Cosmophysics Group Seminar (2015)
24-Mar-2016
Takuya Hasegawa 「On the Li7 problem in SBBN theory」
Abstract： I will introduce my research topic on the Li7 problem in Standard Big Bang Nucleosynthesis(SBBN) theory. Reference： Maxim Pospelov, Josef Pradler "Metastable GeV-scale particles as a solution to the cosmological lithium problem" (2010) : arXiv:1006.4172v2 [hep-ph]
17-Dec-2015
Sanshiro Shibata「Numerical Simulation of Relativistic Jet in Supermassive Star」
Abstract: Recently, supermassive stars (SMS) have been proposed as the seed of supermassive black holes. It is also proposed that such SMS may explodes as supernova or gamma-ray burst. Since the explosion of SMS may affect the subsequent mass accretion, it is important to consider the feedback effect to the host galaxy by the explosion. We focus on the possibility of gamma-ray burst from SMS. We calculate the propagation of relativistic jet in SMS and estimate the feedback of the gamma-ray burst to the host galaxy. Reference:
19-Nov-2015
Mindaugas Karciauskas「Slow-roll corrections to fNL」
Abstract: In a recent paper Kenton and Mulrine (2015) computed a local f_NL parameter in a limit in which the difference between a small and large k modes is several orders of magnitude. The so called extreme squeezing limit. They showed that in this limit f_NL parameter can differ by up to 20% as compared to moderate squeezing, which have been used so far for multi-field models of inflation. We expect that a similar magnitude contribution should also be added if we include slow-roll corrections to such a computation. This intuition is supported by the work of Avgoustidis et al. (2012), where they show that in certain situation slow-roll corrections can give up to 20% contribution to the spectrum. In this talk I will present our work in progress in which we are computing slow-roll corrections to f_NL. Reference:
12-Nov-2015
Jonathan White「Modulated reheating and the Higgs」
Abstract: We discuss the possibility that the Higgs may source primordial density fluctuations via a modulated reheating scenario, focusing in particular on gravitational reheating. We find the scenario to be very tightly constrained by observations. Reference:
22-Oct-2015
Kunihito Ioka 「Strong Limits on High-Energy Neutrino Sources」
Abstract: Recently the IceCube collaboration discovered high-energy cosmic neutrinos. Their origin is a new mystery in astroparticle physics. We discuss simple but strong limits on the high-energy neutrino sources. The limits suggest that the sources are not transients but steady sources, except for gamma-ray bursts. Reference:
15-Oct-2015
Hideo Kodama 「The Evolution of Black Holes in the Axiverse」
Abstract: I summarise the results of our recent work on the dynamics of the BH-axion system, and on the basis of them, I discuss the evolution of black holes in the axiverse. In particular, we focus on the observability of GWs from some candidate black holes and their spin evolution. Reference: Yoshino H, Kodama H: arXiv: 1505.00714; PTEP 1506, 061E01 (2015) PTEP1404, 043E02 (2014) Middleton M: arXiv:1507.06153
17-Sep-2015
Hirotaka Yoshino 「On scalar field in a black ring space-time」
Abstract：I briefly review existing studies on the behavior of a scalar field in a Emparan-Reall black ring spacetime. In particular, I revisit the problem of possible existence of static scalar hair first conjectured by Cardoso, Dias and Yoshida Reference: Phys. Rev. D72, 024025 (2005)
4-Sep-2015
Kentaro Tanabe 「Superluminality in modified gravity」
Abstract：I will give some overviews on the superluminality and causality in modified gravity theory. Reference: R.Geroch, "Faster Than Light ?" arXiv:1005.1614
23-July-2015
Seiju Ohashi 「Axisymmetric stars」
Abstract： I will review the recent progress on axisymmetric perfect fluid stars in general relativity based on Ref[1]. Main content of this talk is to show, "maximum entropy + axisymmetry imply stationarity". Reference：[1] J.S. Schiffrin, [arXiv:1506.00002] [2] L. Lindblom, Astrophys.J.,208,873
9-July-2015
Taro Mori 「Multi-field effects on Non-Gaussianity in Starobinsky inflation」
Abstract:I will discuss a two-field inflation model which contains Starobinsky- like sector and quadratic potential of a scalar field. I will also review and discuss a method to compute Non-Gaussianity of such kind of two-field (or multi-field) model. Reference： [1]C. van de Bruck, L. E. Paduraru(2015):arXiv:1505.01727 [2]D.Kaiser et al(2012):arXiv:1210.7487 [3]J.Elliston et al(2012):arXiv:1208.6011
2-July-2015
Mindaugas Karčiauskas 「f^2F^2 Continued」
Abstract：Supergravity models of inflation are usually plagued by so called eta problem. To remedy this problem additional symmetries are imposed or fine tuning of some parameters is invoked. There is, however, some indication that the eta problem could be solved naturally if the gauge kinetic function is also included in inflationary model building. Many effects of the gauge kinetic function to the dynamics of inflation have been studied in the literature, such as gauge field induced anisotropic inflation, generation of non-Gaussianities and statistical anisotropies. In my work I aim at consistent investigation of these effects including the fact that the inflationary solution is an attractor fixed point. For this purpose I generalise cosmological perturbation theory in an anisotropic background to include a gauge field. Reference：
11-Jun-2015
Shota Kisaka 「Pair cascade in pulsar polar cap」
Abstract： Several features of fast radio bursts resemble those of the observed radio emission from pulsars. Pulsar radio emission is believed to be directly related to the pair production the magnetosphere. I review and discuss the pair production in pulsar polar cap. Reference：Timokhin & Harding (2015) arXiv:1504.02194
4-Jun-2015
Kazunori Kohri 「Wino Dark Matter」
Abstract：I will give a talk on the current status of astrophysical and cosmological constraints on wino dark matter. Reference：
28-May-2015
Hideo Kodama 「No-Go theorems against de Sitter solutions in classical limits of string theory」
Abstract：I overview various no-go theorems against dS solutions in a string theory that take account of flux, branes, higher-order alpha' corrections and gaugino condensates. If I can prepare, I will also discuss a compactification that gives a 4D dS vacuum by a non-compact internal space. Reference：
[1] Quigley C (2015): arXiv:1504.00652 "Gaugino Condensation and the Cosmological Constant"
[2] Gautason FF, Junghans D, Zagermann M (2012): JHEP 1206, 029 [arXiv:1204.0807] "On Cosmological Constants from a’-Correctio"
[3] Kutasov D, Maxfield T, Melnikov I, Sethi S (2015): arXiv:1504.00056 "Constraining de Sitter Space in String Theory"
[4] Dasgupta K, Gwyn R, McDonough E, Mia M, Tatar R (2014): JHEP 1407, 054 [arXiv:1402.5112] "de Sitter Vacua in Type IIB String Theory: Classical Solutions and Quantum Corrections"
[5] Baron H, Dall'Agata G (2015): JHEP1502, 033 [arXiv: 1410.8823] "Uplifting non-compact gauged supergravities"
[6] Hull CM, Warner NP (1988): CQG5, 1517 "Non-compact gaugings from higher dimensions"
21-May-2015
Kunihito Ioka 「The Origin of Cosmic-Ray Hardenings」
Abstract：We will discuss the implications and possible origin of cosmic-ray hardenings recently observed by the AMS-02 experiment. Reference :
AMS-02 Collaboration, "AMS Days at CERN and Latestesults", 15, April, 201
Ohira & Ioka, ApJ 729 (2011) L13
Ohira, Kawanaka & Ioka, in preparation
30-Apr-2015
Jonathan White 「Multi-field formulation of gravitational reheating」
Abstract：It has recently been highlighted that details of the reheating process must be properly taken into account when constraining individual models of inflation with current high-precision CMB data, even in the case of single-field models. As inflation models with a non-minimal coupling between the gravity and inflaton sectors have recently attracted much attention, in this work we discuss reheating in this class of models, allowing for multiple inflaton fields. In such models it is known that even in the absence of explicit interaction terms the inflaton sector can decay into matter as a result of its non-minimal coupling to gravity, thereby reheating the Universe gravitationally. Using the Bogoliubov approach we evaluate the gravitational decay rates of the inflaton fields into both scalars and fermions, and analyse the reheating dynamics. Reference：
16-Apr-2015
Sanshiro Shibata 「Radiative Transfer Simulation for Thermal Radiation from GRB Jet」
Abstract：Recently, thermal radiation from gamma-ray burst (GRB) jets attracts attentions as the origin of the GRB prompt emission. It is expected that the spectrum of thermal radiation from GRB jet is complicated because the jet itself has a very complicated structure. Thus, in order to treat the thermal radiation from GRB jets properly, both the radiative transfer in the jets and the complicated inner structures of the jets should be taken into account. We performed a radiative transfer simulation for thermal photons from GRB jet with a numerical code based on Monte-Carlo method. We will present the details and the results of the simulation. Reference：