## Cosmophysics Group Seminar（2013年度）

**27-Feb-2014**

小玉 英雄「Bosonic Dark Matter -- Review --」

アブストラクト：

I overview the basic idea, research history and present status of bosonic dark matter models, in particular, the BEC SFDM model for galaxy halos.

参考文献：

“A Review on the Scalar Field/ Bose-Einstein Condensate Dark Matter Model” A.Suarez, V.H. Robles, T. Matos: arXiv:1302.0903 ( a bad review).

P.H. Chavanis: PRD84, 043531 (2011);Chavanis PH, Defini L, ibid.,043532 (2011).

Dehen H., Rose B.: Astrophys. Sp Sci., 207, 133-144 (1993).

Wayne Hu, R Barkana, A Gruzinov: PRL85, 1158 (2000).

Guzman FS, Urena-Lopez LA: PRD69, 124033 (2004).

Boehmer CG, Harko T: JCAP 0706, 025 (2007); Harko T: JCAP1105, 022 (2011).

Robles VH, Matos T: MNRAS422, 282 (2012).

F.S. Guzman, F.D. Lora-Clavijo, J.J. Gonzalez-Aviles, F.J. Rivera-Paleo: JCAP 1309, 034 (2013); arXiv: 1310.3909 (2013).

**30-Jan-2014**

吉野 裕高「Discussion on gravitational waves from a BH-axion system 」

アブストラクト：

String theory suggests the possible existence of scalar fields with ultralight mass (string axions). If this is the case, an axion field forms a cloud around a rotating black hole and emit gravitational waves. I would like to discuss whether the gravitational waves from a Cygnus X-1 can be detected with existing gravitational wave interferometers, such as LIGO. The model parameter of the string axions could be constrained from the existing data of their science observation. (Though this work is still incomplete, I would like to explain what I am studying and thinking now.)

**16-Jan-2014**

松本 光洋「Evolution of a black ring by Hawking evaporation」

アブストラクト：

Black objects lose their mass and angular momenta through evaporation by Hawking radiation,and the investigation of their time evolution has a long history. In this paper, we study this problem for a five-dimensional doubly spinning black ring. The black ring is assumed to emit only massless scalar particles. We consider a thin black ring with a small thickness parameter, λ ≪ 1, which can be approximated by a boosted Kerr string locally. We show that a thin black ring evaporates with fixing its thickness parameter λ. Further, in the case of an Emparan-Reall black ring, we derive analytic formulas for the time evolution, which has one parameter to be evaluated numerically. We find that the lifetime of a thin black ring is shorter by a factor of O(λ2) compared to a five-dimensional Schwarzschild black hole with the same initial mass. We also study detailed properties of the Hawking radiation from the thin black ring, including the energy and angular spectra of emitted particles.

参考文献：

Matsumoto, Yoshino, Kodama, arXiv:1205.5715

**19-Dec-2013**

高見 一 「Possible Dust formation in Neutron-Star-Binary Mergers」

アブストラクト：

Recent general relativistic simulations of neutron star (NS) binary (i.e., NS-NS and black hole-NS binary) mergers have revealed significant mass ejection with the total kinetic energy comparable with a supernova (~10 ^{51} erg). The ejecta is inevitably neutron-rich and therefore is a plausible site for r-process nucleosynthesis. Also, an environment with many heavy nuclei is a possible site for cosmic dust formation like supernovae. I will give a talk on our recent progress on dust formation at NS binary mergers with a brief review of this field.

**5-Dec-2013**

Ivan Arraut 「Some pathologies of the dRGT formulation」

アブストラクト：

The non-linear theory of massive gravity has been proposed as an alternative solution for the Dark Energy problem. However, since the moment it was formulated, some pathologies were found inside its formalism. I will make a resume of these pathologies, which include: DVZ discontinuity, BD ghost and finally some pathologies associated with Black Hole solutions and the possibility of exploring Black Hole thermodynamics around this approach.

参考文献：

Van Dam, Veltman, Zhakarov 70's

Vainshtein 72.

Boulware and Deser 72.

Hideo Kodama and Ivan Arraut 2013. arXiv:1312.0370 [hep-th].

Bousso and Hawking, Phys. Rev. D 54(1996).

de Rham and Gabadadze 2010.

**21-Nov-2013**

大橋 勢樹 「Generalization of Horndeski's theory」

アブストラクト：

Recently, "Horndeski's Theory" has attracted much attention in the context of cosmology. In this talk, I will review the Horndeski's original argument on the most general single scalar + tensor theory with second order. Then I will explain its generalization. Mainly I will focus on two (and three) scalars + tensor theory and its application.

参考文献：

G.W. Horndeski, Int. J. Theor. Phys. 10, 363 (1974).

**31-Oct-2013**

野澤 真人 「Dynamical vs thermodynamical stability of relativistic stars」

アブストラクト：

As shown by a series of works of Chandrasekhar, Friedman and Schutz, all rotating stars are unstable under nonaxisymmetric perturbations. I will review the recent development for the dynamical vs thermodynamic stability criteria of relativistic stars in general relativity. Using the Lagrangian displacement framework in the Einstein-perfect-fluid system, one can define the symplectic structure which enables us to introduce the canonical energy. I will discuss how the positivity of canonical energy are utilized to conclude the dynamical stability of relativistic stars.

参考文献：

Dynamic and Thermodynamic Stability of Relativistic, Perfect Fluid Stars, Stephen R. Green, Joshua S. Schiffrin, Robert M. Wald, arXiv:1309.0177

Turning Point Instabilities for Relativistic Stars and Black Holes, Joshua S. Schiffrin, Robert M. Wald, arXiv:1310.5117

**17-Oct-2013**

大山 祥彦 「Constraints on the lepton asymmetry by future 21cm line and CMB observations」

アブストラクト：

Under current observations, relatively large lepton (~neutrino) number asymmetry is still allowed. In addition, there are several baryogenesis models which predict the large lepton asymmetry. I will briefly explain impacts of the lepton asymmetry on the growth of the density fluctuations, and show forecasts of constraints on it by future CMB and 21cm line observations.

**18-July-2013**

松本 光洋 「Time evolution of a thin black ring via Hawking radiation」

アブストラクト：

I will (re-)explain time evolution of evaporation of a black ring. Then, let me discuss a future work for my doctoral thesis.

参考文献：

M. Matsumoto, H. Yoshino, H. Kodama, arXiv:1205.5715

**18-July-2013**

郡 和範 「Gravitational wave and primordial black-hole formations by large fluctuations at small scales in the matter-dominated universe」

アブストラクト：

In some classes of inflation models, large curvature fluctuations are predicted at small scales with satisfying the current observational data reported by the Planck satellite. We show that we can check these types of models by detecting primordial gravitational wave, or a primordial black hole (PBH), both of which may be produced by the large fluctuation at small scales. I will discuss there are definite modifications on productions of the gravitational wave and PBHs if there exists a matter-dominated epoch in the early universe.

**11-July-2013**

井岡 邦仁 「What is the origin of short gamma-ray bursts?」

アブストラクト：

I would like to examine the popular hypothesis that the binary neutron star mergers are the origin of short GRBs.

参考文献：

Kyutoku, Koutarou; Ioka, Kunihito; Shibata, Masaru, arXiv1305.6309;arXiv1209.5747

Mizuta, Akira; Ioka, Kunihito, arXiv1304.0163

Bromberg, O., et al., The Astrophysical Journal, 740, 100 (2011)

Hotokezaka, K., et al., Physical Review D, 87, 024001

**4-July-2013**

小玉 英雄 「IR Problem in Inflation」

アブストラクト：

I overview the current status of the IR divergence issue in the investigation of quantum corrections to the correlation functions for fluctuations created during inflation.

参考文献：

Infrared effects in inflationary correlation functions, D. Seery, Class. Quant. Grav. 27 (2010) 124005 [arXiv:1005.1649]

Semiclassical relations and IR effects in de Sitter and slow-roll space-times, S.B. Giddings and M.S. Sloth, JCAP 01 (2011) 023 [arXiv:1005.1056].

IR divergences in inflation and entropy perturbations, Wei Xue, Xian Gaob, Robert Brandenberger: JCAP1206 (2012) 035 [arXiv: 1201.0768]

Loops in inflationary correlation functions, Takahiro Tanaka, Yuko Urakawa [arXiv: 1306.4461]

**20-June-2013**

吉野 裕高 「Gravitational Aharonov-Bohm effect: A short review」

アブストラクト：

Study on gravity effects on quantum fields is interesting because it might give insights to new physics beyond general relativity and to experimental methods to check it. As one of these issues, I pick up the gravitational Aharonov-Bohm effect and give a short review on old and new discussions.

**23-May-2013**

大橋 勢樹 「Stability of Cauchy horizon in self-similar space-time」

アブストラクト：

The final state of gravitational collapse is one of the unsolved issues in gravitational physics. So far, many solutions which end in naked singular space-time have been reported. Because spherical symmetry is assumed in almost all of these solutions, it is important to ask whether these singularities are stably formed under small perturbations. In this talk, I will report on the recent results on the (in)stability of the naked singularity formation in self-similar space-time.

参考文献：

E.M.Duffy and B.C.Nolan, CQG 28, 105020 (2011)

B.C.Nolan, CQG 24, 177 (2007)

**25-Apr-2013**

Ivan Arraut 「The Cosmologocal Constant in the local physics and massive gravity」

アブストラクト：

I will mention some of my previous results related to the astrophysical scales set by the Cosmological Constant. Then after I will make a review about the basic aspects of massive gravity and its relevant scales. I will also discuss the ghost problem and its solution inside this theory.

参考文献：

Ivan Arraut: Mod.Phys.Lett. A28 (2013) 1350019

Ivan Arraut: arXiv:1305.0475

C. de Rham and G. Gabadadze: Phys. Rev. D 82, 044020 (2010).

C. de Rham, G. Gabadadze and A. J. Tolley: Phys. Rev. Lett. 106, 231101 (2011).

S. F. Hassan and R. A. Rosen: Phys. Rev. Lett. 108, 041101 (2012).

**25-Apr-2013**

野澤 真人 「deSitter vacua in N=2 supergravity」

アブストラクト：

In this talk I will report the recent progress on the construction of deSitter vacua in N=2 supergravity. In particular the necessary conditions for obtaining metastable de Sitter vacua are discussed.

参考文献：

F.Catino, C.A. Scrucca, P. Smyth, JHEP 1304 (2013) 056, Simple metastable de Sitter vacua in N=2 gauged supergravity

M. G-Reino, J. Louis and C.A. Scrucca, JHEP 0902 (2009) 003, No metastable de Sitter vacua in N=2 supergravity with only hypermultiplets

**11-Apr-2013**

高見 一 「Geometrical effects on blazar variability and parameter estimation」

アブストラクト：

Recent progress on blazar observations has revealed very short time variability of flux, down to ~ 10 minutes. Such variability implies particle acceleration sites much smaller than their distances from supermassive black holes and/or the high Lorentz factor of particle acceleration sites close to ~ 100 contrary to direct observations of parsec scale jets by radio interferometers. Here, we revisit the short variability problem and discuss the effects of geometry of particle acceleration sites.

参考文献：

R.J. Protheroe, Publ. Astron. Soc. Aust, 19 (2002) 486-498

M. Salvati, M. Spada, F. Pacini, Astrophys. J., 495 (1998) L19-L21

M. Spada, M. Salvati, F. Pacini, Astrophys. J., 511 (1999) 136-141