The University of Tokyo · 물리·천문학
마사히로 카와사키 교수의 연구실은 초기 우주 물리학과 초입자 이론을 기반으로 한 빅뱅 핵합성(BBN), 중성자열역학, 약한 상호작용을 통한 입자 생성 메커니즘을 중심으로 연구를 전개합니다. 특히 불안정한 초입자(예: 중성미온, 스트레인)의 붕괴가 초기 우주의 원소 비율에 미치는 영향과, 후기 우주의 엔트로피 생산이 우주 배경 복사 및 대규모 구조 형성에 끼치는 영향을 수치적 해석과 이론 모델링을 통해 분석합니다. 또한 초대칭 및 초대칭 이론 기반의 인플레이션 모델, 스트링 이론과 연관된 축상자(axion) 생성 메커니즘 등 고에너지 이론 물리학의 핵심 문제를 다룹니다.
표시된 성과는 수집된 데이터 기준으로 산출되며, 일부 차이가 있을 수 있습니다.
We study the big-bang nucleosynthesis (BBN) with the long-lived exotic particle, called $X$. If the lifetime of $X$ is longer than $\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{sec}$, its decay may cause nonthermal nuclear reactions during or after the BBN, altering the predictions of the standard BBN scenario. We pay particular attention to its hadronic decay modes and calculate the primordial abundances of the light elements. Using the result, we derive constraints on the primordial abundance o
The late-time entropy production by massive particle decay induces various cosmological effects in the early epoch and modifies the standard scenario. We investigate the thermalization process of the neutrinos after entropy production by solving the Boltzmann equations numerically. We find that if the large entropy is produced at $t\ensuremath{\sim}1 \mathrm{sec},$ the neutrinos are not thermalized very well and do not have the perfect Fermi-Dirac distribution. Then the freeze-out value of the n
We propose a chaotic inflation model in supergravity. In the model the Kahler potential has a Nambu-Goldstone-type shift symmetry of the inflaton chiral multiplet which ensures the flatness of the inflaton potential beyond the Planck scale. We show that chaotic inflation naturally takes place by introducing a small breaking term of the shift symmetry in the superpotential. This may open a new branch of model building for inflationary cosmology in the framework of supergravity.
We derive big-bang nucleosynthesis (BBN) constraints on both unstable and stable gravitino taking account of recent progress in theoretical study of the BBN processes as well as observations of primordial light-element abundances. In the case of unstable gravitino, we set the upper limit on the reheating temperature assuming that the primordial gravitinos are mainly produced by the scattering processes of thermal particles. For stable gravitino, we consider $B$-ino, stau, and sneutrino as the ne
We investigate cosmological effects concerning the late-time entropy production due to the decay of nonrelativistic massive particles. The thermalization process of neutrinos after the entropy production is properly solved by using the Boltzmann equation. If a large entropy production takes place at late time $t\ensuremath{\simeq}1\mathrm{sec}$, it is found that a large fraction of neutrinos cannot be thermalized. This fact loosens the tight constraint on the reheating temperature ${T}_{R}$ from
The authors study the cold dark matter axion production in the decay of string-domain wall systems for a scenario where the Peccei-Quinn symmetry remains broken after inflation. Using the most advanced simulations to date, severe constraints for the model parameters (axion mass, decay constant, etc.) are determined, which potentially can be probed in the future experiments.
We review recent developments in axion cosmology. Topics include axion cold dark matter, axions from topological defects, axion isocurvature perturbation and its non-Gaussianity, and axino/saxion cosmology in a supersymmetric axion model.
The authors provide a state of the art analysis of the effects of long-lived non-Standard Model massive particles, decaying during big-bang nucleosynthesis (BBN), on the primordial abundances of light elements. Besides updated standard BBN reaction rates, additional processes and new numerical algorithms are implemented to discuss also solutions to the Lithium problem and the possible gravitino mass for leptogenesis to work.
It has been recently pointed out that the initial value problem in new inflation models is naturally solved by supergravity effects if there exists a preinflation before the new inflation. We study this double inflation model in detail and find that density fluctuations on small cosmological scales are much larger than those on large scales due to the peculiar property of the new inflation. We show that this results in the production of primordial black holes which have $\ensuremath{\sim}{1M}_{\
We argue that the existence of the cold dark matter is explained by primordial black holes. We show that a significant number of primordial black holes can be formed in an axionlike curvaton model, in which the highly blue-tilted power spectrum of primordial curvature perturbations is achieved. It is found that the produced black holes with masses $\ensuremath{\sim}{10}^{20}--{10}^{38}\text{ }\text{ }\mathrm{g}$ account for the present cold dark matter. We also argue the possibility of forming t
Gravitino production and decay in the inflationary universe are reexamined. Assuming that the gravitino mainly decays into a photon and a photino, we calculate the upperbound on the reheating temperature. Compared to previous works, we have essentially improved the following two points: (i) the helicity ±(3/2) gravitino production cross sections are calculated by using the full relevant terms in the supergravity lagrangian, and (ii) the high energy photon spectrum is obtained by solving the Bolt
We propose a novel scenario to produce abundant primordial black holes (PBHs) in new inflation which is a second phase of a double inflation in the supergravity framework. In our model, some preinflation phase before the new inflation is assumed and it would be responsible for the primordial curvature perturbations on the cosmic microwave background scale, while the new inflation produces only the small scale perturbations. Our new inflation model has linear, quadratic, and cubic terms in its po
