C-lab Seminar 2025 | Seminar | Nagoya University Cosmology Group (C-lab)

C-lab Seminar 2026

日本語

Coming Seminars

Seminars in July

Speaker
Kiyotomo Ichiki

Date/Place
10:30-, 1(Wed), July. @ES606

Title
CMB B-mode Foreground and the Deltamap Methods

Abstract
The search for primordial gravitational waves through CMB B-mode polarization is one of the most direct observational probes of inflation in the very early Universe. However, the expected signal is extremely faint and is inevitably contaminated by polarized foreground emission from our Galaxy, mainly synchrotron radiation and thermal dust emission. Understanding and removing these foregrounds is therefore not a technical detail, but a central scientific challenge for future CMB polarization experiments such as LiteBIRD. In this seminar, I will give an overview of why foreground emission limits the detection of primordial B modes, and introduce the “Delta-map” method, a foreground-removal technique developed in Japan. This method, proposed by Ichiki et al. and further developed by Minami and Ichiki in PTEP, reconstructs spatial variations of foreground spectral properties and enables a more flexible separation of CMB and Galactic components.

Seminars in June

Speaker
Masaya Inoue

Date/Place
10:30-, 24(Wed), June. @ES606

Title
Can Ultralight Axion Dark Matter Seed Cosmic Magnetic Fields After Recombination?

Abstract
The existence of weak magnetic fields of approximatel B≳10^(-17)[G] in intergalactic regions of the Universe has been suggested based on gamma-ray observations. While the generation of such magnetic fields is difficult to explain through conventional astrophysical processes, it can be consistently accounted for if they were produced in the early Universe or after recombination. Therefore, a cosmological origin is considered a promising possibility. In a previous study [1], it was argued that ultralight axions, which are light pseudoscalar dark matter candidates, can oscillate as a homogeneous classical field after recombination and amplify one helicity mode of the gauge field through their interaction with the electromagnetic field, thereby generating magnetic fields on cosmological scales. However, the discussion in Ref. [1] is mainly based on order-of-magnitude estimates, and numerical verification of the equations of motion for the axion field and gauge field in an expanding Universe has not been sufficiently carried out. In addition, Ref. [1] treats the post-recombination Universe using a vacuum approximation and neglects the effect of conductivity arising from the residual ionized component. In contrast, Ref. [2] pointed out that the friction term associated with conductivity strongly suppresses the amplification of electromagnetic fields. In this presentation, assuming cosmic expansion during the matter-dominated era, we numerically solve the equations of motion governing the time evolution of the axion field and gauge field, and investigate the magnetic-field amplification for different values of the mass, coupling constant, and wavenumber. As a result, under the vacuum approximation, we confirm that magnetic-field amplification sufficient to satisfy the observational lower bound cannot be obtained within the observationally allowed ranges of the mass and coupling constant. On the other hand, outside these constrained regions, one helicity mode is exponentially amplified, confirming the amplification mechanism proposed in Ref. [1]. Based on these results, we compare the order-of-magnitude estimates in Ref. [1], our numerical calculations based on the equations of motion, and the study in Ref. [2] that takes conductivity into account. We then discuss whether the post-recombination magnetogenesis mechanism driven by ultralight axion dark matter can serve as the origin of magnetic fields in intergalactic regions under observationally allowed conditions. [1] R. Brandenberger, J. Fröhlich and H. Jiao, “Cosmological Magnetic Fields from Ultralight Dark Matter( arXiv:2502.19310) [2] R. Sharma, S. Majumdar and D. Sachdeva, “Suppressed Magnetogenesis from Ultralight Dark Matter due to Finite Conductivity (arXiv:2604.17230)

Speaker
Kauri Ueda

Date/Place
10:30-, 24(Wed), June. @ES606

Title
Numerical Study and Detection Prospects of High-Frequency Gravitational Waves from Reheating

Abstract
This presentation discusses the production and possible detection of high-frequency gravitational waves generated during the reheating phase after inflation. First, the standard inflationary gravitational wave background is reviewed by deriving the evolution equation for tensor perturbations from the Einstein equations and relating the primordial tensor power spectrum to the present gravitational wave energy density spectrum. On subhorizon scales, the tensor modes oscillate rapidly, so their contribution to the energy density can be evaluated using an oscillation average. This provides the connection between the theoretical tensor spectrum and observable quantities. The main focus is then placed on gravitational wave production during reheating. Starting from the Boltzmann equation in a homogeneous and isotropic universe, the phase-space distribution of gravitons is computed by evaluating the collision terms associated with particle reactions. Four production channels are considered: inflaton annihilation, bremsstrahlung from inflaton decay, radiation-catalyzed processes, and radiation scattering in the thermal bath. For each process, the invariant scattering cross section is obtained from the corresponding Feynman diagrams, allowing the gravitational wave spectrum to be calculated numerically. The resulting spectra are compared with previous review results for several values of the coupling parameter y. These computations confirm that reheating can generate gravitational waves in the high-frequency region, typically around the GHz scale. Finally, possible experimental approaches to such high-frequency gravitational waves are introduced, especially resonant cavity searches and photon regeneration experiments based on the inverse Gertsenshtein effect. Although present sensitivities are still far from the predicted signals, these methods provide important theoretical and experimental directions for future searches.

Speaker
Shohei Kanda

Date/Place
10:30-, 17(Wed), June. @ES606

Title
Testing the Cosmological Spatiotemporal Variation of the Fine-Structure Constant Using Subaru/PFS Spectroscopic Observations of [OII] and [OIII] Emission-Line Galaxies

Abstract
The fine-structure constant α is a dimensionless constant that sets the strength of the electromagnetic interaction, and within the Standard Model of particle physics its value is taken to be invariant in space and time. To date, tests have been carried out using laboratory atomic-clock experiments as well as cosmological-scale probes such as quasar absorption lines (e.g., the Many-Multiplet method) and the cosmic microwave background, all of which have suggested its invariance to high precision. Some theories, however, predict a cosmological temporal variation of α. Moreover, if such a temporal variation of α is allowed, it could offer new insights into the nature of dark matter and the Hubble tension problem, making such tests highly significant from the standpoints of both fundamental physics and cosmology. In this study, we test the temporal variation of α on cosmological scales using emission-line galaxy spectra obtained with the Prime Focus Spectrograph (PFS) on the Subaru Telescope. Exploiting the fact that the wavelength separation of the [OII] λλ3726,3729 doublet depends on α², we place statistical constraints on the temporal variation across the broad redshift range 0.6 < z < 2.4. In our method, we use night-sky (airglow) emission lines as the reference for wavelength calibration. These lines, which originate from emission in the Earth's atmosphere, have known rest-frame wavelengths and are detected simultaneously with the observations, allowing us to correct for systematic errors. By combining this approach with the large PFS sample, we aim to place stringent constraints on Δα/α over a wider redshift range and with greater statistical precision than achieved in previous work.

Speaker
Daisuke Watanabe

Date/Place
10:30-, 17(Wed), June. @ES606

Title
Exploring the statistical anisotropy of primordial curvature perturbations with pulsar timing arrays

Abstract
Recent pulsar timing array observations have provided evidence for a stochastic gravitational-wave background, but its origin remains unresolved. Although supermassive black hole binaries are a leading astrophysical candidate, scalar-induced gravitational waves are also a promising cosmological candidate, offering a way to probe small-scale primordial curvature perturbations.Anisotropy provides an additional way to test such a cosmological origin. In PTAs, gravitational waves perturb pulse arrival times, and the resulting deviations from the timing model are called timing residuals. Spatial correlations of these residuals between pulsar pairs are described by the overlap reduction function. If the background is generated by statistically anisotropic primordial perturbations, this anisotropy can leave characteristic signatures in the PTA correlation pattern. Motivated by this, I review Ref. [1], which studies scalar-induced gravitational waves generated from dipole-type anisotropic primordial curvature perturbations. This work shows that primordial anisotropy induces anisotropic scalar-induced gravitational waves and leads to frequency-dependent overlap reduction functions. I summarize the motivation, theoretical framework, modification of the overlap reduction function, and constraints obtained from current PTA data. [1] F. Xie, Z.-C. Zhao, Q.-H. Zhu and X. Li, “Exploring the statistical anisotropy of primordial curvature perturbations with pulsar timing arrays, ”arXiv:2604.21642 (2026).

Speaker
Tomoya Honke

Date/Place
10:30-, 17(Wed), June. @ES606

Title
Investigating Systematic Effects of Fiber Assignment on Cosmological Analysis Using PFS HOD Mocks

Abstract
The spatial distribution of galaxies is an important observable for studying the large-scale structure of the Universe. By measuring statistics such as the correlation function and the power spectrum, we can extract cosmological information from BAO and RSD. The Prime Focus Spectrograph (PFS) on the Subaru Telescope is a next-generation multi-object spectroscopic survey. It aims to measure the three-dimensional distribution of high-redshift galaxies over a wide redshift range. However, in PFS, the target density is high, and not all target galaxies can be assigned to fibers at once. Each fiber has physical size and a limited patrol region, so nearby targets cannot always be observed at the same time. For this reason, PFS uses robotic fiber positioners to change the fiber configuration for each observation. However, this fiber assignment process can change the observed galaxy sample from the original target sample. The goal of this study is to evaluate the systematic effects of fiber assignment on galaxy clustering measurements and cosmological parameter estimation in PFS. To do this, I construct mock galaxy catalogs for PFS using an HOD model, and compare the galaxy distribution and clustering signals before and after fiber assignment. Through this comparison, I investigate how observational selection effects caused by fiber assignment can affect cosmological analysis.

Speaker
Shohei Saga

Date/Place
10:30-, 10(Wed), June. @ES606

Title
Post-collapse Lagrangian perturbation theory

Abstract
The gravitational collapse of collisionless matter leads to shell-crossing singularities that challenge the applicability of standard perturbation theory. Here, we present the first fully perturbative approach in three dimensions by using Lagrangian coordinates that asymptotically captures the highly nonlinear nature of matter evolution after the first shell-crossing (Post-collapse Lagrangian perturbation theory, PCPT). We validate the PCPT predictions against high-resolution Vlasov-Poisson simulations and demonstrate that PCPT provides a robust framework for describing the early stages of post-collapse dynamics. Although I will introduce these our recent results, most of the talk will be devoted to a broader review of the dynamics of cold dark matter and recent theoretical developments in this field.

Speaker
Ziang Yan (Nagoya University)

Date/Place
11:00-, 3(Wed), June. @ES606

Title
The star formation, dust, and abundance of galaxies with CIB cross-correlations

Abstract
The cosmic infrared background (CIB) is the accumulated infrared radiation mainly generated from dust emissions in star-forming galaxies. It bears rich information about star formation history, dust thermodynamics, and galaxy abundance in the distant Universe. The CIB auto- and cross-correlations with other large-scale structure (LSS) tracers have been used to probe the star formation, dust, and abundance of galaxies. In this talk, I will present our recent works (https://arxiv.org/abs/2204.01649, https://arxiv.org/abs/2310.10848) on constraining cosmic star formation history, dust spectral energy distribution (SED), and halo occupation (HOD) model with CIB-galaxy cross-correlations. With the galaxy data from KiDS and unWISE catalogs, we measured very significant (43\sigma with KiDS and 194\sigma with unWISE) cross-correlation signals and made informative constraints on star formation, dust SED, and galaxy HOD. I will also present a forecast on CIB cross-correlations with data from future galaxy surveys. This study, together with related studies, has shown that 1) CIB is a powerful tool to explore the aforementioned topics, and 2) CIB x LSS are generally consistent with IR flux-based studies. We conclude that our understanding of stars, dust, and galaxy abundance from different observations is reaching a converged picture. Future datasets from CSST, LSST, Euclid, etc will yield more precise CIB cross-correlations, thus we will need to improve the CIB models to yield more accurate and comprehensive constraints.

Seminars in May

Speaker
Shingo Akama

Date/Place
11:00-, 27(Wed), May. @ES606

Title
Observational Predictions of General Bounce Cosmology

Abstract
Inflation is widely regarded as the most successful paradigm for the early universe. Nevertheless, alternative scenarios have also been studied, motivated in particular by the conceptual problems of inflation, such as the initial singularity problem and the trans-Planckian problem. In this talk, I will first introduce various aspects of bouncing cosmology as an alternative to inflation, including its successes and challenges. In the latter half, I will present a general framework of bouncing cosmology that admits a parameter space capable of explaining the observed CMB fluctuations, as inflation does. Finally, among its observational predictions, I will discuss the statistical properties of CMB B-mode polarization.

Speaker
Federica Tarsitano

Date/Place
11:00-, 20(Wed), May. @ES606

Title
Optical-to-near-infrared studies of AGN and dual AGN systems with Euclid and the HSC-Niji survey

Abstract
Supermassive black holes (SMBHs), almost ubiquitous in galaxies, are polyglot citizens of the Universe: we observe them igniting as Active Galactic Nuclei (AGN) across the electromagnetic spectrum, and potentially hear the ripples in spacetime, the gravitational waves (GW), they create in collisions. The upcoming Laser Interferometer Space Antenna (LISA) mission will be sensitive to the GW signals from SMBH coalescence, complementing Pulsar Timing Array (PTA) observatories. The ESA Euclid space mission, surveying billions of galaxies in the optical to near-infrared (NIR) with unmatched high-resolution imaging and wide field of view, will bring remarkable advances in the study of the co-evolutionary path of SMBHs and their host galaxies. A fundamental yet poorly explored stage of this evolution is represented by dual AGN systems, which trace the pathway to SMBH coalescence, and can be used to set empirical priors for LISA GW events. Despite their high scientific value, instrumental challenges have limited the number of dual AGN detections confirmed so far, as exquisite spatial and spectral resolution is needed to characterise the galaxy nuclear light, and most surveys lack the necessary statistical power. In this talk, I will discuss recent efforts to build up the first systematic census of dual AGN in Euclid, and to advance our understanding of AGN activity and feedback by exploiting unprecedented synergies with HSC-Niji, a new medium-band survey with the Subaru telescope. The latter will also enable the study of AGN-linked populations, such as Little Red Dots, as well as cosmology-focused analyses, including the multi-wavelength characterisation of galaxy shapes and more accurate galaxy SED reconstruction and photo-z estimates, all relevant for weak-lensing analyses in current and future Stage IV surveys.

Speaker
Shintaro Yoshiura

Date/Place
11:00-, 13(Wed), May. @ES606

Title
Probing the Early Universe with Low-Frequency 21-cm Signals: From Cosmic Reionization to the Dark Ages

Abstract
After the birth of the Universe, the universe went through three important epochs: the Dark Ages, when no astronomical objects had yet formed; the Cosmic Dawn, when the first stars were born; and the Epoch of Reionization, when ultraviolet radiation from young galaxies ionized hydrogen in intergalactic medium. Although these epochs are crucial for understanding the early history of galaxy formation and cosmology, many aspects remain observationally unexplored. One promising approach is to observe the 21-cm line, a radio signal emitted by neutral hydrogen atoms at a rest frequency of approximately 1420 MHz. By detecting the signal from hydrogen that filled the early Universe, we can probe the cosmology of the Dark Ages and the formation of the first cosmic structures. Due to the expansion of the Universe, this 21-cm signal is redshifted to lower frequencies and is observed as low-frequency radio signal below about 200 MHz. In particular, for the Dark Ages, the 21-cm line is almost the only available observational probe. To correctly extract physical information from 21-cm observations, researchers are developing cosmological models as well as instruments and analysis methods that can deal with foreground emission, such as radio emission from our Galaxy. In this seminar, I will first give a brief introduction to studies of the distant Universe using the 21-cm line. I will then explain how the latest observational experiments are working toward the detection of this signal.