Coming Seminars
Seminars in July
Speaker
Yuichiro Tada
Date/Place
13:00-, 10th(Wed.), July. @ES606
Title
Scalar-induced gravitational wave and cosmological crossover
Abstract
Our universe has experienced phase transitions/crossovers several times throughout its thermal history. Gravitational waves (GWs) as a state-of-the-art probe can survey such a history thanks to their high penetration ability. In particular, GWs sourced by oscillations of density contrasts, called scalar-induced GWs (SIGWs), have sensitivity not only to the background expansion rate but also to the plasma sound speed and hence could serve more information of crossovers. Furthermore, they are closely related to the exotic astrophysical object: primordial black holes. In this talk, I’ll review the researches in this context by several authors including Ryoto and Albert, and introduce my ongoing project.
Seminars in June
Speaker
Kohei Onoda
Date/Place
13:00-, 26th(Wed.), June. @ES606
Title
Progress report for summer school
Abstract
The Lyman-alpha (Lyα) forest is a powerful tool for probing the structure formation of the intergalactic medium (IGM) at high redshifts (z > 2). Studying this formation history can help address the S8 tension and explore beyond ΛCDM models. To interpret observations accurately, it is crucial to use hydrodynamical simulations to examine the impact of astrophysical feedback from processes like supernovae and active galactic nuclei (AGN) on the IGM. However, such simulations are computationally expensive, and few examples are conducted at box sizes that can account for cosmological impacts. For instance, the CAMELS project has only performed simulations in volumes of (25 cMpc/h)^3.
At the summer school, I plan to use the CROCODILE simulation, which includes the latest feedback models, to study their impact on the Lyα forest power spectrum with a box size of (100 cMpc/h)^3. As I do not yet have the simulation results, I will introduce the foundational paper for this project, “Probing Feedback via IGM Tomography and the Lyα Forest with Subaru PFS, TMT/ELT, and JWST” (Nagamine et al. 2020), and the CAMELS project. I will also outline the research I intend to undertake in the summer school.
Speaker
Kanta Ito
Date/Place
13:00-, 26th(Wed.), June. @ES606
Title
Progress report for summer school
Abstract
This presentation is a review of [1]. About one fourth of energy budget in the current universe is occupied with the dark matter (DM). Its existence is confirmed in many observations and theories but it is not sure what composes it. One of the most promissing candidates is Axion. Here the word "Axion" means the QCD Axion, which solves strong CP problem in particle physics and the Axion Like Particles, which has a wide range of mass (ALPS). In this paper, Axion graviton coupling, which is less constrained than axion photon coupling, is considered. By such coupling, parity violated gravitational waves(GWs) are produced and GWs produced in the inflation are amplified through the parametric resonance. When considering this model, in high-k
region negative energy modes (ghost mode) appears. By introducing appropriate cutoff in k space, authors succeeded to avoid ghost mode and separate the parameter space into ghost regions and parametric resonance regions. After this regularization, parametric resonance affects a particular mode meets $k/a = m_a/2$ and amplifies such GWs at most 10^6 times in terms of energy density. By such amplification, energy density of GWs by the inflation reaches from 10^-14 to 10^-10 at the specific peak frequency and they find to be detectable by future detectors i.e. Taiji, LISA, IPTA, and SKA. Future GWs detections can lead to constraints to the Axion mass as a DM and specify what the DM is.
Speaker
Shun Yoshioka
Date/Place
13:00-, 26th(Wed.), June. @ES606
Title
Progress report for summer school(Redshift evolution of cosmic birefringence)
Abstract
In summer school, I will review the paper “Redshift evolution of cosmic birefringence in CMB anisotropies” by Galaverni et al., 2023. This paper mainly discuss about cosmic birefringence. Cosmic birefringence is rotation of linear polarization plane of CMB and is considered to be caused by pseudoscalar field. To solve equation of pseudoscalar field, they derive CMB anisotropies C_l. Finally, I will show some numerical calculations using CAMB.
Speaker
Kaito Yura
Date/Place
13:00-, 19th(Wed.), June. @ES606
Title
Progress report for summer school
Abstract
My theme of summer school is to review the paper “Constraint on the primordial vector mode and its magnetic field generation from seven-year Wilkinson Microwave Anisotropy Probe observations” written by Ichiki et al.. In this paper, they discuss the generation of primordial magnetic fields and give constraints on it by primordial vector mode which has a growing mode in the existence of anisotropic stress by free-streaming particles such as neutrinos. I will mainly explain the contents of this paper in this seminar. Also, I have been studying and performing the analyses in the paper using Planck data, so I will present my current progress.
Speaker
Sho Ukai
Date/Place
13:00-, 19th(Wed.), June. @ES606
Title
Progress report for summer school
Abstract
Galaxies and active galactic nuclei(AGN) are thought to be sources of ionizing photons, which trigger cosmic reionization, but the extent of their contributions is still under discussion. For summer school, I will review “Constraining the contribution of galaxies and active galactic nuclei to cosmic reionization” by Yoshiura et al. (2017). In this paper, they calculate the reionization history considering galaxies and AGN as ionizing photon sources, and constrain their contributions using observational data. In addition, I will present the latest observations of galaxies and AGNs by James Webb Space Telescope and update the constraints accordingly.
Speaker
Takahiko Terada
Date/Place
13:00-, 12th(Wed.), June. @ES606
Abstract
Gravitational waves (GWs) emitted in the early Universe can reach us because of their feeble interactions, so they may give us some information about the early Universe. In this talk, we discuss the secondary GWs induced by primordial curvature perturbations, which can have information on inflation as well as post-inflationary cosmic history. Specifically, we focus on effects of an (early) matter-dominated era on the induced GWs. We find that the intensity of the induced GWs sensitively depend on the quickness of the transition from the matter-dominated era to the subsequent cosmic era. In particular, the induced GWs can be significantly enhanced when the transition is rapid (the poltergeist mechanism), examples of which include a primordial-black-hole-dominated scenario and an axion-rotation scenario. If time remains, we also discuss the recent nanohertz GWs found by the worldwide pulsar timing array observations focusing on their spectral index. We introduce three possibilities in which the induced GWs can explain the spectral index.
Speaker
Koki Tokeshi (ICRR)
Date/Place
10:30-, 12th(Wed.), June @ES606
Title
Stochastic methods in inflationary cosmology
Abstract
Nonperturbative behaviours of quantum fluctuations generated
during inflation can be captured by the so-called stochastic formalism
of inflation in a classical but stochastic way. Though the formalism has
extensively been used in the context of primordial black holes in recent
years, other applications where nonperturbative fluctuations give
interesting outcomes will be presented, based on our works using the
stochastic formalism, 2310.16649 and 2304.02592.
Speaker
Ryodai Kawaguchi (Waseda-U.)
Date/Place
15:00-, 10th (Mon.), June @ES606
Title
Higher-order correlation functions in cosmology using path integral formalism
Abstract
In this talk, I will introduce how the correlation function of the cosmological perturbative quantity is evaluated in the path integral formalism and compare it with other formalisms, i.e. field redefinition and direct operator formalisms. The role of the boundary and EOM terms in the Lagrangian, which are negligible in the field redefinition approach, will be clarified and mainly the calculation of the three-point correlation function will be presented. I will also discuss the one-loop correction of the power spectrum.
Speaker
Maxime Paillassa (Nagoya-U.)
Date/Place
13:30-, 5th (Wed), June @ES606
Title
Source detection in wide-field surveys and convolutional neural networks.
Abstract
In the scope of wide-field survey cosmology, millions of galaxies need to be detected and measured. Hence, source detection techniques can present serious limitations in terms of completeness and reliability for further analysis. After briefly reviewing current source detection methods, we will see how neural networks, in particular convolutional neural networks (CNNs), can provide new solutions for source detection. As an application, I will present a project leveraging CNNs to tackle the more specific problem of blending detection in Hyper-Suprime-Cam (HSC) images.
Seminars in May
Speaker
Shun Arai (Kobayashi-Maskawa Institute)
Date/Place
13:30-, 29(Wed), May @ES606
Title
A tale of gravity by a young cosmologist: story never-ending but being better converged
Abstract
Gravity is the fundamental force to form the Universe in its whole lifetime. However, it is complex to capture its essence among from theory, observation, or even from the literature of science, making it challenging to understand how the Universe genuinely looks like. In this seminar, I’d provide a way to involve in exploring gravity by following a bit unconventional path.
In the first part, for a pedagogical purpose, I’d shortly introduce how to derive Einstein’s general relativity in two ways: the conventional derivation invented by A.Einstein and a particle theory approach in R.Feynman’s lecture note. Then we argue possible departure from the general relativity.
In the second part, we face on the problems of cosmic acceleration and quantum gravity in terms of gravitational physics, as long as supporting the general relativity.
In the final part, we describe how the up-to-date cosmology with handful of observables in high precision serve to pin down the true theory of gravity for the first time in the history. Particularly within my scope of research, I’d introduce a state-of-the art model of gravity based on Horndeski’s theory of scalar-tensor system, with remarks of recently-unveiled knowledge and on-going research works. I’d rather go into running projects, if someone may get interested in.
Speaker
Kiyotomo Ichiki
Date/Place
13:30-, 22(Wed), May @ES606
Abstract
The optical depth of cosmic reionization, as estimated from the CMB polarization by the WMAP satellite, was initially found to be larger than expected. However, detailed observations by the Planck satellite later revealed that the value was smaller than anticipated. This smaller value is mainly determined by Planck's large-angle E-mode power spectrum. According to a recent paper by William Giarè, Eleonora Di Valentino, and Alessandro Melchiorri (PRD, 2024), we discuss that the optical depth measurement independently of the E-mode can be done, resulting in slightly larger values of tau=0.07. We also discuss that this optical depth is useful in resolving the lensing and curvature anomalies in the CMB that were discussed a few years ago. On the other hand, these anomalies seem to be relaxed in likelihoods published after the Planck2018 results.
Speaker
Kotoyasu Matsumoto and Sachiko Ishida (Student Counseling Center, Nagoya university)
Date/Place
13:30-, 15(Wed), May @ES606
Title
Wellness Program
Abstract
This ‘Wellness Program’ is one of the tutorial courses to support your university life through think over your research environment (in terms of psychoeducation and organizational development).
In this time, we mainly focus on ‘communication’ and ‘awareness’. Through this course, we aim to improve these abilities.
Good relationships and communications are based on trust and empathy between you and others.
Firstly, we do group work and discussion about the experiences related to these topics and then think again about your research environment and daily life.
Finally, we discuss and consider how you can spend more fulfilling research life.
Speaker
Shuichiro Yokoyama
Date/Place
13:30-, 8(Wed), May @ES606
Title
Primordial non-Gaussianity in Large-Scale Structure
Abstract
Primordial non-Gaussianity has been extensively discussed as a probe of the physics of the early Universe. Planck CMB observations gave a tight constraint on such a primordial non-Gaussianity, but still we have a chance to test it by making use of ongoing/upcoming large-volume and precise measurements of the large-scale structure.
In this seminar, I'd like to give a brief review of the characteristic impacts of primordial non-Gaussianity in large-scale structure, in particular,enhancement/reduction of halo mass function, scale-dependent bias, ...
To evaluate these effects quantitatively to some extent, I would like to employ the so-called peak formalism and explain it by using the blackboard in the seminar. Through this seminar, I hope we can start new collaborations across the subgroups.
Speaker
Shohei Saga
Date/Place
13:30-, 1(Wed), May @ES606
Title
Dynamics of cold dark matters around collapse
Abstract
Cold dark matter (CDM) is microscopically modelled as a self-gravitating collisionless fluid obeying the Vlasov-Poisson equations. The cold nature suggests that the CDM phase-space structure is described as a three-dimensional sheet evolving in six-dimensional phase space. At the initial stage, the phase-space sheet represents a single-stream flow, but as a consequence of the non-linear evolution under self-gravity, the phase-space sheet self-interacts, and subsequently the phase-space sheet becomes a multi-stream flow. I will present how the multi-stream appears, and will discuss the importance of considering the effects of multi-stream regions in analysing the dynamics of CDM.
Seminars in April
Speaker
Kaito Yura
Date/Place
13:30-, 24(Wed), Apr. @ES606
Title
Self Introduction
Abstract
Self Introduction
Speaker
Sho Ukai
Date/Place
13:30-, 24(Wed), Apr. @ES606
Title
Self Introduction
Abstract
Self Introduction
Speaker
Shun Yoshioka
Date/Place
13:30-, 24(Wed), Apr. @ES606
Title
Self Introduction
Abstract
Self Introduction
Speaker
Kanta Ito
Date/Place
13:30-, 17(Wed), Apr. @ES606
Title
Self Introduction
Abstract
Self Introduction
Speaker
Kohei Onoda
Date/Place
13:30-, 17(Wed), Apr. @ES606
Title
Self Introduction
Abstract
Self Introduction
Speaker
Suchetha Cooray
Date/Place
13:30-, 10(Wed), Apr. @ES606
Title
Empirical Galaxy Formation Modeling
Abstract
I will introduce a class of models used for modeling the galaxy-halo connection. The motivation is that we need flexible models that can match all the observations self-consistently, which state-of-the-art cosmological hydrodynamical simulations have been unable to achieve. By extending this framework, we can try to extract the maximal information about galaxy formation and cosmology from observed images of telescopes (field-level inference). I will give an overview of the direction the field is heading towards as well as show some of my progress in the modeling side.
Speaker
Kiyotomo Ichiki
Date/Place
13:30-, 10(Wed), Apr. @ES606
Title
Policy speech
Abstract
Policy speech