Cosmology Group Seminar
10 events
This seminar series is associated with the Cosmology Study Group.
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Seminar
Analytical parametrization of the primordial power spectrum in effective Loop Quantum Cosmology
April 20 (Mon) 13:00 - 14:00, 2026
Almudena Sánchez Guillén (Ph.D. Student, Institute of Structure of Matter (IEM-CSIC), Spain)
We investigate the imprints on the angular power spectra of cosmological perturbations of a pre-inflationary bounce phase, as described by the hybrid and dressed metric approaches to loop quantum cosmology. For this purpose, we derive a new parametrization of the primordial power spectrum at the end of the inflationary regime. Apart from slow-roll coefficients and cosmological parameters that are present in the standard cosmological scenario without quantum modifications, this parametrization additionally depends only on pre-inflationary physics. More specifically, we find a dependence on the number of e-folds during the bounce epoch and on a characteristic suppression scale which, given the e-folds accumulated during cosmic evolution, is determined by the energy density at the bounce.
Venue: via 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
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
Gauge fixing for open systems: A pathway to open gravity EFTs
January 30 (Fri) 14:00 - 16:00, 2026
Maria Mylova (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU))
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.
Venue: #445-447, 4F (Hybrid), Main Research Building
Event Official Language: English
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Seminar
iTHEMS Cosmology Forum n°5 - Effective Field Theory approaches across the Universe
January 29 (Thu) 10:00 - 17:00, 2026
Katsuki Aoki (Research Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
Toshifumi Noumi (Associate Professor, Graduate School of Arts and Sciences, The University of Tokyo)
Lucas Pinol (CNRS Researcher, Laboratoire de Physique de l’École Normale Supérieure (LPENS), CNRS/École Normale Supérieure, France)This fifth workshop will bring together researchers exploring the effective field theory (EFT) framework in diverse cosmological contexts (https://indico2.riken.jp/event/5435/overview). 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.
Venue: Okochi Hall
Event Official Language: English
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Seminar
Two-component dark matter scenario in a pseudo-Nambu-Goldstone dark matter model - Tomohiro Abe
December 18 (Thu) 14:00 - 15:30, 2025
Tomohiro Abe (Assistant Professor, Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI), Nagoya University)
The WIMP dark matter (or thermal dark matter) is one of the leading candidates for dark matter and is widely studied. On the other hand, recent progress in direct detection experiments places severe constraints on WIMP dark matter models. Pseudo-Nambu-Goldstone boson dark matter models (pNG DM models) can explain the measured value of the dark matter energy density via the freeze-out mechanism and also naturally suppress the dark matter-nucleon scattering. The amplitudes for the processes are proportional to the momentum transfer squared from the dark sector to the visible sector; It is suppressed by t ~ 0 for the dark matter-nucleon scattering process, while it is proportional to s ~ 4 m_DM^2 for annihilation processes. The simplest realization faces the domain wall problem and needs to be extended. Several models have been proposed, but each model brings other issues, a large hierarchy in new energy scales, parameter tunings, and a Landau pole of a new gauge coupling. In this talk, I would like to propose a new pNG DM model based on SU(2)_gauge times SU(2)_global symmetry that overcomes those issues. The model predicts a two-component DM scenario, where one is pNG, and the other is an ordinary WIMP. I will show that the effective spin-independent cross section in the direct detection experiments is smaller than the current upper bound and larger than the prospect of the Darwin project in a wide range of parameter space in the two-component DM scenario.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Quantum Information in Scattering: From Amplitude Poles to Entanglement Features
December 4 (Thu) 14:00 - 16:00, 2025
Chon Man Sou (Postdoctoral Researcher, Department of Physics, Tsinghua University, Beijing, China)
Probing quantum entanglement in high-energy collisions has recently become a rapidly growing direction in particle physics, aiming to measure quantum correlations at the highest energy scales accessible to humans. A key question is how such entanglement relies on the analytic structure of scattering amplitudes. In this talk, I will show that the pole structure, associated with intermediate heavy particles, leads to distinctive entanglement features. When a heavy particle mediates inelastic scatterings with three or more final particles, the entanglement entropy between its decay products and the rest exhibits a universal dip as the energy increases, reflecting the limited information flow through on-shell heavy particles and signaling entanglement suppression in the heavy-particle-dominated regime. This reveals entanglement structures beyond the usual “area-law” behavior of 2-to-2 processes. Finally, I will comment on possible ways to probe these features experimentally through analyses of final-state phase-space distributions. This talk is based on JHEP 10 (2025) 003 [arXiv: 2507.03555].
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
The Uchuu simulations data set: large-scale structures and galaxies - Tomoaki Ishiyama
November 13 (Thu) 14:00 - 15:30, 2025
Tomoaki Ishiyama (Associate Professor, Digital Transformation Enhancement Council, Chiba University)
I will introduce the Uchuu suite of large high-resolution cosmological N-body simulations. The largest simulation, named Uchuu, consists of 2.1 trillion dark matter particles in a box of side-length 2.0 Gpc/h, with particle mass of 3.27e8 Msun/h. The highest resolution simulation, Shin-Uchuu, consists of 262 billion particles in a box of side-length 140 Mpc/h, with particle mass of 8.97e5 Msun/h. Combining these simulations, we can follow the evolution of dark matter haloes and subhaloes spanning those hosting dwarf galaxies to massive galaxy clusters across an unprecedented volume from very high-z. We release N-body data (halo/subhalo catalogs and merger trees) and mock galaxy/AGN catalogs constructed using various models, which cover objects from z=0 to very high-z. These catalogs open a new window on understanding the large-scale structures and galaxy formation. In this presentation, I will also introduce results of cosmological simulations adopting a time-varying dark energy, conducted on the supercomputer Fugaku.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Primordial black holes formation and its origin in inflation - Jianing Wang
October 30 (Thu) 14:15 - 16:00, 2025
Jianing Wang (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)
Primordial black holes (PBHs) are thought to form through gravitational collapse of regions with excessively large density in the early universe, and they could serve as seeds for the formation of galaxies. They are also considered one of the important candidates for cold dark matter (DM). Detecting and constraining the abundance of PBHs can provide an effective constraint on realistic inflationary models. In this talk, I will combine inflation models with gravitational waves (GWs) to discuss cosmological phenomena related to primordial black holes. In particular, I will emphasize a simplified toy model of inflation, which naturally enhances the small-scale scalar perturbations by gluing together two linear potentials with different slopes. The enhanced perturbations can not only generate primordial black holes but also emit gravitational waves through higher-order perturbations. This research demonstrates the significant potential of primordial black hole studies, and it naturally leads to a crucial question of how to accurately estimate the PBH abundance. In the latter part of the talk, I will introduce how to use peaks theory to estimate the abundance of primordial black holes. Our new method works well for any form of the power spectrum, and considering the use of more systematic statistical methods, we believe it is currently the most precise approach in the academic community.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Positive neutrino masses with DESI DR2 via matter conversion to dark energy
August 21 (Thu) 14:30 - 16:00, 2025
Kevin Croker (Assistant Research Scientist, Arizona State University, USA)
The Dark Energy Spectroscopic Instrument (DESI) is a massively parallel spectroscopic survey on the Mayall telescope at Kitt Peak, which has released measurements of baryon acoustic oscillations determined from over 14 million extragalactic targets. We combine DESI Data Release 2 with CMB datasets to search for evidence of matter conversion to dark energy (DE), focusing on a scenario mediated by stellar collapse to cosmologically-coupled black holes (CCBH). In this physical model, which has the same number of free parameters as LambdaCDM, DE production is determined by the cosmic star formation rate density (SFRD), allowing for distinct early- and late-time cosmologies. Using two SFRDs to bracket current observations, we find that the CCBH model: accurately recovers the cosmological expansion history, agrees with early-time baryon abundance measured by BBN, reduces tension with the local distance ladder, and relaxes constraints on the summed neutrino mass. For these SFRDs, we find a peaked positive summed mass < 0.149 eV (95% confidence) and summed mass equal to 0.106 +0.050 -0.069 eV respectively, in good agreement with lower limits from neutrino oscillation experiments. A peak in positive summed neutrino mass results from late-time baryon consumption in the CCBH scenario and is expected to be a general feature of any model that converts sufficient matter to dark energy during and after reionization.
Venue: #359, Seminar Room #359
Event Official Language: English
10 events