Seminars in February
Speaker
Maxime Paillassa
Date/Place
13:00-, 18(Thur), Feb.
Title
Robust detection of astronomical sources using convolutional neural
networks
Abstract
Extracting reliable source catalogs from images is crucial for a broad range of astronomical research
topics. However, the efficiency of current source detection methods becomes severely limited in crowded
fields, or when images are contaminated by optical, electronic and environmental defects. Here I propose
new methods to produce more robust and reliable source catalogs using convolutional neural networks
(CNNs). I first present MaxiMask, a CNN trained to automatically identify image defects in astronomical
exposures. Then I introduce a prototype of multi-scale CNN-based source detector robust to image
defects, which is shown to outperform current state-of-the-art algorithms.
Speaker
Kento Sumiya
Date/Place
13:00-, 4(Thur), Feb.
Title
Measuring cosmological density fields twice
Abstract
When we look at things, we can only see them at one time. Therefore, the growth rate cannot be measured
directly. So we aim to establish a method for reconstructing the perturbation at the time of
recombination from the information of CMB photon that is polarized by the galaxy cluster. I will show
concrete theory and simulation results about this method, and discuss the scrutiny of the cosmological
model using this method.
Speaker
Hina Sakamoto
Date/Place
13:00-, 4(Thur), Feb.
Title
Impact of the first star on ionization history proved by
CMB polarization
Abstract
Intergalactic medium (IGM) is likely to have been ionized at z>6 by starlight escaped from early
galaxies, and the process is called cosmic reionization. The Early and young galaxies were born in
minihalos (MHs) and atomic cooling halos (ACHs), where the gas's main coolant is molecular hydrogen and
atomic hydrogen, respectively. Although the gas in MHs was more difficult to form stars due to lower
cooling efficiency than the gas in ACHs, it is not clear whether the contribution of MHs to the
reionization history was so small that we could ignore it. In this study, we consider two models: models
that consider only ACHs as the ionization photon source, and the others are models that consider both
ACHs and MHs as the ionization photon source. We use the results of the ionization histories obtained by
cosmological reionization simulations. By comparing the reionization histories using the PCA analysis of
the CMB E-mode polarization power spectrum, we estimate the impact of MHs on cosmic reionization.
Seminars in January
Speaker
Yuji Okochi
Date/Place
10:00-, 31(Thur), Jan.
Abstract
The possibility of deviations from general relativity on cosmological scales has been motivated by the
expectation that an alternative gravity theory could explain for the accelerated expansion of the
Universe. So, I aim to constrain two phenomenological parameters express deviations from general
relativity. These parameters modify the strength of gravity on non-relativistic and relativistic
particles, respectively. To constrain these parameters, I use measurements of cosmic shear,
galaxy-galaxy clustering, and galaxy-galaxy lensing.
Speaker
Yusuke Mikura
Date/Place
13:00-, 27(Thur), Jan.
Title
Conformal inflation in the metric-affine geometry
Abstract
Systematic understanding for classes of inflationary models is investigated from the viewpoint of the
local conformal symmetry and the slightly broken global symmetry in the framework of the metric-affine
geometry. In the metric-affine geometry, which is a generalization of the Riemannian one adopted in the
ordinary General Relativity, the affine connection is also an independent variable rather than being
given by the Levi-Civita connection. Thanks to this independency, the metric-affine geometry can
preserve the local conformal symmetry in each term of the Lagrangian contrary to the Riemannian
geometry, and then the local conformal symmetry can be compatible with much more kinds of global
symmetries.
Speaker
Yukiyoshi Morishita
Date/Place
13:00-, 14(Thur), Jan.
Title
Multi-field inflation with the Curvaton
Abstract
Inflation models are constrained by the observation of Planck, and VâˆÏ†^n models are seemed to be
disfavoured in the case of single field. So I try to rescue this models by using additional scalar
fields.In my previous study, inflaton+inflaton model was not consistent with Planck ,and then I
attempted using curvaton which is another scalar field uninvolved with inflation. I will introduce
models consists with inflaton and curvaton, and compare my calculations with previous paper.
Speaker
Hiroto Kondo
Date/Place
13:00-, 7(Thur), Jan.
Title
The high-z galaxy cluster cosmology with HSC and Spitzer
data
Abstract
In this work, we are aiming to constrain the cosmological parameters using high-z galaxy clusters
detected with HSC and Spitzer data. With combining infrared data with HSC data, we can improve galaxy
redshift estimation at high redshift. This combining analysis enable to construct more precise galaxy
cluster catalog and constrain cosmological parameters at high redshift. I will talk about cluster
cosmology briefly and report recent progress.
Seminars in December
Speaker
Katsuya.T.Abe
Date/Place
13:00-, 17(Thur), Dec.
Abstract
In this work, we investigate the effect of supernova explosion~(SN) of the Pop III stars on the global
ionization fraction. Due to the explosion, the shock shell will be ionized and expand toward the outside
about sub-kpc in physical scale. Depending on the SNe's number density, the global ionization fraction
can be changed. Then, the CMB photons suffer more scattering, and the E-mode angular power spectrum of
CMB traces it. Using Markov Chain Monte-Carlo(MCMC) method with the Planck 2018 polarization data, we
constrain the amount of SNe events. After showing the result, I will discuss what we can obtain about
Pop III stars through this result.
Speaker
Rika Ando
Date/Place
13:00-, 10(Thur), Dec.
Abstract
Intensity mapping of 21-cm line by several radio telescope experiments will probe the large-scale
structure of the Universe in the post-reionization epoch. It requires a theoretical framework of neutral
hydrogen (HI) clustering, such as modelling of HI power spectrum for Baryon Acoustic Oscillations (BAO)
analysis. We propose a new method for reconstructing the HI map from dark matter distribution using
N-body simulations. Several studies attempt to compute the HI power spectrum with N-body simulations by
pasting HI gas at the dark matter halo centre, assuming the relation between the halo and HI masses. On
the other hand, the method proposed in this paper reproduces the HI power spectrum from simulated dark
matter distribution truncated at specific scales from the halo centre. With this method, the slope of HI
power spectrum is reproduced well at large scales. We discuss how our method works by comparing the
density profiles of HI and dark matter around the dark matter halo.
Seminars in November
Speaker
Kouki Sakoda
Date/Place
13:00-, 26(Thur), Nov.
Title
Measuring CMB lensing
Abstract
The effect of Cosmic Microwave Background (CMB) gravitational lensing is the phenomenon that primordial
CMB is distorted by gravity between Last Scattering Surface and us. From the effect we can obtain
information on matter distribution and primordial gravitational waves.
The Quadratic Estimator method has been used to detect the lensing effect. Since the lensing effect
produces the lensing B-mode from the primordial E-mode, planned B-mode experiments from space, such as
and LiteBird, will significantly enhance the sensitivity of the lensing effect.
I calculate CMB lensing from simulation data (LiteBIRD’s spec).
Speaker
Kunihiko Furugori
Date/Place
13:00-, 19(Thur), Nov.
Title
The 21-cm signals from ultracompact minihalos as a probe
of primordial small-scale fluctuations
Abstract
Ultracompact minihaloes (UCMHs) can form after the epoch of matter–radiation equality, if the density
fluctuations of dark matter have significantly large amplitude on small scales. The constraint on the
UCMH abundance allows us to access such small-scale fluctuations. In this paper, we present that,
through the measurement of 21-cm fluctuations before the epoch of reionization, we can obtain a
constraint on the UCMH abundance. We calculate the 21-cm signal from UCMHs and show that UCMHs provide
the enhancement of the 21-cm fluctuations. We also investigate the constraint on the UCMH abundance and
small-scale curvature perturbations.
Speaker
Ikumi Ueda
Date/Place
13:00-, 19(Thur), Nov.
Abstract
We investigate the measurement of the equation of state parameter w and the sound speed cs of the phase
transition plasma with use of the gravitational waves (GWs) of the universe. Though the propagation of
GW is insensitive tocsin itself, the sound speed value affects the dynamics of primordial density (or
scalar curvature) perturbations and the induced GW by their horizon reentry can thus be an indirect
probe both of w and cs. We numerically reveal the concrete spectrum of the predicted induced GW with two
simple examples of the scalar perturbation spectrum: the monochromatic and scale-invariant spectra. In
the monochromatic case, we see that the resonant amplification and cancellation scales of the induced GW
depend on the cs values at different time respectively. The scale-invariant case gives a more realistic
spectrum and its specific shape will be compared with observations. In particular, the QCD phase
transition corresponds with the frequency range of the pulsar timing array (PTA) observations. If the
amplitude of primordial scalar power is in the range of 10−4.Aζ.10−2, the induced GW is consistent with
current observational constraints and detectable in the future observation in Square Kilometer Array.
Futhermore the recent possible detection of stochastic GWs by NANOGrav 12.5 yr analysis can be explained
by the induced GW if Aζ∼√7×10−3.
Speaker
Nanaka Yamamoto
Date/Place
13:00-, 5(Thur), Nov. @ES635 and with zoom
Title
Cluster mass calibration and galaxies number-mass
relation with CMB gravitational lensing
Abstract
The number density of galaxy clusters as a function of the mass and redshift is sensitive to the
structure growth since clusters are the most massive object. However, most of the mass in the Universe
is thought to consist of dark matter, which cannot be detected directly. Gravitational lensing is a
powerful way to estimate the halo mass including dark matter. In particular, CMB gravitational lensing
that uses the cosmic microwave background as the light source makes it possible to estimate the mass of
high-z (even z>1) clusters. And, the relation between the number of the galaxies contained in each
cluster (richness) and the mass is important to constrain cosmology. I will talk about cluster mass
calibrations and the constraint on the richness-cluster mass relation by using the CMB temperature data
from Atacama Cosmology Telescope (ACT) and CAMIRA S19A cluster catalog from Subaru Hyper Suprime-Cam
(HSC).
Speaker
Koya Murakami
Date/Place
13:00-, 5(Thur), Nov. @ES635 and with zoom
Title
Constraint on dark matter mass with deep learning
Abstract
The mass of dark matter prevent the cosmic large scale structure from glowing at the small scale, so the
analysis of the large scale structure enable us to study the mass of dark matter. In general, the large
scale structure is studied with the two-point correlation function of the matter density fluctuation.
But at the small scale, the non-linear growth of the structure make the information of two-point
correlation escape to the higher order statistics. Therefore, we need the method of analyzing this
higher order statistics and focus on Convolutional Neural Network(CNN). CNN extract the complex
information from images with the filters. In this work, we use the image of the dark matter distribution
from the N-body simulation for training and testing CNN. And then, by CNN and two-point correlation, we
discriminate the model of cold dark matter, which is too heavy to prevent the growth of the structure,
and non-cold dark matter. We compare the results from two methods and show CNN can extract more complex
information than two-point correlation. In addition, we perform the random Gauss simulation which have
only the information of two-point correlation of our N-body simulation, and compare this results with
the results from the other two methods.
Seminars in October
Speaker
Mutsumi Minoguchi
Date/Place
13:00-, 29(Mon), Oct.
Title
Alcock-Paczynski Test with the voids in HI intensity
mapping survey
Abstract
The recent observations with improving accuracy have revealed the issue called “Hubble tension”, which
is the unsolved tension lies between the local measurements of the expansion rate of the universe and
the expansion rate inferred from the cosmic microwave background observation assuming standard
cosmology. Therefore, it is important to test standard cosmology independently from the various
observables. Alcock-Paczynski test is one of the ways of estimating Hubble history, which uses the shape
of the originally isotropic object distorted in redshift space according to universal expansion. We
assume that the (stacked) void in the HI intensity map is the spherical object. In this talk, I will
introduce the details of the methods I use and will present the results currently obtained.
Speaker
Kazuhiro Kogai
Date/Place
13:00-, 22(Mon), Oct.
Title
QCD axion clump formation
Abstract
Axions are some of the hopeful scalar particles motivated beyond the Standard model. Although QCD axion
was introduced to solve the strong-CP problem at first, it has a potential to explain a dark matter.
When the initial field misalignment is large, which corresponds to the fine-tuned initial condition
around the hilltop of the potential, the attractive self-interaction becomes effective. In this case,
considering the adiabatic perturbation, Arvanitaki et al.(20) pointed out that the axion fluctuation
enhanced.
Since the effective potential for QCD axion depends on time, the axion perturbation also depends on the
perturbation of the radiation density. However, because this point was not discussed in the previous
work, we restudy the behavior perturbation including this. Here I will talk about some of the
interesting behavior as possible.
Speaker
Daiki Hashimoto
Date/Place
13:00-, 15(Mon), Oct.
Title
Constraining the Nature of Annihilating Dark Matter with
PDF of LSBG catalog
Abstract
Deriving a number of target regions such as the Galactic Center and Milky Way dwarf spheroidal galaxies
(dSphs), indirect searches for dark matter (DM) annihilation signals have been proved. In our study, we
use a low surface brightness galaxie (LSBGs) catalog as novel targets which contains about 800 objects
discovered by the Hyper Suprime-Cam (HSC). LSBGs are known to have very large DM contents and be less
contaminated by extragalactic gamma-ray sources (e.g., blazars) compared to star forming galaxies or
ordinally galaxies. Unfortunately in the present, most of distances for HSC-LSBGs are not measured
precisely, therefore they are not able to be derived in the analysis for the indirect research. In this
time, we will introduce the method of the new approach using the probability distribution function of
the LSBG catalog.
Speaker
Teppei Minoda
Date/Place
13:00-, 8(Thu), Oct.
Title
Constraining the primordial power spectrum from 21-cm
global signal with ultracompact minihalos
Abstract
The gravitational growth of small-scale primordial density fluctuations possibly leads to the formation
of low-mass halos with dense centers at high redshifts (z>1000), which are called ultracompact minihalos
(UCMHs). We calculate the heating rate for the baryon gas by the dark matter annihilation inside a UCMH
and estimate the abundance of UCMHs from 21-cm line observations to give an upper limit on the amplitude
of small-scale primordial density fluctuations below Mpc. This result is presented in this seminar.
Speaker
Toshiyuki Tanaka
Date/Place
13:00-, 01(Thu), Oct.
Title
Detailed modeling of mini-haloes in cosmological 21-cm
simulation
Abstract
To extract fruitful information on the high-redshift Universe from the neutral hydrogen 21-cm line
signal, theoretically modeling the statistics of the 21-cm signal accurately is the key.
We have conducted the semi-numerical simulations which incorporate the time-evolving escape fraction of
ionizing photons, which reflects increasing minimum halo mass for star formation due to the Lyman-Werner
(LW) negative feedback. We also develop a new methodology to account for the UV photo-heating which has
ever been neglected.
We find that the escape fraction becomes nearly zero until redshift ~35 owing to the growth of LW
intensity, so that the ionization fraction of the Universe remains small compared with the case with
conventionally used constant escape fraction. Such high neutral fraction results in deep absorption in
the 21-cm global signal and negligible impact of UV heating on 21-cm statistics. However, we also find
that if the Universe is ionized more than only percentage level, the UV heating has non-negligible
impact on the global signal to roughly the same extent as ionization on the global signal. Moreover, our
results show that the UV heating steepens the slope of the 21-cm power spectrum by decreasing the power
by nearly one order of magnitude at scale k~0.1 [/Mpc] which is the very target scale of current and
future 21-cm observations. Thus, we suggest that both the time-evolving escape fraction and the UV
heating can be essential to accurately simulate 21-cm signals at high redshift.
Seminar in July
Speaker
Evolution of Supermassive Black Holes inside Dark Matter Halos
Date/Place
13:00-, 27(Mon), June.
Title
Evolution of Supermassive Black Holes inside Dark Matter
Halos
Abstract
There is positive observational evidence that most of the galaxies have a central supermassive black
hole (SMBH) with mass in range of 106 – 109.5 M☉, but no convincing or conclusive theory that proves the
origin of these black holes. Furthermore, there is a clicking proportionality relation between the mass
of central black hole and that of the bulge. This could indicate, although uncertain, the formation of
galaxies from the black holes. Certain quasar observations also proffer the idea of SMBH existing at
very high redshifts. Using merger phenomenon of Black holes and accretion history, we study the
evolution of SMBH inside Dark Matter Halos. We start with the already known relation between masses of
Dark Matter Halos and central Black Holes, which is valid only for z=0. Hence, we modify this known
relation to work for different redshifts. For this purpose, we generate merger histories of 500 Dark
Matter Halos- 100 each of mass 1010, 1011, 1012, 1013 and 1014 solar mass respectively. We find
progenitors of these Halos at various redshifts (z=0.2, z=3, z=6) and assign a black hole to each of
these halos. Then, by evolving these black holes (assuming negligible accretion) we compare our results
with the observations and hence, study the deviation. Through this, we modify the relation to compensate
for the deviation and finally validate the modified relation.
Speaker
Yukiyoshi Morishita
Date/Place
13:00-, 27(Mon), June.
Title
複数場に拡張したインフレーションモデルの検証
Abstract
本研究ではPlanckの観測によるインフレーション理論への制限に着目する。
その観測結果から単一場のV=φ^nモデルは否定的なものと考えられているため、複数場のV = φ^n + σ^n モデルを考える。
φ, σ が共にインフラトンとして振る舞う場合は以前計算したが、パワースペクトルの傾きn_sを下げられる一方、 テンソル・スカラー比rは単一場モデルと同様であった。そのため、本発表では φ がインフラトン、σ
がカーバトンとして振る舞うモデルを考え、rを下げる仕組みについて議論する。
また、カーバトンの特徴である曲率ゆらぎの非ガウス性についても紹介する。
Speaker
Takeshi Kobayashi
Date/Place
13:00-, 20(Mon), June.
Title
アクシオン暗黒物質の宇宙論的帰結
Abstract
暗黒物質としてのアクシオンに対する宇宙論的制限をレビューします。基本的な計算を紹介する事に重点を置きます。時間が余れば、自分の最近の研究についても述べます。
Speaker
Yusuke Mikura
Date/Place
13:00-, 01(Mon), June.
Title
Non-minimal Inflation in the Palatini formalism
Abstract
Einstein Hilbert 作用において, 計量のみを変数として扱う metric formalismと計量と接続を変数として扱う Palatini formalism は完全に等価となる.
しかし作用に非最小結合を持つようなモデルを考える場合, その２つの形式では異なる予言を与える. 重力とスカラー場が φ^2R の形で結合しポテンシャルが φ^n で書かれる時に,
インフレーション理論でよく扱うスペクトル指数 n_s とテンソル・スカラー比 r が e-folds number を用いてどうなるのかを将来の観測も絡めて Palatini formalism
を用いて議論する.
Speaker
Hina Sakamoto
Date/Place
13:00-, 01(Mon), June.
Title
初代星が再電離に与える影響と将来観測機器での観測可能性
Abstract
銀河間物質(IGM)は，宇宙の密度ゆらぎの成長の結果誕生した天体からの電離光子により，z~6より前までに電離が行われたことが観測的に示唆されている．このIGMが電離される過程を宇宙再電離という．IGMを電離する電離光子源の有力な候補は若い銀河で，星形成において水素分子が必要であるミニハロー(MH)と原子冷却ハロー(ACH)で形成される．ところが再電離の一般的なシミュレーションではMHからの寄与は無視されることが多いが，その寄与が無視できるほど小さいかは明らかではない．
本研究では，まず宇宙論的スケールでの輻射流体シミュレーションをMH有無の場合で行い，MHが再電離史に与える影響を見積もる．この電離史の違いがCMBのE-mode偏光のパワースペクトルの大角度スケールに現れると期待される．そのため，CMBを主成分分析(PCA)と呼ばれる統計手法を用い解析することで，MHの影響を調査する．
Speaker
Yuji Okochi
Date/Place
14:30-, 06(Mon), July.
Title
すばるHSCのデータを用いた銀河のクラスタリングと弱重力レンズ効果の二点相関による重力理論の検証法
Abstract
宇宙論的スケールでの重力理論を検証するために、暗黒物質分布を高赤方偏移まで高解像度で測定できるすばる望遠鏡Hyper
Suprime-Cam(HSC)による撮像銀河サーベイのデータを用いる。本研究では非相対論的物質と相対論的物質が受ける重力の強さを修正する現象論的な二つのパラメータを導入して、重力の理論を一般相対論から修正する。galaxy
clustering、galaxy-galaxy lensing、cosmic shearについて観測と理論の比較して、これらのパラメータに制限をつけることが本研究の目標である。
Speaker
Kento Sumiya
Date/Place
14:30-, 06(Mon), July.
Title
銀河団によるCMB偏光を用いたゆらぎの再構築法
Abstract
宇宙論を精査するために、宇宙の大規模構造の進化、ひいては密度ゆらぎの発展を知ることは非常に重要である。しかし、宇宙空間の構造を観測した場合、観測点が遠く離れるほど過去の姿を観測することになり、同じ地点の構造を異なる時刻で観測することができない。そのため異なる地点の構造から推定される物質ゆらぎの成長率は不定性を大きく含んでいる。この問題の解決策として、同じ地点の構造を異なる時刻で観測する新しい手法を確立しゆらぎの成長率を直接測ることが挙げられる。
今回は異なる時刻における構造を測定する方法として銀河団によって散乱されるCMBの偏光を観測し、最終散乱面から我々の間の領域の晴れ上がり時でのCMB温度ゆらぎを再現することを考えた。この手法はCMB光子が銀河団を通過する際に、銀河団内部の自由電子に散乱され、銀河団
まわりの最終散乱面上でのCMB温度ゆらぎの四重極成分に由来する偏光を生じることを用いる。多数の銀河団に対してこの偏光を観測することで、最終散乱面から我々の間の領域の晴れ上がり時でのCMB温度ゆらぎを再構築することが期待できる。
本発表では手法の説明と今後行うシミュレーションの内容についての議論を行う。
Seminar in June
Speaker
Atsushi J. Nishizawa
Date/Place
13:00-, 29(Mon), June.
Title
21cm line cosmology
Abstract
Recent studies related to the 21 cm cosmology, mainly focused on the post reionization epoch will be
introduced. Also we will present our new attempts to remove the foreground emission, such as synchrotron
radiation from our Galaxy in the context of taking a cross-correlation between the CMB-lensing which might
be the unique way to extract the cosmological 21 cm line signals.
Speaker
Yuichiro Tada
Date/Place
13:00-, 15(Mon), June.
Title
A manifestly covariant theory of stochastic inflation in phase
space
Abstract
We formulate the stochastic approach to inflation in a covariant way with respect to the inflatons’ target
space. The stochastic formalism is an effective theory for the superhorizon coarse-grained fields, where
the dynamics is described as a diffusion system with stochastic noise sourced by the subhorizon modes
continuously exiting the horizon. In a previous letter, we pointed out that the naive stochastic approach
suffers from the “stochastic anomaly” due to the non-differentiability of noise. That is, the theory
violates its covariance or exhibits a spurious dependence on the orthonormal decomposition of noise, or
both. In this talk, we show that there is in fact a preferred noise frame corresponding with the basis of
annihilation/creation operators, and then the theory's covariance can be saved if the differential
equation is discretized in the midpoint prescription, i.e., Stratonovich. We also show the equivalent
prepoint (Ito) discretisation for practical usefulness.
Speaker
Kenji Hasegawa
Date/Place
13:00-, 08(Mon), June.
Title
Predicting 21cm-line map from Lyman α emitter distribution
with Generative Adversarial Network
Abstract
The radio observation of 21cm-line signal from the Epoch of Reionization (EoR) is useful to study the
evolution of galaxies and intergalactic medium in the early universe. In our study, we attempt to generate
21cm-line images from the distribution of the Lyman-α emitters (LAEs) through machine learning. We apply
conditional Generative Adversarial Network (cGAN) trained with the results of our numerical simulations
for cosmic reionization. As a result, we find that 21cm-line brightness temperature maps and the neutral
fraction maps can be well predicted by using the distribution of LAEs at large scales. We also show that
the cross correlation between observed 21cm-line data and the generated 21cm-line image is detectable at k
< 0.2 Mpc^-1 by combing 400 hours of MWA Phase II observation and LAE deep survey of the Subaru Hyper
Suprime Camera.
Speaker
Hironao Miyatake
Date/Place
13:00-, 01(Mon), June.
Title
Enlightening the Dark Universe with Weak Gravitational
Lensing
Abstract
Weak gravitational lensing, the coherent distortion of distant galaxy images caused by massive structures
in the Universe, is one of the powerful probes of cosmic acceleration. I will start my talk with basics of
weak lensing, highlight cosmology results from the Subaru Hyper Suprime-Cam (HSC) survey, and show the
current status of the HSC survey. I will then introduce my ongoing projects; cosmological constraints from
galaxy-galaxy lensing and clustering using the Baryon Oscillation Spectroscopic Survey (BOSS)
spectroscopic sample at z~0.55 and LBG Lyman Break Galaxies (LBGs) at z~4. I will also discuss Japanese
participation plan for upcoming weak lensing surveys.
Seminar in May
Speaker
Shuichiro Yokoyama
Date/Place
13:00-, 28(Thu), May.
Abstract
We present an impact of coupling between dark matter and a scalar field, which might be responsible for
dark energy, on measurements of redshift-space distortions (RSDs). We point out that, in the presence of
conformal and/or disformal coupling, linearized continuity and Euler equations for total matter fluid
significantly deviate from the standard ones even in the sub-horizon scales. In such a case, a standard
relation between a peculiar velocity of total matter field and growth of matter density field would be
modified, and then RSDs are not a direct probe of growth of structure any longer. As a result, we’d like
to stress the importance of the direct measurement of the time-evolution of structure.
Speaker
Tomohiro Abe
Date/Place
13:00-, 21(Thu), May.
Title
The effect of the early kinetic decoupling in a fermionic dark
matter model
Abstract
We study the effect of the early kinetic decoupling in a model of fermionic dark matter (DM) that
interacts with the standard model particles only by exchanging the Higgs boson. There are two DM-Higgs
couplings, namely CP-conserving and CP-violating couplings. If the mass of the DM is slightly below half
of the Higgs boson mass, then the couplings are suppressed to obtain the measured value of the DM energy
density by the freeze-out mechanism. In addition, the scattering processes of DM off particles in the
thermal bath are suppressed by the small momentum transfer if the CP-violating DM-Higgs coupling is
larger than the CP-conserving one. Due to the suppression, the temperature of the DM can differ from the
temperature of the thermal bath. By solving coupled equations for the number density and temperature of
the DM, we calculate the DM-Higgs couplings that reproduce the right amount of the DM relic abundance.
We find that the couplings have to be larger than the one obtained without taking into account the
difference in the temperatures. A consequence of the enhancement of the DM-Higgs couplings is the
enhancement of the Higgs invisible decay branching ratio. The enhancement is testable at current and
future collider experiments.
Speaker
Hiroyuki Tashiro
Date/Place
13:00-, 14(Thu), May.
Title
Gravitational waves and small-scale magnetic fields
Abstract
Gravitational waves can be used as powerful probes of magnetic fields in the early universe. In this
talk, considering the anisotropic stress of magnetic fields as a source of gravitational waves, we sho
w that pulsar timing arrays have a potential to provide a strong constraint on magnetic fields in the
early universe. We also demonstrate that the turbulent motion due to magnetic fields could be an inte
resting source of gravitational waves.
Speaker
Kiyotomo Ichiki
Date/Place
13:00-, 07(Thu), May.
Title
Observing cosmological density fluctuation twice- A new
test of dark energy using CMB quadrupoles.
Abstract
Scattering of cosmic microwave background (CMB) radiation in galaxy clusters induces polarization
signals according to the quadrupole anisotropy in the photon distribution at the location of clusters.
This `remote quadrupole' derived from the measurements of the induced polarization in galaxy clusters
provides an opportunity to reconstruct primordial fluctuations on large scales. Here we discuss that
comparing the CMB quadrupoles predicted by these reconstructed primordial fluctuations and the direct
measurements done by the CMB satellites may enable us to test dark energy beyond cosmic variance limits.
Seminars in April
Speaker
Tomomi Sunayama
Date/Place
13:00-, 30(Thu), Apr.
Title
Modeling projection effects for more accurate cluster
cosmology
Abstract
Galaxy clusters constitute a natural cosmological probe for constraining the properties of the
primordial fluctuations as well as cosmological parameters including the nature of dark energy. Many
current and future surveys, such as the Hyper Suprime-Cam (HSC) survey, the Dark Energy Survey (DES),
the LSST, and Euclid, are aiming to capitalize on this prospect by utilizing clusters as one of their
key cosmological probes.We expect these surveys to find galaxy clusters in unprecedented numbers and
carry out cluster-based cosmology analyses with great statistical precision if systematic effects are
well under control. The major systematic effect for optical cluster cosmology is mis-identification of
member galaxies along the line-of-sight, known as projection effects. This effect alters the
mass-observable relation for optical clusters. In this talk, I will present the impact of projection
effect on mass-observable relationship as well as other observables such as lensing and clustering.
Speaker
Yuji Okochi
Date/Place
13:00-, 23(Thu), Apr.
Title
銀河とcosmic-shearの二点相関を用いた修正重力理論の検証法
Abstract
修正重力理論の検証法として、BOSSとHSCから得られる銀河数密度ゆらぎとcosmic-shearの二点相関（galaxy clustering、galaxy-galaxay
lensing、shear-shear correlation）について、理論と観測の比較を行った。
本研究では、修正重力理論において線形摂動の時間変化を記述する４つのα関数はダークエネルギー成分に比例することを仮定した。そして、それらの比例係数のうち、α_Bとα_Mを一般相対論における値から変更した場合について、ΛCDMモデルと合わせて観測結果と比較することにより、修正重力理論の検証が可能であることを示せた。
Speaker
Hina Sakamoto
Date/Place
13:00-, 23(Thu), Apr.
Title
Lyα線の有効光学的厚みを用いての宇宙再電離のモデルの検証
Abstract
Lyα線の光学的厚みはある赤方偏移で中性水素の割合と密度の関数であるが、z∼5.5で密度揺らぎのみから推定される光学的厚みのばらつきより観測される光学的厚みのばらつきが大きいことがわかっている。そこで、再電離のモデルの違いからこのばらつきを説明することを考える。再電離のモデルとして密度の多いところから電離するモデルと、密度の低いところから電離するモデルが存在する。今回これらのモデルで電離史を数値的に計算し、観測の光学的厚みと比較することで、どちらのモデルがより尤もらしいかを考察した。
Speaker
Kento Sumiya
Date/Place
13:00-, 23(Thu), Apr.
Title
同一初期ゆらぎからの密度ゆらぎの発展シミュレーションと各流体成分の可視化
Abstract
宇宙論的摂動論の議論はフーリエ空間上でなされることが多い。しかし多流体成分の相互作用を見たい時などは実空間上で議論した方が直感的な理解を得やすい。そこで、任意の時刻・任意の空間スケールで宇宙の任意の流体成分の発展を実空間で記述できるツールの開発を目指す。さらに開発したツールによって個別に可視化された流体成分の発展がどのような物理効果によってもたられているかを検証する。
Speaker
Yukiyoshi Morishita
Date/Place
13:00-, 16(Thu), Apr.
Title
δNを用いたmulti-fieldモデルの検証
Abstract
現在、理論的に様々なインフレーションモデルが考案されている。その一方で、観測によってそれらのモデルには制限が課されている。今回はPlanckによる観測に注目した。Planckの観測により、インフレーションモデルのspectral
indexとtensor fractionには制限が課される。そこで今回は、Planck
の観測により否定されているsingle-fieldのモデルをmulti-fieldのモデルに拡張することでPlanckの観測に一致するモデルができるか否かを検証した。また、数値計算にはmulti-fieldを扱う上で便利とされているδN
formulaを用いた。
Speaker
Yusuke Mikura
Date/Place
13:00-, 16(Thu), Apr.
Title
自己紹介と卒業研究
Abstract
ビックバン宇宙論にはいくつか問題点があり、その問題を解決するためにインフレーションと呼ばれる加速膨張が導入された。卒業研究では線形の摂動方程式の導出を行い、最も単純なモデルである単一スカラー場によるインフレーションでの揺らぎの成長を調べた。
Speaker
Naoshi Sugiyama
Date/Place
13:00-, 9(Thu), Apr.
Title
New year's impression
Abstract
New year's impression