tiny density perturbations to the cosmic web of voids, clusters and filaments of galaxies tells us about what the universe is made of. There are many ways to utilise this large scale structure to probe cosmology, but measuring the growth rate itself in the nearby universe is particularly powerful. In this talk I will cover how we can measure the growth rate from the velocities of supernovae. We now have access to a large set of galaxy and velocity data, with much more to come in the near future from various different surveys. Using these new and upcoming data, we may be able to distinguish between the effects of basic cosmological constant dark energy or modifications to our theory or gravity.

Speaker: Anthony Scott Carr (KASI)

15:55 Holographic Entanglement Entropy in the FLRW Universe

We compute a holographic entanglement entropy via Ryu-Takayanagi prescription in the three-dimensional Friedmann-Lemaître-Robertson-Walker universe. We consider two types of holographic scenarios analogous to the static patch holography and the half de Sitter holography, in which the holographic boundary is timelike and placed in the bulk. We find in general that the strong subadditivity can be satisfied only in the former type, and in addition, the holographic boundary has to fit inside the apparent horizon. Also, for the universe filled with an ideal fluid of constant equation of state w<−1, the condition is sharpened as that the holographic boundary has to fit inside the event horizon instead. These conditions provide a necessary condition for the dual quantum field theory to be standard and compatible with the strong subadditivity. This talk is based on the paper arXiv:2504.10457.

Speaker: Fumiya Sano (IBS CTPU-CGA)

16:20 → 16:50 break

16:50 → 18:30 100: chaired by Atsuhisa Ota

16:50 A photon cloud induced from an axion cloud

Speaker: Zi-Yu Tang (IBS CTPU-CGA)

17:15 Signatures of Primordial Gravitational Waves on the Large-Scale Structure of the Universe.

Primordial gravitational waves (GWs), beyond their direct detection prospects, can induce second-order scalar perturbations. These tensor-induced scalar modes evolve similarly to standard matter perturbations and leave distinct imprints on the large-scale structure (LSS). In this talk, I will present a detailed study of these effects, including analytical results for the evolution of induced density contrasts across radiation- and matter-dominated eras, and the role of GWs energy density fluctuations as a source. I will also discuss how the resulting non-Gaussianity reflects the nature of the primordial GWs spectrum, ranging from scale-invariant to sharply peaked models. These findings suggest a novel observational avenue to probe primordial GWs using galaxy surveys and LSS data, opening a complementary window to the early Universe beyond traditional GWs detectors.

Speaker: Pritha Bari (IBS CTPU-CGA)

17:40 Superradiant Bosons Driving Supermassive Black Hole Mergers

Ultralight bosons, predicted in scenarios beyond the Standard Model and viable dark matter (DM)
Candidates can form superradiant clouds around spinning black holes, influencing their dynamics.
Using century-long monitored OJ287 supermassive black hole binary, we set the first DM–DM-independent,
dynamical constraints on their masses μ = (8.5−22)×10−22 eV. These dynamical constraints, driven
by boson cloud friction, are robust against DM-model uncertainties and offer a novel ultralight boson
probe. We show that analogous superradiant dynamics across the cosmic population of supermassive
black hole systems could help resolve the final-parsec evolution stalling problem and imprint a detectable
suppression and break in the gravitational wave background.

Speaker: Huiyu Zhu (IBS CTPU-CGA)

18:05 Ultra-Light Dark Matter and its Dynamical Friction

Speaker: Hyeonmo Koo (U of Seoul)

Wednesday, 30 July

10:00 → 11:00 35+25: chaired by Junggi Yoon

10:00 Quantum physics seen through a gravitational lens

Speaker: Prof. Keun-Young Kim (GIST)

10:35 String theory and black hole microstructure: Towards resolving horizons and singularities

Speaker: Shaun D. Hampton (KIAS)

11:00 → 11:30 break

11:30 → 12:30 35+25: chaired by David Parkinson

11:30 Mapping the Universe of Dark Matter: Tensions in the Standard Cosmological Model

Speaker: Prof. Ho Seong Hwang (SNU)

12:05 Peak Sphericity of non-Gaussian random fields

Speaker: Michiru Uwabo-Niibo

12:30 → 18:00 Free discussion

18:00 → 20:00 Banquet

Thursday, 31 July

10:00 → 11:00 35+25: chaired by Piljin Yi

10:00 Scattering Amplitudes for Gravitational Wave

Speaker: Prof. Sangmin Lee (SNU)

10:35 Gravitational Binary Dynamics in Post-Minkowskian Expansion and Poincaré Symmetry

Speaker: Hojin Lee (SNU)

11:00 → 11:30 break

11:30 → 12:30 35+25: chaired by Dong-Won Jung

11:30 Cosmological stimulated emission

We study stimulated emission and absorption of gravitons in a squeezed vacuum
state immersed in a thermal radiation bath. Employing one-loop
interaction-picture perturbation theory, we track the time evolution of the
graviton number operator and its expectation value in the squeezed vacuum,
which characterizes the inflationary graviton state. In a Minkowski background
with a thermal bath as a toy example, we demonstrate that the net graviton
emission or absorption rate depends sensitively on the initial squeezing
parameters. As a thought experiment, we consider LIGO/Virgo-like detectors
operating in radiation at temperatures of order 0.1 GeV and find that graviton
occupation numbers at frequencies of order 100 Hz can be significantly
enhanced, suggesting a novel mechanism for amplifying gravitational-wave
signals. Although these conditions exceed current experimental capabilities,
they point toward potential future advances in detection. Extending our
analysis to an expanding, radiation-dominated universe, we show that subhorizon
gravitons undergo stimulated absorption, while superhorizon modes exhibit
secular logarithmic growth, indicating the breakdown of perturbative methods
and motivating further investigation. These findings open a new direction for
exploring graviton coherence effects in realistic cosmological and laboratory
settings.

Speaker: Prof. Atsuhisa Ota (Chongqing U)

12:05 Cosmological Implications of Unification: Topological Structures and Gravitational Waves

Speaker: Rinku Maji (IBS CTPU-CGA)

14:30 → 16:20 35+75: chaired by Hyun Seok Yang

14:30 Cosmic Mystery of Supermassive Black Holes: Perspectives in the Era of High-resolution Simulations

Speaker: Ji-Hoon Kim (SNU)

15:05 Decay and lifetime of oscillons coupled to an external scalar field: Insights from instability band analysis

Oscillons are long-lived, spherically symmetric solitons that can arise in real scalar field theories with potentials shallower than quadratic ones. They are considered to form via parametric resonance during the preheating stage after inflation and have extended lifetimes. However, the estimation of their lifespan becomes complicated when taking into account the interactions between the inflaton field and other fields, as naturally expected in realistic reheating scenarios. In this study, we investigate how the lifetime of a single oscillon is affected by the coupling to the external real scalar field. By numerically computing the instability bands of the external field with the inhomogeneous oscillon profile as background, we show that the resonance behavior depends intricately on the coupling strength and shape of the oscillon. We analyze distinct instability mechanisms that dominate across different regimes of the coupling strength and oscillon shapes. Especially, we show that the parametric resonance fails to occur when the oscillon size is too limited to drive enhancement of the external field. Furthermore, our simulations show that as the oscillon loses energy, the exponential growth of the external field can terminate before the oscillon reaches its critical energy for collapse, which indicates that the external field does not necessarily lead to rapid destruction of oscillons even in the presence of strong coupling or with large amplitudes. These results suggest that oscillons can remain long-lived across a wide range of coupling strengths, with potential implications for their role in cosmological evolution.

Speaker: Siyao Li (IBS CTPU-CGA)

15:30 Detecting the ISW effect using CMB-CIB cross-correlation

Speaker: Dongkok Kim (SNU)

15:55 Cosmological Aspects of Stochastic Inflation

Speaker: Tomotaka Kuroda (IBS CTPU-CGA)

16:20 → 16:50 break

16:50 → 18:30 100: chaired by Nils Albin Nilsson

16:50 Searching for Dark Neighbors: DL-based local DM density estimation in sub-kpc scale

Speaker: Yigon Kim (SNU)

17:15 Constraining the Dark Energy Equation of State with the Extended Alcock–Paczyński test

Speaker: Minseong Kwon (SNU)

17:40 Realizing Non-Singular Cosmologies within Horndeski Gravity

We present a minimal setup within the framework of Horndeski gravity that describes a nonpathological Genesis scenario. Our setup allows for a fully stable transition to the kination epoch, during which General Relativity (GR) is restored. This Genesis scenario circumvents the no-go theorem, albeit at the cost of encountering the risk of strong coupling in the past. Interestingly, our scenario admits two distinct regimes for the background evolution of the Hubble parameter during the Genesis phase: one with power-law behavior and one with manifestly non-power-law behavior. We explicitly show that, in both regimes, our model remains within unitarity bounds. However, in most cases, the resulting tensor spectrum is blue-tilted. We then investigate an alternative model in which the Genesis phase is followed by Starobinsky inflation. We find that corrections from the Genesis phase to Starobinsky inflation can account for the ACT data.

Speaker: Pavel Petrov (IBS CTPU-CGA)

18:05 Primordial Black Hole: Future Perspective in Gravitational Wave Astronomy

Speaker: Qianhang Ding (IBS CTPU-CGA)

Friday, 1 August

10:00 → 11:00 35+25: chaired by Sebastain Bahamonde

10:00 Classical Geometry from Matrix Model

Speaker: Hyun Seok Yang (GIST)

10:35 Stability and cosmological dynamics of the Bumblebee field

Speaker: Nils Albin Nilsson (IBS CTPU-CGA)

11:00 → 11:30 break

11:30 → 12:30 35+25: chaired by Masahide Yamaguchi

11:30 Foundations of Metric Affine theories of gravity and black holes

Speaker: Prof. Sebastian Bahamonde (IPMU/CTPU-CGA)

12:05 The Tale of Cosmological Collider

Speaker: Yuhang Zhu (IBS CTPU-CGA)