Cセミナー 2017

English

4月 | 5月 | 6月 | 7月 | 8月 | 9月 | 10月 | 11月 | 12月 | 1月 | 2月 | 3月

＊プレゼンテーションファイルは研究室内部ネットワークのみで閲覧可能です。

## 3月のセミナー

Purnendu Karmakar (Universita degli Studi di Padova)

3月12日(月)16:00-＠ES635

We study cosmic structures in the quadratic Degenerate Higher Order Scalar Tensor (qDHOST) model, which has been proposed as the most general scalar-tensor theory (up to quadratic dependence on the covariant derivatives of the scalar field), which is not plagued by the presence of ghost instabilities. We then study a static, spherically sym- metric object embedded in de Sitter space-time for the qDHOST model. This model exhibits breaking of the Vainshtein mechanism inside the cosmic structure and Schwarzschild-de Sitter space-time outside, where General Relativity (GR) can be recovered within the Vainshtein radius. We then look for the conditions on the parameters on the considered qDHOST scenario which ensure the validity of the Vainshtein screening mechanism inside the object and the fulfilment of the recent GW170817/GRB170817A constraint on the speed of propagation of gravitational waves. We find that these two constraints rule out the same set of parameters, corresponding to the Lagrangians that are quadratic in second-order derivatives of the scalar field, for the shift symmetric qDHOST.

Victor Bonjean

3月8日(木)11:00-＠ES606

The star formation rates (SFR) and the stellar masses (Mstar) of galaxies hold a lot of information about the evolution and the history of the cosmic structures they belong to. These quantities are well estimated for star-forming galaxies, but not so much for passive galaxies, which are of particular interest to study and characterise the galaxy clusters and the nature of cosmic filaments. Recently, machine learning algorithms have been more and more developed and have now become reliable tools to classify or estimate physical properties of astrophysical objects. I will present a machine-learning based estimation of SFR and Mstar for any types of galaxies in the all sky.

3月1日(木)11:00-＠ES606

A cluster of galaxies is both a magnetized relativistic and non relativistic plasma. Thus, it acts as a birefringent medium for the light that goes through this kind of structures. In particular, CMB photons cross clusters of galaxies on their way to us. Depending on the relativistic nature or not of the free electrons in the plasma, the effect on the CMB polarization will be different. In the case of a cold plasma, the polarization plane is rotated after going through a cluster of galaxies (Faraday Rotation). On the contrary, when the plasma is relativistic, CMB photons experience Faraday Conversion which transfers linear polarization into circular polarization. I looked at a statistical study of this effect, by first focusing on predicting the angular power spectra of such effects due to clusters, using a line-of-sight approach and the halo model, implementing the cosmological principle for the statistics of the directions of the magnetic fields inside clusters. These effects are both sensitive to the cosmology through the mass function of the clusters and so parameters such as σ8, and to the distribution of the magnetic field and electrons inside a cluster, and can be used as a probe both for cosmology and for clusters of galaxies. Then I looked at the consequences of these effects on the angular power spectra of the CMB polarized anisotropies: the Faraday rotation effect mixes the E and B modes (creating secondary B modes in addition to the primordial and lensing ones), whereas the Faraday conversion effect transfers E and B anisotropies into the V mode, creating circular polarization which has no primordial origin in the standard cosmological scenario. Finally, I will sketch how polarized CMB anisotropies can be used for reconstructing the Faraday rotation and conversion fields.

## 2月のセミナー

2月22日(木)10:00-＠ES606

Almost isotropic gamma-ray background(EGB) is observed by Fermi gamma-ray sky survey. Good understandings of its origin are helpful for clarifying astronomical objects radiating gamma-rays. the EGB is thought to originate from objects such as blazars and star-forming galaxies or from exotic components such as annihilating or decaying dark matters. These gamma-ray sources are concentrated on halos. Therefore researches have been performed to probe the cross correlation signals with the gamma-ray sky map and the matter distributions of sky in the EGB. In this research, we focus on the red shift evolution of signals. Actually, each gamma-ray source candidate has a different red shift dependence on the gamma-ray intensity. Therefore, the clusters catalog with clusters of the wide red shift distribution is expected to arrow us to clarify the red shift evolution of the cross correlation signal and we can approach the properties of the EGB itself and the gamma-ray sources in the EGB by comparing the red shift dependence of each gamma-ray source.
In my presentation, I present the cross correlation signal with Fermi gamma-ray sky survey and HSC clusters catalog(CAMIRA) from the two point cross correlation analysis and the stacking analysis with introductions for how to analyze.

2月22日(木)10:00-＠ES606

Type Ia supernova (SN Ia) is a well-known standard candle, and is useful for the constraining cosmological parameters. The cosmological parameters can be obtained from the distance moduli of SNe Ia only after the precise measurement of photometry (e.g. Suzuki et al., 2012). However, the current observations do not correct for the magnification of SNe flux caused by gravitational lensing due to large-scale structure. The aim of this work is to study the effect of gravitational lensing on the constraint of cosmological parameters. First, we estimate the effect of the magnification on the distance modulus by N-body simulation (Takahashi et al., 2017) and find it is less than 3%. Second, we estimate the magnification by two estimators using HSC galaxy catalogs; one is convergence map reconstructed from HSC shear catalog, and the other is mass distribution anticipating the profile of galaxies and stellar to halo mass relation combined with photometric redshift of galaxies. Although it is naively expected that scatter of distance modulus around the best fit theoretical curve can partly be explained by the gravitational magnification, we find no significant signature of it. Last, we show the result of the measurement of cosmological parameters by Markov Chain Monte Carlo (MCMC) method and rest of the time will be devoted for the discussion about the lensing effect on the cosmological parameters.

2月22日(木)10:00-＠ES606

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 statistically isotropic Gaussian. In this study, I compared the CMB Monte Carlo simulation maps and the real CMB map, and tested the statistical anisotropy of the mean of the CMB temperature fluctuations in spherical harmonics space.

2月21日(水)10:30-＠ES606

Cosmology from cosmic microwave background polarization

One of the promising cosmological probes in the next decades is the CMB polarization. While CMB temperature anisotropies have been already measured very precisely, CMB polarization, in particular a twisting pattern in the polarization map (B mode) is not well measured. The detection of B mode at more than degree angular scale opens new window into the inflationary universe and high energy physics beyond the standard model. Precise polarization data also enables us to measure gravitational lensing of CMB which is a key probe to understand the properties of neutrinos, dark matter and dark energy. In this talk, I will first present my analysis works within the BICEP2/Keck Array collaborations. I will also talk about synergy between CMB experiments and galaxy surveys such as the galaxy-lensing cross correlation with Subaru-Hyper Suprime Cam and CMB experiments, and delensing B mode with mass tracers.

2月13日(火)15:00-＠ES606

Electromagnetic Wave Observations of GW170817

The first gravitational wave (GW) observationfrom a neutron star merger was successfully made for GW170817. The detection triggered electromagnetic (EM) wave observations over the entire wavelength range, which enabled the first identification of an EM counterpart of a GW source. Observed properties of optical and near-infrared emission are similar to those of theoretically-expected “kilonova” which is powered by radioactive decays of r-process nuclei. In this seminar, I review EM observations of GW170817 and discuss implications for the origin of heavy elements in the Universe.

2月1日(木)13:00-＠ES606

It is proposed that there are one dimensional high energy objects called cosmic strings and cosmic superstrings in the universe. Cosmic strings are formed at the GUT phase transition and some inflation models and cosmic superstrings are suggested by superstring theory. Cosmic superstrings have Y-junctions in the configuration and low reconnection probability.
It is difficult to observe these strings directly, but we know that they make the gravitational wave background from the sharp structure called kinks. However, it is complex to calculate the gravitational wave background of cosmic superstrings because there are Y-junctions in the cosmic superstrings, so it has not been clarified properly. Therefore, we focus on the specific tensions of cosmic superstrings, and calculate the number of kinks and the power spectrum of the gravitational wave background. Then, we discuss the observational probability of the gravitational wave background.

## 1月のセミナー

1月25日(木)13:00-＠ES606

Underdense regions in the large scale structure of the universe, namely voids, are expected to be a probe of cosmology. Recent works have reported that the evolution of voids represents differences of dark energy or gravity models. To identify the void structure, we need tracers of dark matter because we do not observe it directly. In Stark et al.,(2015) they present a method to identify voids by using Lyα flux. In this work we consider the neutral hydrogen (HI) gas to be one of tracers of dark matter. The HI gas emit the 21cm line which is expected to be detected by Square Kilometer Array. Our goal is to identify void structures in the distribution of HI gas provided by the ILLUSTRIS simulation and inspect whether HI voids are consistent with dark matter voids. In this seminar I explain our procedure in this work via reviewing previous researches.

Patel Teerthal

1月18日(木)10:00-＠ES606

Magnetogenesis in Axion Monodromy Inflation

Magnetic fields are ubiquitous in the Universe and are present at various scales. Recent observations has suggested the presence of cosmological magnetic fields in the inter galactic medium and indicate that the magnetic fields were generated in the Early Universe. The ambiguity surrounding the high energy physics fundamental to inflation has motivated us to explore a string theory inspired inflation model, namely, Axion Monodromy. We quantitatively compute, not only the primordial field strength generated in this inflation model, but also the corresponding scale dependence. The magnetic fields are generated in this setup via a helical interaction term between the scalar inflaton and the electromagnetic vector field. Quantitative comparison is conducted between the magnetic fields generated with an inflationary background driven by a quadratic scalar potential and the ones generated with an oscillatory contribution to the inflaton potential. This suggests that the oscillating background does not contribute any significant changes to the generated magnetic fields over the plain quadratic potential case and a qualitative analysis will be performed.

1月18日(木)10:00-＠ES606

Non-linear effects during inflation can generate primordial non-Gaussianities. Arkani-Hamed & Maldacena showed that an interaction between the inflation and higher spin fields, which may be predicted in string theory, can generate angular dependent non-Gaussianities, 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 non-Gaussianity from massive spin 2 fields can be observationally distinguished from the anisotropic non-Gaussianity with a violation of the global isotropy, which can be generated e.g., by a vector field. As a result, we found that the violation of the global anisotropy leads to non-vanishing off-diagonal components of multipole moment ell and B mode, which cannot be generated from the angular dependent non-Gaussianity with the global isotropy. In addition, if possible, the previous study will be shown with more precisely.

1月18日(木)10:00-＠ES606

Dark energy is considered as a strong candidate that causes accelerating expansion of the universe. By constraining the parameters of the equation of state of dark energy, it is possible to constrain the properties of dark energy. By using baryon acoustic oscillation(BAO), we can constrain the models of dark energy from the expansion history of the universe.
Square Kilometre Array (SKA) which is a large-scale radio interferometer will observe the 21-cm line emitted from neutral hydrogen(HI). By Intensity mapping survey, SKA will measure the intensity from unspecified sources. Therefore, we can obtain the information of BAO from HI.
In the previous research (Bull et al 2015), they predicted the constraint on the parameters of dark energy by SKA. The parameters are constrained by comparing the power spectra obtained by observations with that calculated using the model. They forecasted using a model where the HI bias is linear. For modeling the HI bias, we used the illustris simulation and osaka model simulation.

## 12月のセミナー

12月21日(木)13:00-＠ES606

We study Planck 2015 cosmic microwave background anisotropy data using the energy density inhomogeneity power spectrum generated by quantum fluctuations during an early epoch of inflation in the non-flat ΛCDM model.
Unlike earlier analyses of non-flat models, which assumed an inconsistent power-law power spectrum of energy density inhomogeneities, we find that the Planck 2015 data alone, and also in conjunction with baryon acoustic oscillation measurements, are reasonably well fit by a closed ΛCDM model in which spatial curvature contributes a few percent of the current cosmological energy density budget.

Tanmay Vachaspati (Arizona State University)

12月19日(火)16:00-＠ES606

The ongoing search for intergalactic magnetic fields using blazar observations suggests field strength in the 10^(-14) to 10^{-17} G range on tens of Mpc length scales. Such fields can have important implications for the early universe and also raises some puzzles.

12月7日(火)9:00-＠ES606

The existence of the primordial magnetic fields (PMFs) is suggested for the origin of the cosmic magnetic fields, but there are no direct evidence for the PMFs. Now the upper limit of the strength for the PMFs is about 4.4 nano-Gauss obtained by the CMB anisotropies (Planck 2015 results XIX). On the other hand, (Marinacci & Vogelsberger, 2016) has claimed that the PMF stronger than 1 nano-Gauss can dramatically impact the cosmic star formation and the galaxy formation history based on the cosmological MHD simulation. Therefore the above constraint on the PMFs with CMB observation is not strong enough with respect to the cosmological structure formation. In addition, the current upper limit of the PMFs with CMB observation is obtained with only the primary component of the CMB anisotropy at large-angle, where $\ell \lesssim 3 \times 10^3$.
Therefore, we investigated the two different works on the PMFs and the CMB anisotropies: (1)The constraint on PMFs with primary component of the CMB anisotropy at small-angle, $\ell \lesssim 10^4$. (2)The constraint on PMFs with secondary component of the CMB anisotropy, especially thermal Sunyaev-Zel’dovich effect. Consequently, we obtained the tighter constraint on the PMFs of $\mathcal{O}(0.1)$ nano-Gauss with work (1). Also, we concluded that the constraint with work (2) is much weaker than that with (1), but we found that the non-linear evolution in the high-z universe can affect the results of (2) even with the $\mathcal{O}(0.1)$ nano-Gauss PMF. The numerical calculation including the non-liner structure formation with the PMFs is beyond the scope of our works, so we put this on the future work.

12月7日(火)9:00-＠ES606

Owing to resent redshift surveys, we now have a lot amount of data on galaxy clusters’ positions. Accordingly, the number of voids between galaxy clusters also can be treated statistically. This means voids also can be one of the important prove for testing cosmological models. So it is important to construct model for describing the evolution of void’s quantities such as sizes and shapes. Icke 1984 reported voids tend to be expand and be spherical in very simple case as isolated top-hat model, whereas it seems not necessarily the case in cosmological circumstances, as shown by Nadathur 2016, for example. I found out that individual void can grows to be either spherical or distorted in cosmological case using N-body simulation. Because we distinguish the void from density peaks around it, the model with density peaks, rather than top-hat model, is worth examining. In this talk, I will show the methods and results on cosmological simulation and isolated void simulation, and discuss on the evolution of individual voids.

12月7日(火)9:00-＠ES606

The first stars are the first luminous objects in the universe, which marked the end of the dark ages. Although they are thought to have affected the following structure formation, their mass spectrum has not been clarified very much as well as other properties. Since the mass spectrum can be investigated through the 21-cm signature emerged in the vicinity of the first stars, some radiative transfer simulations have been done to predict the structure of the 21-cm signal in previous works. However, halos are not resolved and the constant escape fraction of ionizing photons is assumed in these previous works, nevertheless, the escape fraction changes with time so that the ionization structure resulted from the time-varying escape fraction would give different structures of the 21-cm signal from the case of constant escape fraction.
In order to investigate the impact of evolving escape fraction on the 21-cm signal, we conduct one-dimensional Lagrangian hydrodynamics simulations incorporating radiative transfer and non-equilibrium chemistry self-consistently, considering gas density profile in a halo. In this talk, we will quantitatively show the impact of the time-changing escape fraction on the signal. Also, we will discuss the dependences of stellar masses, halo masses, and redshifts on the signal.

12月7日(火)9:00-＠ES606

Observations of quasars at z>6 reveal the existence of supermassive black holes (SMBHs) with a few billion solar masses in 1 Gyr after the Big Bang. Because of the difficulty in observing AGNs in higher redshifts, we have not yet got the details of their formation process. However it can be expected that there exist a number of AGNs as the seeds of SMBHs before the epoch of reionization.
In this talk, we focus on redshifted 21 cm lines originated from neutral hydrogen hyperfine structure as a probe of seed AGNs in high redshifts. These signals depend on the physical conditions of neutral hydrogen gas. Since AGNs emit UV and X-ray photons which can ionize and heat surrounding neutral hydrogen gases, AGNs can produce the specific spatial structures of 21 cm signals around them. Therefore, we can probe the redshift evolution of AGN number count on redshifted 21 cm maps.
In our study, we assume a simple AGN luminosity function model based on the Press-Schechter formalism. Then, we evaluate the AGN number count survey by the SKA observation, and estimate parameter values in our model using Fisher analysis. I’m going to talk about this result and potential of constraining the AGN luminosity function by the future SKA observation.

12月7日(火)9:00-＠ES606

The Lyman continuum (LyC) escape fraction is a key quantity to determine the contri- bution of galaxies to cosmic reionization. It has been known that the escape fractions estimated by observations and numerical simulations show a large diversity. However, the origins of the diversity are still uncertain. In this work, to understand what quan- tities of galaxies are responsible for controlling the escape fraction, we numerically evaluate the escape fraction by performing ray-tracing calculation with simplified disc galaxy models. With a smooth disc model, we explore the dependence of the escape fraction on the disposition of ionizing sources, and find that the escape fraction varies up to ? 3 orders of magnitude. It is also found that the halo mass dependence of disc scale height determines whether the escape fraction increases or decreases with halo mass. With a clumpy disc model, it turns out that the escape fraction increases as the clump mass fraction increases because the density in the inter-clump region decreases. In addition, we find that clumpiness regulates the escape fraction via two ways when the total clump mass dominates the total gas mass; the escape fraction is controlled by the covering factor of clumps if the clumps are dense sufficient to block LyC photons, otherwise the clumpiness works to reduce the escape fraction by increasing the total number of recombination events in a galaxy.

12月7日(火)9:00-＠ES606

## 11月のセミナー

11月21日(火)17:00-＠ES606

Formation of primordial black holes from cosmological fluctuations

After a brief introduction of primordial black holes, I will talk our recent results on primordial black hole formation by primordial fluctuations both in the radiation-dominated era and matter-dominated era. In the former, I will discuss the formation threshold in several aspects, while in the latter, I will discuss nonspherical effects in the formation probability and the spins of formed primordial black holes.

11月16日(木)13:00-＠ES606

My talk consists of two parts. First, I will discuss cosmological evolution of the QCD axion dark matter coupled to (hidden) photons. The motion of the axion field leads to explosive production of hidden photons by tachyonic instability. We find that the axion abundance is suppressed compared to the case without the coupling. Next, I will discuss the cosmological implications of the string axion whose potential has a pleateu region far from the minimum. Depending on the parameters and the initial value, the axion self interaction can cause strong parametric resonances and, as a result, a significant amount of gravitational waves can be emitted followed by oscillon formations ? a quasi-stable soliton-like object.

11月9日(木)13:00-＠ES606

In the standard cosmology, there could be a lot of cosmological phase transitions in the early universe and these phase transitions produce some kinds of cosmic defects. In this talk, we investigate the cosmic defect called global textures by using weak lensing signals, CMB lensing curl and cosmic shear B modes. Here we introduce the evolution of vector and tensor modes perturbations of textures at first, and we will see the generation of weak lensing signals by textures the next. Finally, we discuss the detectability of textures using weak lensing signals.

Jean-Baptiste Durrive

11月2日(木)13:00-＠ES606

Gravitational instability is a key process in astrophysics and cosmology. Formally, the study of gravitational instability is an eigenvalue problem, which is not easy to solve because the corresponding system of equations is of fourth order, with complicated coefficients. In this talk, I will focus on the relatively simple case of a static self-gravitating planar structure, but with a general polytropic equation of state. This problem has already been solved by other authors in the past, but in most part numerically. Here I will present a way to solve it analytically. More precisely, I will explicit the discrete part of the spectrum (eigenfunctions and eigenfrequencies), by reformulating and decomposing the full fourth-order problem into a sequence of second-order problems that can be solved separately. I will also mention how to generalize these results in order to describe in a more realistic way the graviational fragmentation of baryons in cosmic walls and filaments, in the context of the cosmic web.

## 10月のセミナー

10月31日(火)16:00-＠ES606

Clusters of galaxies have been thought as a powerful cosmological probe, yet for accurate cosmological studies it is important to understand selection effects inherent to individual cluster finding methods. I will discuss possible selection biases in various cluster samples selected in different wavelenths, and then talk about our ongoing efforts to construct new cluster catalogs in the Subaru Hyper Suprime-Cam survey

10月26日(木)13:00-＠ES606

Recent radio observations show that giant molecular cloud (GMC) mass functions noticeably vary across galactic disks (e.g., Colombo et al. 2014). High-resolution magnetohydrodynamics simulations show that multiple episodes of compression are required for creating a molecular cloud in the magnetized interstellar medium (e.g., Inoue et al. 2012). We formulate a semi-analytical time evolution equation for GMCs including cloud-cloud collision process, which represents GMC populations in Milky Way and nearby galaxies. Our analysis shows that large radio observations are capable of constraining GMC formation timescale, self-dispersal timescale, and gas resurrecting factor in different galactic environment by measuring GMC mass function slopes. Our results also indicate that star formation triggered by cloud-cloud collision may amount to a few 10 percent of total star formation in galactic disks.
In addition, I am explaining some ongoing pilot study of galactic-scale simulations that employ the GMC time evolution prescription as a sub-grid model. Lastly, I also briefly mention some ongoing and upcoming observational collaboration both in Milky Way and in external galaxies.

10月19日(木)13:00-＠ES606

Observations of quasars at z>6 reveal the existence of supermassive black holes (SMBHs) with a few billion solar masses in 1 Gyr after the Big Bang. Because of the difficulty in observing quasars in higher redshifts, we have not yet got the details of their formation process. However it can be expected that there exist a number of quasars as the seeds of SMBHs before the epoch of reionization. In this talk, we focus on redshifted 21 cm lines originated from neutral hydrogen hyperfine structure as a probe of seed quasars in high redshifts. These signals depend on the physical conditions of neutral hydrogen gas, e.g., density, temperature etc. Since quasars emit UV and X-ray photons which can ionize and heat surrounding neutral hydrogen gases, quasars can produce the specific spatial structures of 21 cm signals around them. Therefore, we can probe the redshift evolution of quasar number count on redshifted 21 cm maps. Assuming a simple model for the high redshift quasar distribution based on the Press-Shechter formalism, we evaluate the feasibility of the quasar number count survey by the SKA observation. We also discuss the potential of the SKA observation to probe the redshift evolution of the quasar mass function which is deeply related to the formation of SMBH in high redshifts.

10月19日(木)13:00-＠ES606

From the observation of Cosmic microwave background (CMB) by the Planck andWMAP ,they indicate the CMB angular power-spectrum has CMB anomaly. There are about 3sigma deviation between observational data sets and theoretical prediction from simple inflation models at large scale. This is CMB anomaly. CMB anomaly may indicate that primordial power spectrum is differ from power law primordial power spectrum.
In my work,I search the inflation model by axion field which comes from super string theory. Typically potential of axion field has oscillation and generally there are some oscillations in the potential. I use potential which has 2 oscillations . Then the potential has the interference . And firstly I can get a primordial power spectrum which has the interference. Second, I used the CosmoMC which is the MCMC analysis code and I researched how improved chi^2 .Then we set a limit to the model's parameter region. In this talk,I will talk about these results and problems.

10月12日(木)13:00-＠ES606

Recent detections of gravitational waves (GW) emitted from binary black holes (BBH) imply that a BBH is a dominant source in event rate for the second-generation GW detectors like advanced LIGO and that they can be detected out to cosmological distance. This opens up a new window to utilize BBH as a probe for cosmology. In particular, their angular and distance distributions give information about large-scale structures of the Universe. In this presentation, we propose new methods to measure the anisotropies of source luminosity distance and the matter inhomogeneities in the Universe by cross-correlating GW and galaxy observations. First, we show that the second-generation GW detector network is able to measure the angular clustering of BBH and test how they trace the matter density fluctuations. This information can be an important clue to distinguish the possible origins of stellar-mass BBH. Second, we show that future GW detectors such as Einstein Telescope are able to precisely measure source luminosity distances out to high redshifts and extract cosmological information about the large-scale structures around z~2 without any help of redshift information. The expected constraints on the primordial non-Gaussianity with the Einstein Telescope would be comparable to or even better than the other large-scale structure probes at the same epoch. Also the cross-correlation with other cosmological observations is found to have high-statistical significance, providing additional cosmological information at very high redshifts.

10月5日(木)13:00-＠ES606

Gauge fields with non-minimally coupled gravities are often contained in the Kaluza-Klein theory of higher dimensional gravity or a quantum effect of gravity. The non-minimaly coupled fields cause some interesting phenomena such as an equivalence principle violation.
We consider a Horndeski vecter-tensor theory, which is a non-minimal coupling theory, in the radiation dominated universe. In the curved space time, on this theory, the dispersion relation of the Gauge particles depend on the direction of travel, and birefringence occurs when the metric perturbations have quadrupole moments. Due to these effects, the gravitational waves are generated by the anisotropic stress of the gauge particle in the thermal bath. We find that the equation of the gravitational wave is non-linear. It shows the equations are significantly different from the minimal coupling theory.
In this talk, I will introduce this model and show the parameter regime of this model required to detect gravitational wave by future observations.

## 9月のセミナー

9月20日(水)14:00-＠ES606

Stochastic Formalism in Curved Field Space

In multi-field inflation scenarios, the scalar fields can span curved field spaces in general.
Recently it was pointed out that the sufficiently large field curvature can make the entropic degrees of freedom tachyonic during inflation and alter the inflation dynamics [PRL117, 141301]. This geometrical destabilization can be applied to a large class of inflationary models.
To analyze the destabilization, we formalize the stochastic approach in the general curved field space from first principles.
Also I’ll briefly mention the slow-roll phase well after the geometrical destabilization.

## 8月のセミナー

Jean Coupon (Geneva)

8月8日(木)11:00-＠ES606

The past decade has seen the emergence of new techniques and exciting discoveries powered by wide-field imaging surveys from the UV to the near-IR domain. Owing to gravitational lensing, galaxy clustering and abundance matching (to name but a few), coupled with advanced statistical interpretation, the informative power of astronomical imaging surveys has significantly increased. In particular, the connection between galaxies and dark matter, a keystone in cosmology and the study of galaxy evolution, has widely gained from this "scale revolution" and the future is bright, as the next experiments such as HSC, LSST, Euclid or WFIRST are dedicated "survey" machines that will further increase imaging data by orders of magnitude (without mentioning the tremendous gain in image resolution, time domain and deep near-IR imaging). I will focus my talk on reviewing the main techniques to connect galaxies and dark matter in the context of wide-field surveys and I will show some concrete examples of applied data analysis in the CFHTLenS and COSMOS projects, showing that these techniques are now well proven, although the challenges in reducing some critical systematic uncertainties are ahead of us.

## 7月のセミナー

Victor Bonjean

7月26日(水)10:30-＠ES606

Multi-wavelength analysis of inter-cluster filaments

I will present a multi-wavelength analysis of two galaxy-cluster pairs detected in SZ by Planck with an excess of SZ signal between clusters. For the first time, we have combined SZ and optical analysis, to derive gas pressure and galaxy properties such as SFRs and stellar masses in the filamentary regions between the clusters. We propose a possible scenario for the filament in the cluster pair A399-A401.

Bharat Ratra

7月25日(火)16:00-＠ES606

Dark Energy: constant or time variable? (... and other open questions)

Experiments and observations over the two last decades have persuaded cosmologists that (as yet undetected) dark energy is by far the main component of the energy budget of the current universe. I review a few simple dark energy models and compare their predictions to observational data, to derive dark energy model-parameter constraints and to test consistency of different data sets. I conclude with a list of open cosmological questions.

7月19日(水)11:15-＠ES606

As a way of proving dark matter, one can use the it’s property that gamma rays are emmited when a dark matter annihilates (Dark Matter Annihilation (DMA)) or decays (Decaying Dark Matter (DDM)). Therefore, as a method of indirectly searching dark matter, there is a method of searching signals of these gamma rays in Extragalactic Gamma-ray Background (EGB) . The properties of dark matter can be restricted from the intensity distribution of EGB and the gamma ray intensity from dark matter predicted theoretically. In this study, I use the intensity distribution of EGB from observation of Fermi GRST and the distribution of the galaxy clusters and the redshift of each cluster from HSC.
These observation bring the intensity distribution of the EGB in the direction of the cluster with information of the depth on the line of sight. On the other hand, In theory, I obtain the total gamma ray intensity distribution from one cluster as a function of the observed the gamma ray energy and the redshift of the gamma ray from DMA and DDM.
In this presentation I will report current progress on the properties of DMA and DDM using the cross-correlation signal obtained from observation and the gamma ray intensity from dark matter predicted theoretically.

7月19日(水)11:15-＠ES606

Primordial black hole (PBH) is formed out of the high density region by the gravitational collapse in the early Universe. There is no evidence indicating the existence of PBH, but limiting the abundance of PBH may solve the cosmological problems such as identifying the candidate of dark matter. A previous work (Carr et al. 2010) calculated the flux of gamma-rays emitted from PBHs in the Universe, and imposed the limit on the ratio of the density of the PBHs at their formation time to Universe’s density by using the observation data of gamma-ray background radiation. In this work, we focus on the dark matter halos of the galaxy clusters which are expected to have many PBHs because of their high density. We examine whether we can update the constraint of the PBH abundance more strongly by considering the gamma-ray from dark matter halos. As a result, we impose stronger limit on the ratio of the PBH density parameter to the dark matter density parameter. We discuss the result in comparison with that in the previous work.

7月19日(水)11:15-＠ES606

LIGO directly detected Gravitational waves (GWs) from Binary Black Holes (BBHs) coalescence in 2015.This demonstrates that it is possible to observe the Universe using GWs as an observation method independent of electromagnetic wave observation. GWs gives us measurement of the cosmic expansion rate. I review the papers which estimated the measurement precision of the cosmic expansion rate using GWs, (Holz & Hughes 2005) and (Nishizawa 2016). In the former paper, the space GW telescope LISA determines the luminosity distance up to the supermassive BBH. In addition, the electromagnetic counterpart to the BBH GW event gives the redshift of the BBH. So it is possible to restrict the state equation of dark energy independently of the observation the standard candle. In the latter paper, GW detector networks detects GWs from stellar-mass BBHs and the redshift of them is obtained from the host galaxy. This paper shows Hubble constant can be determined with high accuracy, and it can be a means to resolve the discrepancy of the Hubble constant from the observation of CMB and Cepheid variables. Through these papers we discuss limitations on cosmological parameters by GWs.

7月19日(水)11:15-＠ES606

The existence of CMB is considered to be one of the most powerful evidences that support Big Bang theory. In the early universe, the blackbody spectrum of CMB is held by processes such as Compton scattering, bremsstrahlung, and double Compton scattering. However, by cooling of the universe due to the expansion, these processes become inefficient gradually. Then, if any energy is released at those epochs, the CMB distortions are created. Anisotropies of CMB distortions can be created by spatially varying heating processes. For example, the dissipation of small scale perturbations can create distortion anisotropies. As Cross-correlation of distortion parameters and temperature can be regarded as three-point correlation of temperature, we can estimate primordial non-Gaussianity. In this talk, I review the paper that estimates primordial non-Gaussianity using CMB spectral distortion anisotropies (Chluba et al, 2016). This work describes the energy injections more factually than previous similar works, and derives more factual conclusion.

7月19日(水)11:15-＠ES606

It is confirmed by observations that the expansion of the universe is accelerating. However, we can not explain the cause. Dark energy is considered as a candidate which causes the accelerating expansion. there are a lot of dark energy models. In order to find the most suitable models for our universe, it is necessary to constrain the parameters that characterize dark energy. Since the expansion history depends on the value of the parameters of Equation of State of dark energy “w”, we can constrain the models from the expansion rate of the universe.
Baryon acoustic oscillation (BAO) can be used as a standard ruler of the universe. It is possible to measure the expansion rate of the universe by measuring BAO scale. Information of BAO will be obtained by galaxy redshift survey and HI intensity mapping survey by Square Kilometer Array (SKA). In previous research, Fisher forecast was used to predict the constraint on parameters of dark energy based on expected specifications of SKA. In this presentation, I explain the mechanism of the constraint on the parameters ,and reproduce the previous results.

Surhud More (Kavli IPMU)

7月13日(木)11:00-＠ES606

Halo assembly bias -- the dependence of the large scale clustering amplitude of halos on secondary parameters other than the halo mass -- is a well established prediction of the standard structure formation model. However it has been notoriously hard to confirm, observationally. I will present a 6.6 sigma detection of difference in the clustering of redmapper galaxy clusters which share the same weak lensing mass. I will also present a detection of the splashback radius or the edges of redmapper galaxy clusters in observations. I will discuss some recent progress in understanding these observations in light of selection effects induced by optical cluster finding.

Patel Teerthal

7月12日(水)10:30-＠ES606

Cosmological magnetic fields are ubiquitous in the Universe and are observed from astronomical to the largest scales. Numerous studies involving the possible origin of the magnetic fields span various epochs and employ a wide range of mechanisms, yet no conclusive theory or model has been established. It has been shown that the standard inflationary approach with generic coupling between the scalar(pseudo-scalar) fields cannot produce the required magnetic field strength. In this study, the generation of magnetic field in Axion Monodromy Inflation is explored. The axion driven inflaton field in this scenario has a modulation term and can contribute significantly to primordial field generation through resonant enhancement. The effects of the backreaction are studied to constrain the modulation and coupling parameters but, it is assumed to have insignificant impact on the evolution of the background inflaton field. The resulting primordial magnetic field is used to perform order estimation of the present day magnetic field strength. A thorough analysis can thus not only provide a way to generate the required primordial magnetic field but also provide phenomenological constraints on high energy physics models inspired by string theory.

Clément Walter

7月12日(水)10:30-＠ES606

With a ground-based detector network composed of advanced LIGO, VIRGO, and KAGRA, we can measure the speed of a gravitational wave directly. Currently gravitational wave speed has been weakly constrained because there are only two detectors operating. But in the next couple of years, we will be able to obtain a much better constraint from direct measurements of gravitational waves. We resolve to what extent we can achieve in measurement precision and how we can optimize the method.

7月12日(水)10:30-＠ES606

Escape fraction is a key quantity to determine the contribution of galaxies to cosmic reionization. Though many numerical simulations have predicted the LyC escape fraction, the predicted values are different for each study. Since characteristics of the simulations, e.g., resolution, implemented physics and numerical algorithm, are quite different each other, it is usually difficult to understand what causes the diversity. In this work, to understand what quantities of a galaxy are responsible for controlling the escape fraction, we numerically evaluate the escape fraction by performing ray-tracing calculations with simplified disk galaxy models. With a smooth disk model, we first explore the dependence of the escape fraction on the disposition of ionizing sources, and find that the escape fraction varies up to ~ 3 orders of magnitude. With a clumpy disk model, it turns out that clumps basically increase the escape fraction as far as the total clump mass is less dominant, because LyC photons can effectively escape through gaps among the clumps. We also find that the escape fraction is controlled by the covering factor of clumps if the clumps are dense enough.

7月5日(水)13:00-＠ES606

According to many previous observations, magnetic fields are found on various astronomical objects in the universe, from asteroids to clusters of galaxies. One possible origin of these cosmic magnetic fields is the magnetogenesis in the primordial universe. Magnetic fields generated by such a cosmological mechanism are called primordial magnetic field (PMFs), and are considered to affect the evolution of matter density fluctuations due to the Lorentz force and the thermal history of the intergalactic medium (IGM) gas due to the so-called ambipolar diffusion. Hence the information of PMFs is expected to be imprinted on the anisotropies of the cosmic microwave background through the thermal Sunyaev-Zel’dovich (tSZ) effect in the IGM. In this talk, given an initial power spectrum of PMFs, we show dynamical and thermal evolutions of the IGM with PMFs, and the resultant tSZ angular power spectrum. As a result, we find that the tSZ angular power spectrum induced by the PMFs becomes more remarkable on small scales than that by galaxy clusters even with PMFs below the current cosmological constraint (Minoda et al., 2017, arXiv:1705.10054). The measurement of the tSZ angular power spectrum on small scales can provide the stringent constraint on PMFs.

7月5日(水)13:00-＠ES606

Cosmic voids can be used as a prove of the cosmological model and it has been studied eagerly as we obtain statistically enough data about void structures from observation.
I tried to explain the mechanical process of individual void using simplified isolated void model, while there are many study about voids focusing on statistical profile and using N-body simulation.
I analyzed the redshift evolution of individual voids in N-body simulation and compared the result to that of isolated void models with numerically calculation.

7月5日(水)13:00-＠ES606

The first stars are the first luminous objects in our universe, which marked the end of the dark age. They are thought to have formed as a result of growth of density fluctuation, and have affected following structure formation such as galaxies. However, its properties have not been clarified very much even though many previous studies conducted numerical simulations on the first stars’ formation. Therefore, there are still many theoretical models of the first stars. Some characteristics (e.g. its masses), can be investigated through observations of 21-cm line emitted from the vicinity of the first stars. The 21-cm line signals would be detected by forthcoming radio interferometers such as Square Kilometre Array. In order to interpret those observations’ data and extract the physical properties of the first stars, we need to investigate what each model of the first stars looks like on the celestial sphere.
I conducted radiative hydrodynamics simulation to obtain the distribution of 21-cm signals in various masses of the first stars and some different redshifts. We took gas density profile into consideration and investigated spatial distribution of 21-cm line signals. In this talk, I will compare my results with the radiative transfer simulation of Yajima & Li (2014) in whose setup gas is treated as static and uniform intergalactic medium. Also, we will discuss detectability of the first stars with SKA.

## 6月のセミナー

6月26日(月)17:00-＠ES606

Lyman-alpha emitting galaxies (LAEs) are key objects to understand cosmic reionization and galaxy formation in the early Universe. Combining cosmological hydrodynamics simulations and multi-wavelength radiation transfer calculations, we investigate the formation mechanism of LAEs and their contribution to cosmic reionization. Our findings are as follows : (1) cold gas accretion produces bright LAEs via the excitation cooling process; (2) observed LAEs can keep the ionization of IGM at z < 7; (3) some of high-z LAEs evolve to local L* galaxies (Yajima et al. 2015, ApJ, 801, 52). We also study impacts of star formation and stellar feedback on the galaxy formation. We find the star formation efficiency affects the star formation history and galactic morphology significantly (Yajima et al. 2017 arXiv:1704.03117). In addition, as galaxies evolve, massive black holes are likely to form at galactic centers. I will introduce our recent studies about the formation of massive black holes in high-redshift galaxies.

6月21日(水)13:00-＠ES606

The redshifted 21cm signal from neutral hydrogen in the intergalactic medium (IGM) during the Epoch of Reionization (EoR) is difficult to detect due to intense foreground radiation. Taking cross-correlation between the 21cm signal and Lyman Alpha Emitters (LAEs) is one of the ways to reduce the foreground effects because the spatial distribution of high-z LAEs is not correlated with the foreground emission. In this work, we investigate the detectability of the 21cm- signal with the 21cm-LAE cross-correlation by using our state-of-the-art reionization simulation data. As a result, we find that the combination of Murchison Widefield Array(MWA) and Subaru Hyper Suprime-Cam (HSC) has the ability to detect the cross-power spectrum signal on large scales. It is also found that the sensitivity on small scales is dramatically improved by the spectroscopic observations with the Prime Focus Spectrograph (PFS) because the redshifts of LAEs are accurately determined. Finally, we find that upcoming observations with the Square Kilometre Array(SKA) and PFS has a potential to accurately determine the typical size of ionized bubbles by measuring the turnover scale of the cross-power spectrum.

6月14日(水)13:00-＠ES606

The Lambda CDM model, which provides a consistent prediction at larger scales than ~10kpc, has not succeeded to match the observational data at smaller scales. While this is usually attributed to the computational issue of baryon physics, the discrepancy from the Lambda CDM model below kpc scales may suggest the presence of an ultra-light axion whose mass scale is of O(10^{-22}eV). In this talk, I will review a recent progress on the cosmological consequences of the ultra-light axion. Then, I will explore another cosmological consequence on cosmic microwave background and large scale structures.

## 5月のセミナー

5月24日(水)13:00-＠ES606

The first data release of the HSC survey has been public since Feb 2017. The data covers more than 100 square degrees of sky. I will briefly describe the HSC survey and data products of the HSC DR1.
On the last half of the seminar, I will give a summary of the collection of various results including, cluster evolution, photometric redshift and synergy with CMB observations, Planck and ACTPol.

5月22日(月)13:30-＠ES606

Kinematic Sunyaev-Zel'dovich effect

Over the past few years, cosmologists have been able to make the first detections of the kinematic Snuyaev-Zel'dovich (kSZ) effect by combining galaxy data with measurements from CMB experiments.　 The kSZ effect is well-suited for studying properties of the optical depth of halos hosting galaxies or galaxy clusters. As the measured optical depth via the kSZ effect is insensitive to gas temperature and redshift, the kSZ effect can be used to detect ionized gas that is difficult to observe through its emission, so-called "missing baryons". This work presents the first measurement of the kSZ effect in Fourier space. While the current analysis results in the kSZ signals with only evidence for a detection, the combination of future CMB and spectroscopic galaxy surveys should enable precision measurements. This talk emphasizes the potential scientific return from these future measurements.

5月17日(水)13:00-＠ES606

We discuss a new detection method for ultra-low frequency gravitational waves (GWs) with a frequency much lower than the observational range of pulsar timing arrays. Ultra-low frequency GWs (< 10^-10 Hz) are not detectable with the conventional method of pulsar timing arrays, because the effect of the GWs is exactly the same as the pulsar spin-down rate, and it is absorbed when we fit the pulsar spin-down rate for individual pulsars. However, the GWs from a single source should leave the effect with a quadrupole pattern in the sky, which depends on relative direction of a pulsar and GW source, while the spin-down rate of pulsars is randomly distributed. Thus, in principle, ultra-low frequency GWs can be detected by looking for a quadrupole pattern in all-sky distribution of the pulsar spin-down rates. We evaluate the potential of this method by employing Monte-Carlo simulations and estimate the sensitivity of pulsar observations by the Square Kilometre Array.

5月10日(水)13:00-＠ES606

The energy injection in the early universe (before the redshift $z \sim 10^6$) heat up the CMB. If the energy injection is inhomogeneous, the additional temperature fluctuations are generated. In this talk, we consider the dissipation of the primordial magnetic fields in the early universe as the energy injection. We show that the measurement of the CMB temperature anisotropy can provide the constraint on the primordial magnetic fields on small scales ($10^4 Mpc^-1 < k < 10^8 Mpc^-1$).

## 4月のセミナー

4月27日(木)10:00-＠ES606

Detecting gravitational wave background generated during inflation through B-mode polarization of the cosmic microwave background (CMB) is one of the main scientific goals of future CMB experiments. However, it has already become clear that synchrotron and thermal dust emissions from our galaxy hinder the cosmological B-mode signal across the sky. Here, assuming only Gaussianity and statistical isotropy and homogeneity of the CMB map, we develop a new template cleaning method taking into account spatial variations of spectral indices of the foreground emissions, and apply it to various foreground models using Monte-Carlo simulation technique. We find that the method successfully remove the unwanted bias in determining the tensor-to-scalar ratio $r$ that was found in the simple template cleaning method presented in Katayama and Komatsu 2011. In this poster, we present how small $r$ we will be able to reach according to the frequency bands and noise specification proposed for the LiteBIRD experiment.

4月20日(木)10:00-＠ES606

The existance of scalar fields whose vacuum expectation values determine the physical constants is generically predicted by the recent attempts toward unifying all elementary forces in nature based on string theory. In scalar-tensor theories of gravity, a scalar field couples to the Ricci scalar, which provides a natural framework for realizing the time variation of the gravitational constant via the dynamics of the scalar field. I present cosmological constraints on the scalar-tensor theory of gravity by analyzing the angular power spectrum data of the cosmic microwave background (CMB) obtained from the Planck 2015 results together with the baryon acoustic oscillations (BAO) data. We find that the inclusion of the BAO data makes more than 10% improvement on the constraints on the time variation of the effective gravitational constant.

4月13日(木)10:00-＠ES606

4月13日(木)10:00-＠ES606

The distance modulus is defined as a difference between apparent magnitude and absolute magnitude of the celestial object, and is related to the luminosity distance. The distance modulus from type Ia supernovae surveys has been compared with the theoretical prediction which depends on cosmological parameters (e.g. J. Guy et al., 2010). It is known that there is a discrepancy between the distance modulus from observation and LambdaCDM prediction. The discrepancy can partly be accounted for by a flux magnification due to gravitational lensing. In this work, first we estimate the effect of the magnification on the distance modulus using N-body simulation and analytic calculation. We find the effect of the magnification is less than 3%. Second, from Supernova Legacy Survey (SNLS) data, we impose constraints on the cosmological parameters by Markov chain Monte Carlo (MCMC) method and obtain a consistent result with the previous work. Finally, we reconstruct the projected mass distribution from Canada France Hawaii Telescope Lensing Survey (CFHTLenS) data and find no significant cross correlation between the distance modulus.

4月13日(木)10:00-＠ES606

Arkani-Hamed and Maldacena showed that massive higher spin fields, whose presence may be predicted in string theory, can generate angular dependent primordial non-Gaussianity, while the global isotropy is still preserved.
As was argued by F.Schmidt et al., such an angular dependent non-Gaussianity can be explored through an observation of galaxy shapes.
Meanwhile, an angular dependent non-Gaussianity can be generated also in case the global isotropy is broken, e.g., by a vector field.
We studied whether the anisotropic bi-spectrum generated in a setup with the global isotropy can be distinguished from the one in another setup without the global isotropy by observing galaxy shapes.
Then, we found that the B-mode of the galaxy lensing and the off-diagonal component of ell can be generated only in the latter case. This can provide a crucial difference between these two cases.

4月13日(木)10:00-＠ES606

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 anisotropy of the mean of the CMB temperature fluctuations in spherical harmonics space.

4月13日(木)10:00-＠ES606

Dark matter is one of the most important factors in considering the evolution of the universe, and accounts for 90 % or more of the mass of the cluster of galaxies. If we know how density perturbations of dark matter evolve, we can determine perturbations in early universe from the structure of the present universe. I assumed that the perturbations in ellipsoids are all uniform. I derived differential equations of the time evolution about their shapes, and solved them numerically. I calculated them for various axis ratios using the probability distribution of the axial ratio around the peak of the random gaussian field.

4月13日(木)10:00-＠ES606

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