標準的な宇宙の構造形成理論を中心に観測的宇宙論の解説を行う。フリードマン宇宙モデルにもとづく膨張宇宙の進化について述べたあと、宇宙論的摂動論にもとづいて、宇宙初期で作られた原始ゆらぎがどのように進化し、観測される宇宙の大規模構造や宇宙マイクロ波背景放射などとどう結びつくのかを説明する。その上で、観測から宇宙論的情報を引き出す際に重要な効果（重力レンズ、赤方偏移空間ゆがみなど）について述べる。さらに、銀河サーベイなどの観測データの理解に不可欠な、重力進化が進んだ非線形な構造形成について、基礎的概念から最近の進展までを紹介し、観測からボトムアップ的に宇宙の成り立ちを理解するために必要な基礎の習得を目指す。 授業の内容は以下の通りである。おおむね、それぞれのトピックにつき１～２回程度の講義をおこなう。 　１）オーバービュー 　２）一様等方宇宙モデル 　３）構造形成の線形理論 　４）非一様宇宙の観測 　５）非線形構造形成

It is shown that the existence of the background magnetic field affects the spectrum and quantum state of primordial gravitational waves (PGWs).
In this talk, the basic prescription to analyze the PGWs with the existence of the background magnetic field is introduced.
Especially, the correlation between a graviton and a photon during inflation is discussed based on results shown in the paper arXiv:2211.05576v2.

Dynamics of fields around black holes, such as Hawking radiation and superradiance, have attracted much interest.
In this talk, I'll discuss about the effect of self-interaction on these phenomena.
For the Hawking radiation, I review about the secular divergence of the loop correction to the two-point function.
In addition, I'll introduce the large-N method to resum these secular growths.
For the superradiance, I'll discuss our previous works 2211.01949 and 2201.04382.
I'll show that the self-interaction has significant impact on the evolution of the scalar condensate.

Abstract of the talk.

The discovery of the sphaleron has opened up various possibilities of baryogenesis.
One of the leading proposals among them is baryogenesis through lepton number violation, i.e., leptogenesis.
After reviewing the basic concepts of baryogenesis via leptogenesis, we discuss recent ideas based on 2011.09347 and 2111.03082.

Abstract of the talk.

Over the next few decades, we will have an exciting opportunity to probe the early Universe with gravitational wave (GW) observations using space interferometers. A particularly interesting scenario is that of a first-order phase transition, in which bubbles of true vacuum nucleate, expand, and collide, involving the dynamics of surrounding plasma particles. In this talk, we review both the microphysical and macrophysical aspects of this scenario, from the high-energy physics that triggers phase transition to the production of GWs and their observational prospects, and then discuss some recent developments.

A gravitational wave signal from a binary black hole merger is an important probe to test gravity. Especially, the observation of ringdown may allow us to perform a robust test of gravity in strong gravity regimes as it is a superposition of excited quasi-normal (QN) modes of a Kerr black hole. In this talk, I will discuss the modeling of ringdown not only with QN modes but also with the reflectivity of the black hole geometry, i.e., the greybody factors, as an alternative modeling of the ringdown. First, I will discuss the importance of highly damped QN modes, i.e., overtones, for comparable and extreme mass ratio mergers. In the latter part of the talk, I will show that the greybody factor is imprinted on the ringdown spectrum. The greybody factor is another "no-hair quantity" of a black hole. As such, independently of the measurement of QN modes, one can test strong gravity with the greybody factors. Furthermore, I will show that the ringdown spectrum for near extremal cases is similar to the Fermi-Dirac distribution characterized by the Hawking frequency.

Abstract of the talk.

It is known that magnetic fields exist near black holes and photons can go around black holes due to strong gravity. Utilizing these facts, we can probe hypothetical pseudoscalar particles, so-called axions. In fact, photons can be converted into axions when they propagate in a magnetic field. The conversion of such photons into axions leads to a dimming of the photon ring around the black hole shadow. We show that photon ring dimming can occur efficiently for supermassive black holes. Remarkably, it turns out that the maximal dimming rate of the photon ring is 25%. The frequency band and the magnitude of the dimming depend on the axion-photon coupling and axion mass. Hence, the distorted spectrum of the photon ring provides a novel tool for detecting axions.

Galaxy formation is an important topic of current astrophysics. Numerical simulations play a crucial role in investigating physical processes of galaxy formation due to its fully non-linear nature. Here, I provide the brief overview of the historical development of numerical simulations of galaxy formation. Then I introduce several studies based on high resolution simulations of galaxy formation and evolution, such as simulations resolving individual stars and simulations using Fugaku.

Primordial gravitational waves (GWs) are generated in the early Universe through processes such as inflation and reheating, and their observations allow us to probe the physics of the early universe in a direct way. In particular, the circular polarization modes of primordial GWs can be a crucial probe as they reflect the parity violation in the early Universe. In this seminar, I show that the superposition of GWs from numerous compact binary mergers generates a circularly-polarized GW background, though it is not expected for an isotropic distribution of sources. Then the astrophysical polarized foreground limits the sensitivity of the future searches for the circular polarizations of a primordial GW background. I also discuss the implication to the recent detection of the GW background from supermassive black hole binaries by NANOGrav.

TBA