Cosmophysics Group Seminar (2016)
23-Feb-2017
Nagisa Hiroshima「DM Indirect detection with dSph」
Abstract：The dark matter (DM) dominate the baryons in our universe, however, we do not know what it is. Among several approaches to explore the nature of the DM, indirect detection experiments which search the DM annihilating signals into the standard model particles are very powerful probe. We can access the energy higher than TeV in gamma-ray search of the DM, for example, and we have a good target to see such as dwarf spheroidal galaxies (dSphs). In this talk, I will talk about the basics of the gamma-ray DM search and report my recent progress about the study which focus on the dSph. Reference：Charles et al., 2016, Phys. Rept 636 Ackermann et al., 2014, Phys. Rev. D., 042001 Geringer-Sameth et al., 2015, ApJ, 801
9-Feb-2017
Jonathan White「Slow-roll corrections in multi-field inflation」
Abstract： In this era of precision cosmology, theoretical predictions for inflationary observables must be accurate at the percent level, meaning that often-neglected slow-roll corrections should be accounted for. Using the δN formalism, we obtain compact expressions for slow-roll corrections to the power spectrum and bispectrum of the curvature perturbation in multi-field inflation, written in terms of quantities that in principle are observable. We find that corrections to the power spectrum are constrained to be at the percent level, while corrections to the tilt and bispectrum can potentially be much larger. An example model where this is the case is presented.
2-Feb-2017
Kazunori Kohri「IR background observations and axion-photon conversions」
Abstract：The amount of the IR background is sensitive to an opaqueness of the TeV gamma-rays emitted from distant sources such as blazars and so on. Through scatterings of TeV gamma-rays off the IR background photons, the TeV gamma-rays are partly absorbed, which can be measured by gamma-ray observations such as Fermi or H.E.S.S. By checking consistencies between the TeV gamma-rays and the IR background, we can obtain various information for physical processes. In this talk I discuss a possible interrelation for features of the observational data in terms of Axion-Like Particles (ALPs) which induces photon-axion conversions during the propagation of the TeV gamma-rays from the distant sources. Reference： D. Hooper and P. D. Serpico, ``Detecting Axion-Like Particles With Gamma Ray Telescopes,'' Phys. Rev. Lett. 99, 231102 (2007), [arXiv:0706. 3203 [hep-ph]]
26-Jan-2017
Takuya Hasegawa「Neutrino oscillation in the early universe」
Abstract：Recently some kinds of neutrino experiments show anomalous results called SBN anomaly. And this anomaly can be explained by the existence of eV-scale mass sterile neutrino and their mixing with active neutrinos. In this scenario, the light sterile neutrino might change the physics in the early universe. In this talk, I will introduce the neutrino oscillation in matter for this study and explain the paper in which cosmological effects of sterile neutrinos are discussed. Reference： S.Hannestad et al. (2012) : arXiv 1204.5861 G.M. Fuller et al. (2016) : arXiv 1608.01336 A.Mirizzi et al. (2013) : arXiv 1302.1200
12-Jan-2017
Koutarou Kyutoku「Introduction to gravitational waves」
Abstract：In this talk, I will present basics of gravitational-wave theory as dynamical degrees freedom of gravity. I will also explain why the quadrupole formula should not have been applicable (although turned out to be applicable in reality) to compact object binaries. Reference：Misner, Thorne, Wheeler "Gravitation" Maggiore "Gravitational Waves" Flanagan, Hughes, New. J. Phys. 7, 204 (2005)
5-Jan-2017
Taro Mori 「Axion-Moduli Inflation」
Abstract：I will introduce multi-field inflation model which contains both of moduli and axion behave as inflatons. In the first part of this talk, I'II briefly review what are moduli fields and why we need to stabilize them. After that, I'll talk about Kahler Moduli Inflation. This model is based on stabilized moduli potential. In the end of this talk, I'll briefly report the analysis on Axion-Moduli Inflation by numerical approach. Reference：Kahler Moduli Inflation/J.Conlon, F.Quevedo hep-th:0509012
13-Oct-2016
Kazunori Kohri 「Effects of electrically charged dark matter on cosmology」
Abstract：We examine the possibility that dark matter (DM) consists of charged massive particles (CHAMPs) in view of the cosmic microwave background ( CMB) anisotropies. The evolution of cosmological perturbations of CHAMP with other components is followed in a self-consistent manner, without assuming that CHAMP and baryons are tightly coupled. We incorporate for the first time the "kinetic re-coupling" of the Coulomb scattering, which is characteristic of heavy CHAMPs. By a direct comparison of the predicted CMB temperature/polarization auto-correlations in CHAMP models and the observed spectra in the Planck mission, we show that CHAMPs leave sizable effects on CMB spectra if they are lighter than 1011GeV. Our result can be applicable to any CHAMP as long as its lifetime is much longer than the cosmic time at the recombination (∼4×105yr). An application to millicharged particles is also discussed.
8-Sept-2016
Jonathan White「Slow-roll corrections in multi-field inflation」
Abstract：Multi-field inflation models allow for a rich variety of inflationary dynamics, and even in the simplest of models it is not unusual to encounter at least temporary violations of slow-roll. However, the slow-roll approximation is often made when calculating the spectrum of perturbations generated in multi-field inflation models. In this talk, we will give an overview of the methods used to calculate perturbation spectra in multi-field inflation models and investigate the potential importance of slow-roll corrections, focussing on the power spectrum and bispectrum in the squeezed limit.
7-July-2016
Takuya Hasegawa 「On the Li7 problem in BBN」
Abstract：In this talk, I will introduce the long-standing Li7 problem in SBBN and a possible solution. Pospelov et al. (2010) have proposed decays of GeV-scale BSM particle can solve the problem. However, according to the recent observations, we need fine tuning of parameters in the model. I am going to discuss the effects on this scenario taking into account neutrino oscillation. Reference：M. Pospelov, J. Pradler (2010) arXiv:1011.1054 G. Mangano, P. D. Serpico et al. (2005) arXiv:0506164 P. F. Salasa, S.Pastora (2016) arXiv:1606.06986
30-June-2016
Taro Mori「Multi-field effects on Starobinsky inflation」
Abstract：I will discuss a two-field inflation model which contains Starobinsky- like sector and quadratic potential of another scalar field. In general we cannot calculate Power Spectrum and Non-Gaussianity analytically if two fields are interacting.In this talk I will introduce a numerical approach to compute these quantities and investigate allowed parameter regions in field space. Reference：C. van de Bruck, L. E. Paduraru(2015):arXiv:1505.01727 D.Kaiser et al(2012):arXiv:1210.7487
16-June-2016
Yoshihisa Kitazawa「Effective Lagrangian in de Sitter Spacetime 」
Abstract：We have proposed that scale invariant quantum fluctuations of the metric render fundamental couplings time dependent. We construct effective Lagrangian in de Sitter spacetime to make point more clearly. We argue effective Lorentz invariance at sub-horizon scale leads to unique physical predictions. Reference："Time Dependent Couplings as Observables in de Sitter Space" Hiroyuki Kitamoto, Yoshihisa Kitazawa (2014) arXiv:1402.2443 [hep-th]
9-June-2016
Yuta Hamada「Reheating-era leptogenesis in various seesaw models 」
Abstract：First we show that observed baryon asymmetry can be produced by scattering processes between the standard model particles, which we call the reheating era leptogenesis scenario.Then, we consider the concrete embedding of our scenario in various seesaw models. Reference：Hamada Kawana arXiv:1510.05186 , Hamada Tsumura Yasuhara to appear
12-May-2016
28-Apr-2016Ryusuke Jinno 「Analytic gravitational-wave spectrum from bubble collisions 」
Abstract：We consider gravitational wave production by bubble collisions during a cosmological first-order phase transition. In the literature, such spectra have been estimated by simulating the bubble dynamics, under so- called thin-wall and envelope approximations in a flat background metric. However, we show that the gravitational wave spectrum can be estimated in an analytic way under these assumptions. As a result, it is found that the most of the contributions to the spectrum come from single- bubble contribution to the correlator of the energy-momentum tensor, and in addition the fall-off of the spectrum at high frequencies is found to be proportional to f^{－1}. Reference： Ryusuke Jinno, Masahiro Takimoto, 2016 , arXiv:1605.01403 [astro-ph.CO]
21-Apr-2016Nagisa Hiroshima 「Time variability analysis of the Crab nebula based on giant radio pulse observations」
Abstract：Observations of radio pulses provides us with information about plasma structure on our lines of sight. The Crab pulsar and its nebula are very variable and interesting objects in this sentence. We conducted series of observations of the Crab pulsar at 325 MHz and detected change of pulse light-curve shape. This phenomena can interpret with filaments in the nebula crossing our line of sight. Reference： Kuzmin et al., 2008, A&A, 483, 13 Cronyn, 1970, Sci, 168, 1453
Masahiro Takimoto「Suppressed Non-Gaussianity in the Curvaton Model」
Abstract： We show that the local type non-Gaussianity in a class of curvaton model is suppressed, i.e. the non-linearity parameters f_{NL} and those related with higher statistics can be at most O(1), even if the curvaton energy density is subdominant at the decay, This situation is naturally realized in a very simple curvaton potential with quadratic term plus quartic term. Reference： Phys. Rev. D 89, 123515, JCAP 1406 (2014) 013)