Cセミナー 2019

English

今後のセミナー

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＊プレゼンテーションファイルは研究室の内部向けリンクから閲覧可能です。

7月のセミナー

7月24日(水)15:00-＠ES606

We consider the constraints on Degenerate Higher-Order Scalar-Tensor (DHOST) theories with matter density fluctuations. The DHOST theory is one of the most interesting model of modified gravity which is an origin of the late-time acceleration. Its equations of motion has higher derivatives more than second order, but there is no ghost modes due to its degeneracy. These derivative interactions are imprinted on the growth of density fluctuations at both linear and non-linear levels. At a linear level, the effect of these interactions appears as the new friction term on the evolution equation of linear density fluctuations. Then, we derive the growth index of the linear growth rate in the DHOST theories and can constrain it from LSS observations. At a non-linear level, the second-order kernel is modified from that of GR and the shape dependence of matter bispectrum is peaked at the folded triangle in the DHOST theories.

7月24日(水)13:00-＠ES606

21-cm線を用いた小スケールの初期パワースペクトルへの制限

ビッグバン理論の宇宙初期に関する諸問題の解決策としてインフレーション理論がある。インフレーションモデルは無数に存在するため、そのモデルを制限する必要がある。その方法の一つに初期パワースペクトルによる制限がある。初期パワースペクトルの制限は宇宙マイクロ波背景放射の観測などから得られてきたが、小スケール側 ($波数 k \ge 1Mpc^{－1}$) は不定性が大きい。近年注目されている Ultracompact minihalo (UCMH) という天体を用いることで小スケール側に制限を課せる可能性がある。 UCMH は、赤方偏移 z=1000 に形成される高密度天体であり、密度揺らぎの大きさが $10^{-3} \le \delta \le 0.3$ を 満たす時に構造形成されうる。高赤方偏移において形成される UCMH は、空間的にコンパクトであり、初期パワースペクトルの短波長側の波数範囲に制限をつけることが期待される。

7月24日(水)13:00-＠ES606

Non Bunch-Davies真空における単一スカラー場インフレーションモデルの観測量の推定

インフレーション理論はビッグバン理論の抱える問題を解決するために生まれた理論であり、多様なモデルが存在する。その中でも最も単純なモデルとして、単一スカラー場インフレーションモデルがある。インフレーションモデルを特徴付ける観測量として、テンソル・スカラー比 r と非ガウス性パラメータ fNL に着目する。ここでテンソル・スカラー比とは、テンソル型の揺らぎ (初期重力波を生む時空の揺らぎ) とスカラー型の揺らぎ (曲率揺らぎと呼ばれる揺らぎで、密度揺らぎの起源となる) それぞれの二点相関関数の大きさの比に対応する量で、非ガウス性パラメータ fNL はインフレーション期につくられる初期揺らぎの三点相関の大きさを表す量である。このモデルでは r はスローロールパラメーター ε を用いて理論的に r = 16ε で表され、実際の CMB 観測では、その値は r < 0.07(⇒ ε < 0.07/16) に制限されている。また、fNL は無視できるほど小さな値となる。

7月17日(水)13:00-＠ES606

クォーク・ハドロン相転移による背景重力波への影響

7月17日(水)13:00-＠ES606

21cm線とCMB弱重力レンズ効果の相互相関による中性水素存在量の推定

HIは波長21cmの電磁波(21cm線)を放出する。従って21cm線を観測すば、HIの存在量の時間発展を得ることができる。しかし、21cm線のシグナルは前景放射によって汚染されてしまう。この問題を解決するために、21cm線と、大規模構造がCMBに及ぼす重力レンズ効果による収束場との相互相関をとるという手法を採用する。[Tanaka et al 2019]では、今後観測が始まる望遠鏡であるSKA(Square Kilometre Array)を想定した場合に、HIの存在量の推定精度をFisher解析により予測している。その結果、SKA観測による、相互相関を用いたHIの存在量の推定は、従来の望遠鏡による他の手法を用いた制限よりも、高赤方偏移まで精度よくHIの存在量を推定できる可能性を示している。

7月17日(水)13:00-＠ES606

CMBレンジングを用いた銀河団の質量推

7月10日(水)13:00-＠ES606

Sub-grid model of escape fraction for cosmological 21-cm signal simulation

Based on the standard model of structure formation, the first stars have formed at around redshift 20-30 in mini-haloes. They play important roles in the history of the universe such as emitting ionizing photons and providing metals into intergalactic medium. Although resent theoretical studies remarkably improve our understanding on the stellar mass function (SMF) of the first stars, these studies have not reached consensus on the SMF. On the other hand, forthcoming 21-cm observation by SKA is expected to obtain 21-cm signal, originated from neutral hydrogen, from the first stars’ era, which would enables us to approach the SMF observationally. However, the relation between the SMF and cosmological 21-cm signal is poorly understood. Therefore, we model the relation focusing on the escape fraction of the ionizing photons for mini-haloes hosing the first stars. In this talk, I will explain, in detail, the model of escape fraction dependent on the SMF, background density, and redshift. Finally, I will show the results from test calculations of cosmological 21-cm signal with 21cmFAST in order to demonstrate how important the escape fraction is on the large scale 21-cm signal distribution.

7月3日(水)13:00-＠ES606

Dynamical Evolution of Voids with Surrounding Gravitational Tidal Field

The underdense regions of the Universe are called voids. While they are promising probe of gravity and cosmology on a larger scale than the galaxy, there is not enough agreement on what is the appropriate model for individual voids' evolution. It is known that the underdense region itself tends to grow to be spherical (V. Icke 1984), but the voids in cosmic web rather distort as it grows (e.g. E.G.P. Bos et al. 2012). Though there are some possibilities to cause this, here I follow the description that voids are distorted by the tidal field on their effective radii, and examine the correlation between time derivative of void shape and the tidal field. In this talk, I will first briefly introduce some previous works on the nature of void and related topics, and discuss the main cause of void shape evolution.

6月のセミナー

6月26日(水)13:00-＠ES606

CMB constraint on primordial magnetic fields

Large-scale structure (e.g., voids and galaxy clusters) is magnetized. The primordial magnetic fields (PMFs) are expected to explain the origin of these magnetic fields. If the PMFs exist, they possibly create remarkable effects on some cosmological observables, including the cosmic microwave background (CMB) anisotropy. PMFs create additional metric perturbations in the early universe, and induce the CMB temperature and polarization anisotropy. As these effects should include the information about the epoch or mechanisms of the primordial magnetogenesis, we can put a constraint on the PMF model parameters by analyzing the CMB data from Planck satellite and South Pole Telescope. In this seminar, I mainly review the effect of PMFs on the CMB anisotropy, and show the constraint on PMFs that we obtained.

Alexis Boudon

6月26日(水)13:00-＠ES606

Non-minimally coupled quintessence and the evolution of the present day equation of state

Quintessence, a canonical scalar tensor theory, is one of the simplest candidates to explain dark energy. During this presentation, we will look at the different changes that a non-minimally coupled theory of the quintessence can bring to the cosmological equation of state and to the gravitational "constant". In the first part, I will present the modifications on the Friedmann and Klein-Gordon equations as well as important formulas. In a second part, I will explain the approach used to build a code capable of numerically solving these equations according to different parameters. Finally, I will show that the equation of state and the gravitational "constant" depend a lot of these parameters but that within the limits of current observations, these differences are minimal.

6月19日(水)13:00-＠ES606

Aspects of primordial black holes and implication to multi-phase inflation

As proposed by Hawking and Carr more than four decades ago, black holes can be formed even during the early radiation-dominated era in advance of the ordinary star formation, called “primordial black hole (PBH)”. The possible mass of PBHs covers a very wide range, and the extremely light one (~10^-12 M_\odot) attracts attention as a candidate of the main component of dark matters. Moreover recent LIGO/VIRGO’s gravitational wave detections have revealed the ubiquitous existence of slightly massive and less spinning BHs, which also can be explained by the primordial ones. I first briefly review such astrophysical motivations of PBHs and current constraints on those. Then I show that PBHs can be easily realized if inflation is not a single continuous phase but can be divided into multiple phases. Interestingly recently proposed “dS conjecture” in the context of string theory may also support such a multi-phase inflation scenario.

6月12日(水)13:00-＠ES606

Where are metals originated in massive Population III stars?

Theoretical studies have shown that Population III stars are massive. Since massive metal-free stars are expected to eject metals with a characteristic pattern when the stars explode, detecting such a characteristic abundance pattern would be direct evidence for massive Pop III stars. In this study, by using numerical simulation data, we explore the distribution of metals ejected by Pop III stars, and compare it to that by Pop I (or II) galaxies. As a result, we find that metals originated in Pop III stars are dominant in low-density regions in the Universe. The absorption feature imprinted on QSO spectra could be detected by spectroscopic observations with Subaru PFS.

6月5日(水)13:30-＠ES606

Photometric Redshift for Hyper Suprime Cam Survey

Photometric redshift (photo-z) is a method to measure the distance to galaxies. Although the photo-z measurement is a crude estimate, particularly for the photometric surveys where no spectrograph is available, the photo-z is the only tool to measure the distance. To alleviate the uncertainty of the photo-z, various approaches have been established. In this seminar, we will review the methods based on the traditional template fitting and empirical method including machine learning. Moreover, the image based analysis using deep neural network is recently proposed and approved by the practical data of BOSS survey. Finally, as an alternative approach, clustering redshift method is introduced where no photometric information is used. As an application of the clustering-z, we will also discuss how we quantify the effect of cosmic variance due to the limited region of the calibration sample, by use of Illustris TNG simulation with super survey modes.

5月のセミナー

5月29日(水)13:00-＠ES606

Clustering of PBHs

We formulate two-point correlation function of primordial black holes (PBHs) at the formation, based on the functional integration approach which has been often used in the context of clustering of the halos. We found that the PBH clustering on large scales could be never induced in the case where the initial smoothed density fluctuations are Gaussian, while it can be enhanced by the so-called local-type trispectrum (4-point correlation function) of the primordial curvature perturbations.

5月22日(水)13:00-＠ES606

Charge excess in the Universe

It is widely accepted that the Universe is electrically neutral. So far, cosmological analysis are almost performed, based on the charge neutrality, and there are no observational results which is conflict with this assumption. However no one knows that the universe is perfectly neutral. In this seminar, we discuss the observational constraint on the charge excess in the Universe.

5月15日(水)13:00-＠ES606

Although the CDM paradigm is consistent with a wide variety of existing observations, it has yet to be sufficiently tested on scales smaller than those of massive galaxies. Here we show that the future 21cm forest observations may offer a powerful test for various alternative dark matter models such as WDM, ULA and PBHs.

5月8日(水)13:00-＠ES606

Weak lensing cosmology with ongoing and upcoming imaging surveys

Weak gravitational lensing is one of the most powerful cosmological probes because of its direct sensitivity to dark matter distribution, and thus quite a few galaxy imaging surveys are carried out and being planned. In this talk, I will give a broad overview of ongoing and upcoming weak lensing surveys, including the Subaru Hyper Suprime-Cam survey, and major challenges towards high-precision weak lensing measurement. I will also present the current status of cosmological analysis I am recently working on, especially about the combined galaxy-galaxy clustering and lensing analysis.

4月のセミナー

4月24日(水)13:00-＠ES606

4月24日(水)13:00-＠ES606

The cosmological principle is the Cosmology’s standard assumption. The cosmological principle is the idea that the Universe is isotropic and homogeneous. Isotropy of the universe is supported by some direct evidence. (CMB, Galaxy, etc. ). But homogeneity of the universe is NOT supported by any direct evidence. It is announced that distant Supernovae are darker than expected by matter dominated epoch in the universe under cosmological principle. The homogenous universe is necessary a dark energy to explain it. (Perlmutter(1998)) But the inhomogeneous universe causes it without a dark energy. I think the constrained (LTB) spacetime as the inhomogeneous universe. I verified that constrained LTB spacetime satisfied both SN-Ia and kinematic Sunyaev Zel’dovich effect, So the constrained LTB metric was excluded. I review the paper “Looking the void in the eyes – the kSZ effect in LTB models”(Garcia, Haugboelle(2008).

4月24日(水)13:00-＠ES606

TiO2に担持されたPtナノ粒子触媒（以下Pt/TiO2と略記）は、CO酸化反応に高い活性を示し、COガスによって強い金属担体間相互作用（SMSI）を引き起こす。SMSIは、加熱処理や反応ガスの導入によって著しい活性の促進や劣化が起きる現象である。O2とCO導入におけるPt/TiO2のSMSI効果は報告されていないため、本研究では、Pt/TiO2触媒のCO,O2ガス導入による構造変化をTEMで観察した。その結果、O2ガス導入で、Pt/TiO2界面のピラー成長が見られた。

4月17日(水)13:00-＠ES606

The analysis of large-scale structure is performed by calculating two-point correlation function, and most of the information on density fluctuations known to be approximately Gaussian can be obtained from the two-point correlation function. However, in order to further restrict the cosmological model, it is necessary to investigate non-Gaussianity contained in density fluctuation, and in order to extract this information, it is necessary to investigate the correlation of three or more points. The problem is that we need to increase the number of points in order to obtain sufficient information, but the information doesn’t increase greatly even if we increase the point which is calculated. On the other hand, it is difficult to calculate multi-point correlation. In this study, we focused on the analysis of the image of density distribution using machine learning. In the Convolutional Neural Network (CNN) method used in this research, the image is filtered and analyzed to extract the information of multi-point correlation at a small computational cost and classify the image. In this study, images of density distribution are created from simulations of the universe with cold dark matter (cdm), and with warm dark matter (wdm), and CNN is trained to use classification of cdm model and wdm model, or the estimation of mass of warm dark matter. In addition, the difference of results of the classification between when the value of density is converted and when it is not is examined. As a result of learning, the classifications of cdm and wdm of all the masses examined in this study can be correctly classified with probability of 80% or more, and the mass estimation of wdm is also much more accurate than randomly classifying.

4月17日(水)13:00-＠ES606

In order to solve the problem of pure big bang theory, it is thought that inflation is necessary. In particular, slow-roll inflation (SR), which is one of the inflation models, is widely accepted. SR is characterized by the fact that initial perturbations, which are the seeds of large-scale structures, are preserved after horizon cross. However, in recent years, an inflation model with potential with an inflection point has been considered. Inflation with a flat potential is called ultra slow-roll inflation (USR). USR has the feature that the perturbation grows after the horizon cross. Therefore, it has recently been attracting attention as a theory for forming initial black holes. In this study, first, we explain the characteristics of the power spectrum in each model, and show that USR cannot end inflation. Next, we connect two models to solve this problem, and show the result of analytically investigating the influence of the connection on the power spectrum for the curvature perturbation.

4月17日(水)13:00-＠ES606

4月15日(月)16:00-＠ES606

In the standard cosmology, it is believed that the first self-evolving objects are born within the first billion years after the CMB is emitted in the bulk Universe. The objects have gravitationally and thermally grown over the past 10 billion years as the CMB has passed in the structures formed, during which local physical phenomena in the structures are imprinted on the CMB. One of the imprints created by the structures is the gravitational lensing of the CMB (CMB lensing).
In this talk, we report properties of CMB lensing presumably sourced by radio galaxies. We compute the angular cross power spectrum between radio galaxies in NRAO VLA Sky Survey (NVSS) and CMB measured by Planck. We estimate the redshift distribution of the radio galaxies in the lensing kernels via the galaxies in Kilo-Square Degree Survey (KiDS) catalog. To be precise, we use radio galaxies in CENSORS as a reference to the redshift distribution of radio galaxies in high precision.
The theoretical predictions via the Planck 2015 standard cosmological model with assuming the redshift distributions the estimation from KiDS and CENSORS, the angular auto power spectrum of radio galaxies is C_l = \cal O(10^{-3}), and the theories predict that the correlation coefficient is significantly high over 0.4 at l <= 300. The auto power of the radio galaxies is in good agreement with the first measurement in C.Blake et al 2004 at l <=100. Meanwhile, in our measurement of C_l at l>=100, the data is completely covered by shot noise, ending up underestimating the coefficient well down the theoretical predictions. This suggests that a more careful subtraction of the noise is necessary to test the cosmology and the formation history of radio galaxies.

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