Cセミナー 2018 | セミナー | 名古屋大学 宇宙論研究室(C研)

Cセミナー 2018

English

次回のセミナー

発表者
簑口 睦美

日程/場所
1月23日(水)13:00-@ES606

概要

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1月のセミナー

発表者
箕田 鉄兵

日程/場所
1月16日(水)13:00-@ES606

A constraint on primordial magnetic fields from 21-cm global signal

概要
In the present universe, magnetic fields are found on various scales, from planets and stars, to galaxies and clusters of galaxies. One possible origin of these cosmic magnetic fields is a weak seed field generated in the early universe, which is called "Primordial Magnetic Fields" (PMFs). If they exist, they affect the thermal history of intergalactic medium (IGM) gas via magnetohydrodynamic effects. Especially ohmic diffusion and ambipolar diffusion are significant processes to heat up the IGM gas. The recent 21-cm global absorption signal is useful to constrain these additional heating mechanisms for the IGM gas because they can be detected only when the IGM kinetic temperature is lower than the CMB temperature. We have calculated these heating efficiencies by assuming the distribution of PMFs as following a single power-law power spectrum, and performed a numerical calculation to solve the evolutionary equations for kinetic temperature, ionization fraction of IGM gas, and the PMF energy density. As a result, we have obtained the upper limit of parameters characterizing the PMF power spectrum from the 21-cm global absorption signal. In this seminar, I introduce the 21-cm line observation, IGM thermal history, and PMFs at first. Then I talk about the IGM heating mechanisms due to PMFs and a novel constraint on PMFs obtained in our recent work.

発表者
遠藤 隆夫

日程/場所
1月9日(水)13:00-@ES606

The Alcock Pazcynski test with stacked voids in HI distributions.

概要
The Alcock Paczynski test is one of ways to probe cosmological models via the expansion history of the universe. The AP test demands objects whose physical size are known or objects which are statistically isotropic such as correlation function or power spectrum. The shape of cosmic voids is also expected to be one of candidates for the AP test since a stacking process ensures their average shape to be spherical, even though each void is not spherical. There are several works that apply stacked voids to the AP test. While previous works detected voids from galaxy distributions, we try to detect them from HI distribution, which is also expected to be a tracer of dark matter distributions. In this seminar, I would like to introduce our procedure to detect voids from HI distributions and discuss whether voids in HI distributions can be applied to the AP test by using cosmological hydrodynamics simulation data.

12月のセミナー

発表者
橋本大輝

日程/場所
12月14日(金)13:00-@ES606

Constraint on the Nature of the Dark Matter Annihilation from HSC Lower Surface Brightness Galaxies and Fermi Gamma-ray Sky

概要
In the context of high energy astrophysics, natures of dark matter annihilation expected to emit gamma rays has been studied by utilizing observed gamma-ray flux in a direction of dark matter halos. Recently, some studies have been performed probing the natures focusing on nearby dwarf galaxies or the Galactic center region or individual massive object (ex. M31 or the Virgo cluster). In those studies, compering the expected gamma-ray flux induced by dark matter annihilation from interest objects with observed one, upper limit on the annihilation cross-section have been researched. In our study, we focus on Lower Surface Brightness Galaxy (LSBG) found by Hyper Suprime-Cam (HSC). LSBG can be a novel target of dark matter annihilation signal because of some seasons. One is that those are more massive than dwarf spheroids, which lead to producing more luminous dark matter flux if happened. Second is that gamma rays from those are expected having less astronomical contaminations such as gamma rays originated from supernova remnants, star-forming regions or pulsars due to relatively quiescent states of those, which can perform robuster research for dark matter annihilation signals. We model gamma-ray flux from 8 HSC-LSBGs with known redshifts. Compering with observed gamma-ray flux by Fermi Large Area Telescope, we limit annihilation cross-section. On our presentation, we will report the methods and the results currently in progress.

11月のセミナー

発表者
安藤 梨花

日程/場所
11月28日(水)13:00-@ES606

Effect of the astrophysics on the large-scale clustering of HI

概要
The observation of the large-scale structure of the universe is the good for the exploration of dark energy or modified gravity. Although the observations of the galaxy and QSO distributions have been done so far, it is difficult to take the spectrum to accurately measure the redshift of the sources. Then the 21-cm line emitted from the neutral hydrogen gas (HI) draw attention as a new tracer of the large scale structure, and several experiments to map the 21 cm distribution by intensity mapping such as Square Kilometre Array (SKA) are proposed. We measured the scale dependence and redshift dependence of the HI bias at redshift 1

発表者
加藤 健太

日程/場所
11月28日(水)11:00-@ES606

The test for the anisotropy in the mean of the CMB fluctuations in spherical harmonics space

概要
The existence of CMB is considered to be one of the most powerful evidences that support Big Bang theory. The CMB temperature field has about 0.01% fluctuations and it results from the primordial density perturbations predicted by the inflation theory. The inflation theory predicts that the perturbations obey the statically isotropic Gaussian. In this study, I compared the CMB Monte Carlo simulation maps and the real CMB map, and tested the statical isotropy of the mean of the CMB fluctuations (of temperature and E mode polarization) in spherical harmonics space.

発表者
榊原 日菜子

日程/場所
11月21日(水)13:00-@ES606

Effect of gravitational lensing on cosmological parameter from SNe Ia

概要
Since the discovery of the acceleration expansion of the universe in 1998, observations of type Ia supernovae (SNe Ia) covering z<1 have imposed on the cosmological model. However, it is known that Hubble residuals used for cosmological parameter estimation have scatters around the theoretical prediction of ΛCDM model. In previous works, gravitational lensing effect, which is one of causes of dispersion, is not corrected for individual supernovae in parameter estimation. In this work, we study the correlation between the magnification by the gravitational lensing and the Hubble residual of the SN and investigate the influence of the gravitational lensing effect on the estimate of cosmological parameter. For the analysis, we use SNLS3 supernovae data set and HSC S16A shape catalog, S17A galaxy catalog and photo-z catalog. To estimate the magnification by the gravitational lensing along the line of sight of the SN, we consider two methods: one reconstructs the projected mass distribution from the distortion of the shape of the galaxy assuming a weak gravitational lensing approximation and another one estimates the magnification from the matter density distribution along the line of sight. In this talk we discuss the correlation between magnification and Hubble residual estimated by these two methods. We also show the effect of the magnification on the Ωm estimation.

発表者
小粥 一寛

日程/場所
11月21日(水)13:00-@ES606

Approach to Angular dependent Primordial Non-Gaussianity with Intrinsic Galaxy Alignments

概要
Non-linear effects during inflation can generate primordial non-Gaussianities(NG). Arkani-Hamed & Maldacena (A&M) showed that an interaction between the inflation and higher spin fields, which may be predicted in string theory, can generate angular dependent NG, while preserving the global isotropy. As was argued by Schmidt et al., this anisotropy can be explored by observing galaxy alignments. In this talk, we study whether the angular dependent NG from massive spin-2 fields can be observationally distinguished from the anisotropic NG with a violation of the global isotropy, which can be generated e.g., by a vector field. And we examine the closer to case massive spin-2 angular PNG. As the former result, we found that the violation of the global anisotropy leads to non-vanishing off-diagonal components of multipole moments ell and B mode, which cannot be generated from the angular dependent NG with the global isotropy. The latter result reveals that the imprint of PNG appears in small-scale. If I have time, I will talk about the relation between the higher moments with galaxy alignments and the PNG from massive particles with higher spins.

発表者
Wentao Luo (IPMU)

日程/場所
11月5日(月)14:00-@ES606

Galaxy-Galaxy Lensing and its Applications

概要
Galaxy-Galaxy lensing is now a major probe of dark matter halos around galaxies. We apply this galaxy-galaxy lensing technich to study the properties of subhalos as well as the shape of halos hosting clusters of galaxies.

10月のセミナー

発表者
大場淳平

日程/場所
10月24日(水)13:00-@ES606

題名
Cosmological constraints on the velocity-dependent Baryon-Dark matter coupling

概要
The modern cosmology indicates that the usual baryonic matter is forming only 5% of the energy budget of our universe. And it is also considered that galaxies, galaxy clusters and the large scale structure are mainly dominated by the cold dark matter. Although there are a lot of astronomical evidences which suggesting the existence of the dark matter, to reveal its nature is still a main goal of the modern cosmology. Here, we focus on a possibility to have a baryon-dark matter coupling which is naturally realized by several dark matter models, e.g. the dark matter with a small electric charge. In this talk, I will talk about the velocity-dependent baryon-dark matter coupling and its effects on the cosmology. Also I will discuss about results of the constraint on a cross section of such a coupling by using cosmological observables including CMB and Lyman-alpha forest observation.

発表者
多田 祐一郎

日程/場所
10月17日(水)13:00-@ES606

題名
Stochastic inflation and curvature perturbations

概要
I will introduce a method to calculate inflationary curvature perturbations without perturbative expansions by combining the stochastic and delta N formalisms. Several results by my recent numerical code will be shown as well.Regarding the stochastic formalism itself, I will also describe the subtleties we found in this formalism.

発表者
西澤 淳

日程/場所
10月10日(水)13:00-@ES606

題名
Map reconstruction with Iterative Harmonic Transformation and Gaussian Process

概要
I introduce how to recover the missing data in 2 dimensional map such as CMB by use of two different methods, iterative harmonic transformation and Gaussian Process.

9月のセミナー

発表者
Jean-Baptiste Durrive

日程/場所
9月26日(水)16:00-@ES606

題名
The Cosmic Web as a probe for Modified Gravity

概要
Structure formation at non-linear scales may provide strong constrains on gravity theories. In this (farewell) talk, I will first discuss how Modified Gravity can be investigated by studying the properties of the Cosmic Web in general. Then, I will present several reasons why focusing on Cosmic Walls is particularly interesting for this purpose. Finally, I will introduce an analytic model of the accretion of matter inside Walls, based on the powerful method of conformal mapping often used in mathematics and hydrodynamics. This toy model constitutes a complementary tool to numerical simulations, and is useful to help us better understand the subtle dynamics of halos and galaxies along the Cosmic Web.

発表者
Anastasia Fialkov

日程/場所
9月12日(水)10:30-@ES606

題名
Shining Light Into Cosmic Dark Ages

概要
The first billion years is the least-explored epoch in cosmic history. The first claimed detection of the 21 cm line of neutral hydrogen by EDGES (announced at the end of February this year), if confirmed, would be the first time ever that we witness star formation at cosmic dawn. In my talk I will discuss theoretical modeling of the 21 cm signal. I will also summarize the status of the field after the EDGES detection, and discuss prospects for future detections of this line.

発表者
Jordi Miralda Escudé

日程/場所
9月7日(金)13:30-@ES606

題名
Highly magnified gravitationally lensed images: a probe to small-scale clumpiness and the nature of dark matter

概要
Different hypotheses on the nature of dark matter imply a diversity of possibilities for the small-scale substructure in the mass distribution: primordial black holes, axion mini-clusters, scalar waves, or cold dark matter subhalos. I will discuss possible ways to probe these small-scale clumpiness with gravitational lensing, especially using highly magnified multiple images.

発表者
Chen Heinrich

日程/場所
9月6日(木)13:30-@ES606

題名
WFIRST high Latitude Survey and Synergies with Ground-based Surveys

概要
The Wide-Field Infrared Survey Telescope (WFIRST) is a top-rated space-based mission planned to launch in the mid-2020’s with capabilities to study dark energy. It is designed to constrain dark energy evolution and deviations from GR with exquisite control of systematics, and offers a wide range of synergies with the ground-based surveys. In this talk, I will demonstrate the capabilities of the WFIRST spectroscopic and imaging surveys, by presenting the latest forecasts on a variety of dark energy probes in the nominal survey as well as in potential survey extensions. I will also comment on exciting synergies expected with LSST.

7月のセミナー

発表者
阿部克哉

日程/場所
7月18日(水)13:00-@ES606

題名
スニヤエフ・ゼルドビッチ効果を用いた原始ブラックホールの密度パラメータへの制限

概要
宇宙マイクロ波背景放射(CMB)などの観測により、宇宙の構成要素のうちおよそ27パーセントがダークマターであることが判明している。ダークマターとは宇宙の構造形成を司る非常に重要な物質である。しかし、未だその実態は明らかにされておらず、ダークマターの解明は宇宙論における重要課題の1つとなっている。そのダークマターの候補の1つに原始ブラックホール(PBH)がある。PBHとは、宇宙初期に高密度領域が重力崩壊して形成されるブラックホールである。未だに観測例がなく、現在様々な観測によってその存在量に制限を与える取り組みがされている。今回私はスニヤエフ・ゼルドビッチ効果(SZ効果)と呼ばれる現象の観測により、PBHの密度パラメータに制限をつけることを考えた。SZ効果とはCMB光子が高温プラズマを通過した時に、逆コンプトン散乱を通じて電子からエネルギーを受け取り、CMBの温度スペクトルが歪む現象である。PBHごく近傍のガスは、降着時の重力エネルギー解放により温度が上昇し熱制動放射を引き起こす。そのためPBHはUVやX線の放射源となり、周囲のガスを加熱し電離する。これらの加熱、電離されたガスがSZ効果を引き起こす。本研究ではまず、PBH周辺の電離・温度構造を明らかにするため、PBHの放射光度を仮定し輻射輸送シミュレーションを実施した。それによるとエディントン光度の1パーセント程度の放射で数kpcスケールの電離領域を作ることがわかった。この結果をもとにPBHによるSZ効果を計算し、さらにはCMBの温度ゆらぎのスペクトルに与える影響を見積もった。この結果をSouth Pole Telescope(SPT)の観測結果と比較することによりPBHの密度パラメータに新しい制限を与えた。

発表者
近藤寛人

日程/場所
7月18日(水)13:00-@ES606

題名
弱い重力レンズからの銀河バイアス推定

概要
宇宙の構造形成において、はじめにダークマターの密度ゆらぎが成長し、その高密度領域で銀河が形成される。その後、銀河進化が進んでも物質とダークマターのゆらぎの間にはずれが存在し、そのずれを銀河バイアスと呼ぶ。銀河バイアスは、銀河の個数密度分布からダークマターの密度分布を推定することを可能にし、銀河進化などを研究する上でも有用である。銀河バイアスの主流の推定手法は銀河の二点相関を用いたものであるが、これは銀河の観測のみによるものであり、ダークマターの分布については観測量から直接的に求めてはいない。 一方で弱い重力レンズ効果用いた手法では、観測された銀河の形状データを解析することで質量分布を推定しており、宇宙論モデルへの依存性は小さい。本発表では、弱い重力レンズ効果から赤方偏移依存した銀河バイアスを推定した論文をレビューを行う。 レビューする論文中では、Dark Energy Survey Science Verificationのデータを解析し、弱い重力レンズ効果から得られるシアーマップと銀河とマップを相関させることで、赤方偏移に依存したバイアスを求めている。また解析と並行してシミュレーションを行い、解析工程ごとの誤差を評価している。推定された銀河バイアスの値は、他の銀河の二点相関を用いた手法とCMBレンズを用いた用いた手法からの推定値と比較され、2σレベルで一致している。将来的にDESは5000平方度まで観測領域が拡大されるため、より統計誤差の少ない銀河バイアスの推定が可能になることが期待される。

発表者
福永颯斗

日程/場所
7月18日(水)13:00-@ES606

題名
アクシオンの自己相互作用による宇宙の構造進化

概要
アクシオンとは素粒子物理学から予言されるスカラー粒子である.QCD アクシオンが導入されて以降,超弦理論からもアクシオンの生成が予言されている.多くの場合アクシオンの質量は軽く,持ちうる質量の範囲は広いと考えられている.一方で,宇宙論においてアクシオンのようなスカラー場が宇宙に存在すれば,インフレーションやダークマターといった,宇宙論における未解明の物理を解決することができる.以上のことからアクシオンの宇宙論的な性質について調べ,観測的に制限をつけることは有意義である.本発表では,まず先行研究で用いられている解析手法を用いて,アクシオンの密度ゆらぎの成長について議論する.そこではアクシオンは質量を持つ自由場として振る舞うと仮定した上で,密度ゆらぎの時間発展が導かれている.しかし,一般にはアクシオンの非線形な自己相互作用が存在する.そこで,自己相互作用を考慮したアクシオンのポテンシャルを与えて,その場合の密度ゆらぎの進化について議論する.

発表者
吉田貴一

日程/場所
7月18日(水)13:00-@ES606

題名
Lyα線を用いた小スケールの等曲率ゆらぎへの制限

概要
現在観測される宇宙の大規模構造は、宇宙初期に存在した密度ゆらぎが時 間とともに成長して形成されたものである。初期のゆらぎには、断熱ゆらぎ と等曲率ゆらぎがある。インフレーションにより生成されるゆらぎは断熱ゆ らぎであるため、多くの研究でこのゆらぎが用いられている。実際、Planck 衛星による宇宙マイクロ波背景放射 (CMB) の温度ゆらぎの観測結果は、大 スケール k ∼ O(0.1) h/Mpc において断熱ゆらぎのみを考慮した場合とよく 一致している。一方、等曲率ゆらぎも様々な宇宙の構造形成シナリオで生ま れる。このゆらぎは例えば、原始ブラックホール (PBH) があると小スケール で卓越すると予言されており [2]、本研究ではこのような小スケールにおける 等曲率ゆらぎに着目した。CMB や物質の密度ゆらぎのパワースペクトルは、断熱ゆらぎと等曲率ゆら ぎの重ね合わせで与えられる。従って、観測から得られるデータと理論計算 を比較することで、初期条件に制限を与えることができる。しかし、上で述べたような等曲率ゆらぎは Planck 衛星の観測領域よりも小 さなスケールに痕跡を残す。このモードを制限するためには、Planck 衛星の データよりも小さなスケールの情報が必要である。このような小スケールの 情報を得るために、ライマンα線を用いることができる。ライマンα線は高 分解能で観測できており、この吸収線は小スケールの密度ゆらぎの情報を含 んでいる。本発表ではクエーサーからのライマンα線のデータと Planck 衛星 のデータを合わせることで、小スケール k ∼ 1 h/Mpc における等曲率ゆらぎ に与えた制限について報告をする。

発表者
西澤篤志

日程/場所
7月11日(水)13:00-@ES606

題名
Polarization test of gravitational waves from compact binary coalescences

概要
Gravitational waves have only two polarization modes in general relativity. However, there are at most six possible modes of polarization in metric theory of gravity in general. The test of gravitational-wave polarizations can be a powerful tool to probe for gravity theories. In the coming years, the observations of gravitational waves with a world-wide network of interferometric detectors such as Advanced LIGO, Advanced Virgo, and KAGRA will make it possible to obtain the information of gravitational wave polarizations. Here we systematically study the separability of the polarization modes for the inspiral gravitational waves from the compact binary coalescences, which have already been observed but not studied well the distinguishability of the polarization modes because of the complexity of the waveform. We clarify the condition to break the parameter degeneracies and to detect the polarization modes separately. We also show how KAGRA contributes to the joint observation with a global detector network.

発表者
北嶋 直弥

日程/場所
7月11日(水)13:00-@ES606

題名
Distribution of primordial black holes and small-scale structure formation

概要
I discuss small-scale structure formation sourced by primordial black holes (PBHs) which constitute a small part of present cold dark matter component.First, I will show that PBHs originated from primordial density perturbations with moderately-tilted power spectrum have number fluctuations following the log-normal distribution, while they follow the Poisson distribution if the spectrum is steeply blue. Those fluctuations behave as an isocurvature mode and affect the matter power spectrum in different ways. In particular, the halo mass function can be significantly modified in the presence of PBHs even if their fraction accounts for only 0.1 percent of total dark matter abundance. I will then discuss the subsequent 21cm signature from those minihaloes. The future 21cm observation can put a stronger constraint on the PBH fraction than the current one in a wide mass range. I will also show a specific example of cosmological scenarios which predicts the PBH formation with log-normal fluctuation

6月のセミナー

発表者
宮武 広直

日程/場所
6月20日(水)13:00-@ES606

題名
Hyper Suprime-Cam Weak Lensing Measurement of Galaxy Clusters selected by Atacama Cosmology Telescope Polarimeter

概要
Galaxy clusters are one of the most powerful cosmological probes because of an exponential tail at the high-mass end of the cluster mass function. The accuracy of mass function measurement is currently limited by systematic uncertainties in cluster mass estimates due to an uncertain physical assumption, e.g., hydrostatic equilibrium when using X-ray observables. On the other hand, weak gravitational lensing can provide unbiased mass estimates because of its direct sensitivity to the dark matter distribution around galaxy clusters. In this talk, I will present the weak lensing mass calibration of galaxy clusters detected by Atacama Cosmology Telescope Polarimeter (ACTPol) through the thermal Sunyaev-Zel'dovich (SZ) effect, using the Hyper Suprime-Cam (HSC) Subaru Strategic Program (SSP) first-year data. The HSC SSP is an ongoing galaxy imaging survey that will cover 1,400 deg^2 of the sky down to the limiting magnitude i~26. The superb image quality, wide field-of-view, and tremendous light-collecting power make HSC one of the best weak lensing instruments in the world.I will discuss detailed studies of systematic uncertainties and comparison of our mass calibration to previous results in the literature.

発表者
平野 信吾(九州大学)

日程/場所
6月18日(月)15:00-@ES606

題名
Formation of first stars with ten to one million solar-masses

概要
First stars (Population III stars) play vital roles in the early cosmic evolution by initiating cosmic reionization and chemical enrichment of the intergalactic medium. The characteristic mass of first stars and its initial mass function are thus essential to understand the formation and evolution of the first galaxy, one of the important observational targets by the next-generation telescopes. The numerous theoretical/numerical works suggest various paths of first star formation and final masses depending on the star-forming environment properties. A large cosmological sample of first star formation shows a wide range of stellar masses from 10 to 1000 Msun depending on the properties of star-forming gas cloud. Under a quite limited condition, very massive first stars can form with 1e5-1e6 Msun and leave a promising seed for the formation of observed high-z quasars. Furthermore, a recent study shows the formation of the first star clusters and massive star binaries. The eventual formation of the remnant black holes will leave a close binary of massive black holes, which can be a progenitor of strong gravitational wave sources.

発表者
柏野 大地

日程/場所
6月12日(火)15:00-@ES606

題名
Exploring the end tail of reionization with JWST

概要
The James Webb Space Telescope (JWST), which is a 6.5-m space telescope and covers a wide range of infrared wavelengths from 0.6 to 28.8 micron. The launch is so far scheduled in 2020, with an expected Cycle 1 proposal deadline around Feb 2019 for general observers. JWST is expected to well characterize the properties of a number of z>10 objects and revolutionize our understanding on cosmic reionization and galaxy evolution and formation in the early Universe. In the talk, I would introduce JWST and our GTO program of Wide-Field Slitless Spectroscopy (WFSS) with the NIR-infrared Camera (NIRCam). We aim to study the end of reionization at redshift around 6 by searching for strong [OIII]-emission galaxies, focusing especially on the structures of galaxy large scale distribution and the ionized intergalactic medium.

発表者
黒柳幸子

日程/場所
6月6日(水)13:00-@ES606

題名
Matched filtering search for gravitational wave bursts from cosmic strings

概要
Cosmic strings are one-dimensional topological defects, which arise naturally in cosmological phase transitions, as well as in inflation models based on superstring theory. They are known to emit strong gravitational wave bursts from singular points on string loops, called cusps, and it has a particular spectral shape of f^{-4/3}. Since the waveform is known, we can perform matched filtering search using templates of cosmic string bursts. In my talk, I will report the progress of the code development with KAGALI (KAGRA Algorithmic Library) for matched filtering search of cosmic string gravitational waves. I also would like to discuss how to remove fake events, which in fact arise a lot when one performs the analysis with real data.

発表者
柿市 航樹 (University College London)

日程/場所
6月5日(火)16:00-@ES606

題名
On the Role of Galaxies and AGN in Reionizing the IGM and 21cm Cosmology

概要
Study of reionization has now experiencing a surge of new insights, but the long-standing problem ‘what reionized the universe?’ still remains unsolved. Hubble observations have placed a now-commonly-held view that the intrinsically faint galaxies are responsible for driving the reionization process, but with a fundamental assumption of large escape fractions. A further puzzle comes from the recent deep spectroscopy of luminous galaxies and the spatial opacity fluctuation of the intergalactic medium (IGM) at z>5.7, which suggests a possibly important contribution of luminous systems and active galactic nuclei (AGN). To address these issues, I discuss our recent results from a new Keck spectroscopic survey of 56 QSO fields, allowing us to gain new insights into the reionization era before the arrival of JWST and SKA. I will finish by discussing how can the future 21cm tomography help to understand the role of galaxies and AGN at the heart of reionization era, 7

5月のセミナー

発表者
長谷川賢二

日程/場所
5月30日(水)13:00-@ES606

題名
A new method to identify the spectral type of ionizing sources during the epoch of reionization

概要
Many observational studies have indicated that the Universe was ionized at z>6. However our understating regarding the ionizing sources is not yet enough because there is no conclusive evidence for identifying the sources. In this talk we suggest a new approach, in which we focus on the distribution of HeII, to distinguishing the types of ionizing sources. The new idea is based on the fact that the spatial correlation between HI and HeII is expected to reflect the spectral type of ionizing sources owing to the difference between the ionization energies of hydrogen and helium. Our radiative transfer simulations with various spectral types indeed show that the distributions of HI/HeII are quite sensitive to the spectral type. In addition we find that the hyperfine structure line of 3HeII (an isotope of HeII) is detectable with the SKA2 if the spin temperature is completely coupled with the IGM gas temperature.

発表者
浦川優子

日程/場所
5月23日(水)13:00-@ES606

題名
Gravitational wave forest from string axiverse

概要
It has been claimed that axions predicted in string theory may have a scalar potential which has a much shallower potential region than the conventional cosine potential. We first show that such axions generically undergo prominent instabilities after the onset of the oscillation based on both analytic argument and lattice simulation. We further show that because of these instabilities, string axions in various mass ranges lead to a generation of gravitational waves (GWs) whose spectrum has peaks at various frequencies determined by the mass scales, dubbed the GW forest. This may allow us to explore the string axiverse through future multi-frequency gravitational wave observations.

発表者
真喜屋龍 (Kavli IPMU)

日程/場所
5月22日(火)15:00-@ES606

題名
Cross-correlation between the thermal Sunyaev-Zeldovich effect and galaxy number density: Probing gas physics in the local Universe and beyond

概要
The thermal Sunyaev-Zeldovich (tSZ) is the spectral distortion of the CMB spectrum due to the hot thermal electrons in intra-cluster medium (ICM). Although the angular power spectrum of the tSZ effect is a sensitive probe of the amplitude of matter density fluctuation, it also depends significantly on gas physics of galaxy clusters. To investigate the physical properties of ICM gas, we have performed a tomographic study of the tSZ effect by cross-correlating it with the galaxy catalog obtained by the 2MASS redshift survey, as well as the SDSS galaxies. In this talk I will briefly review the current measurements and analysis of the tSZ effect and will show our latest results on the cross-correlation study of the tSZ effect.

発表者
田代寛之

日程/場所
5月16日(水)13:00-@ES606

題名
The EDGES anomaly - a solution from small-scale structure formation

概要
The detection of a global 21-cm absorption cosmic dawn signal by the EDGES experiment has stimulated a number of interpretations, most of which rely on exotic dark matter particles. However we suggest here that there is a simple cosmological explanation of the 21-cm absorption signal, which is based on the standard model of small-scale structure formation. We demonstrate that minivoids with a few kpc comoving scale can account for the amplitude of the EDGES global 21-cm absorption signal.

発表者
市來淨與

日程/場所
5月9日(水)13:00-@ES606

題名
Noise bias in the template cleaning method for the LiteBIRD experiment

概要
We have been developing a template-based foreground cleaning method for LiteBIRD, which is a satellite that will search for primordial gravitational waves emitted during the cosmic inflation. In this method, it now becomes clear that the instrumental noise introduces a significant bias in the estimation of foreground parameters and the tensor-to-scalar ratio r. To mitigate this bias problem, we improve the algorithm of the method. We discuss how small r can be recovered based on the new algorithm with the current specification of the LiteBIRD experiment.

4月のセミナー

発表者
新居舜

日程/場所
4月25日(水)13:00-@ES606

題名
On gravitational interactions at cosmological scales through scalar-tensor portals

概要
Gravity ubiquitously interacts with all the matters in the universe. According to General Relativity, gravitational interactions between the matters are given as a geometrical force with the single constant coupling. In some alternative theories of gravity, however, it is known that gravitational couplings are multiplicable. For instance, scalar-tensor theories with a dynamical scalar field predict various types of interactions depending on scales and time. As a result, gravitational couplings are no longer constant depending on matters and vary in time. In addition, some types of the scalar-tensor couplings are able to explain the accelerating universe such as the inflation and the late-time acceleration. To this end, it is curious to study those interactions from both theoretical and observational point of view.
We consider the gravitational interactions between a scalar and a tensor mode. Currently we work on Horndeski theory. We performed a systematic analysis to search the characteristics of models which are explainable the current observations. From the observation of gravitational-waves propagation, we prove that non-minimal interaction with the tensor sector are severely constrained. In fact by the latest of observation of GW170817 the model-space is significantly limited. Furthermore, we compute the CMB angular power spectrum for the models in the Horndeski theory, seeing the difference of Hubble expansion and gravitational constant in time.
In the end, we discuss what types of theory remains to be constrained in the future observation and theoretical developments.

発表者
Fabian Schmidt (Max Planck institute for Astrophysics)

日程/場所
4月18日(水)10:30-@ES606

題名
Galaxy clustering: an effective field theory approach

概要
Our theoretical understanding of galaxy clustering has improved significantly over the past 5 years. In the context of standard cosmological models based on GR, we now have a well-defined approach for absorbing all the complicated, incompletely understood physics of galaxy formation into a set of free parameters, the bias parameters. Following the effective field theory paradigm, the various contributions are classified by order in perturbation theory as well as number of spatial derivatives. I will review these developments, and discuss the implications for galaxy clustering as a robust probe of cosmology.

発表者
吉田貴一

日程/場所
4月11日(水)13:00-@ES606

題名
Reconstruction of density fluctuation of large scale structure and estimation of parameters

概要
It is important to know about the distribution and feature of fluctuation of matter density because it has much information related to cosmology. However, it’s difficult to reconstruct the initial density fluctuation, because we can only observe the fluctuation on the light cone, so if we observe fluctuation at farther place, it means we observe that of past.
How can we reconstruct density? If there is no restriction, we can’t reconstruct the fluctuation because of less information. Therefore, in this research, I used a method of reconstructing the density fluctuation that reproduces the power spectrum. To estimate parameter, I calculated the likelihood test using chi-square distribution and also calculated chi-square test, approximating that fluctuation obeys the gaussian distribution.
We could estimate cosmological parameters reliably 68% as a result and know the difference between likelihood test to chi-square test. I used power spectrum in this time, but I want to reconstruct without using this.

発表者
福永颯斗

日程/場所
4月11日(水)13:00-@ES606

題名
Exploring sting axiverse using gravitational waves

概要
According to string theory, axions are ubiquitous in the universe and their mass range is broad. However, we have not verified string theory by observation. I explore verifiability of string theory using gravitational waves. Once the onset of oscillation of axions are delayed, parametric resonance can enhance the inhomogeneous component of axions and that can generate detectable gravitational waves. I show roughly behavior of gravitational wave about simple model of axions using linear analysis. I also argue universal character of potentials so as to make observable gravitational wave.

発表者
近藤寛人

日程/場所
4月11日(水)13:00-@ES606

題名
Limit of abundance of PBHs by gravitational wave

概要
When PBHs as candidate of Dark Matter make binaries, they radiate gravitational wave. Small mass binaries make continuous wave. Gravitational waves from pulsars have already searched using data from the first run of the Advanced LIGO, but continuous wave is not found. If PBHs exist densely, the amplitude is enough large to be searched. So I estimated gravitational wave amplitude containing PBHs’ fraction in Dark matter, and tried to set limit of abundance of PBHs and to consider observability in future.

発表者
阿部克哉

日程/場所
4月11日(水)13:00-@ES606

題名
Gas accretion onto PBHs

概要
Primordial black holes(PBHs) are one of the candidates of the dark matter. Many researches have done to limit on the PBHs’ abundance by utilizing a wide range of observations.In this talk, we focus on a study which limited the abundance of PBHs by theoretically modeling the gas accretion onto PBHs. However, it is difficult to model theoretically because the accretion processes are highly complicated. Therefore, we conducted hydrodynamics simulations in order to check whether their model is reproducible or not. We found that numerical calculations can produce the same structures of some physical quantities such as gas density and gas velocity, with better accuracy in outer region.

発表者
杉山直

日程/場所
4月11日(水)13:00-@ES606

題名
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概要
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