# KOBAYASHI Tsutomu

Last updated: Sep 21, 2019 at 18:16

Name KOBAYASHI Tsutomu tsutomurikkyo.ac.jp Rikkyo University College of Science Department of Physics Professor Rikkyo University, Rikkyo University 40580212

## Research Areas

Apr 2012
-
Mar 2019
Associate Professor, College of Science Department of Physics, RIKKYO UNIVERSITY

Apr 2019
-
Today
Professor, Graduate School of Science Field of Study: Physics, RIKKYO UNIVERSITY

Apr 2019
-
Today
Professor, Graduate School of Science Field of Study: Physics, RIKKYO UNIVERSITY

Apr 2019
-
Today
Professor, College of Science Department of Physics, RIKKYO UNIVERSITY

## Education

-
Mar 2001
Faculty of Science, Kyoto University

-
Mar 2006
Graduate School, Division of Natural Science, Kyoto University

## Published Papers

Shin'ichi Hirano, Tsutomu Kobayashi, Daisuke Yamauchi
Phys. Rev. D 99, 104073 (2019)      Mar 2019
We consider a subclass of degenerate higher-order scalar-tensor (DHOST) theories in which gravitational waves propagate at the speed of light and do not decay into scalar fluctuations. The screening mechanism in DHOST theories evading these two gr...
Shin'ichi Hirano, Tsutomu Kobayashi, Daisuke Yamauchi, Shuichiro Yokoyama
Phys. Rev. D 99, 104051 (2019)      Feb 2019
We investigate the potential of cosmological observations, such as galaxy surveys, for constraining degenerate higher-order scalar-tensor (DHOST) theories, focusing in particular on the linear growth of the matter density fluctuations. We develop ...
Tsutomu Kobayashi
Jan 2019
This article is intended to review the recent developments in the Horndeski theory and its generalization, which provide us with a systematic understanding of scalar-tensor theories of gravity as well as a powerful tool to explore astrophysics and...
Kazufumi Takahashi, Tsutomu Kobayashi
Class. Quant. Grav. 36 (2019) no.9, 095003      Dec 2018
We show explicitly that the nonminimal coupling between the scalar field and the Ricci scalar in 2D dilaton gravity can be recast in the form of kinetic gravity braiding (KGB). This is as it should be, because KGB is the 2D version of the Horndesk...
Shingo Akama, Tsutomu Kobayashi
Phys. Rev. D 99, 043522 (2019)      Oct 2018
Our Universe is nearly spatially flat, but this does not mean that it is exactly spatially flat. In this paper we derive general quadratic actions for cosmological perturbations in non-flat models from the Horndeski theory. This allows us to study...
Aya Iyonaga, Kazufumi Takahashi, Tsutomu Kobayashi
JCAP 12 (2018) 002      Sep 2018
Among single-field scalar-tensor theories, there is a special class called "cuscuton," which is represented as some limiting case of k-essence in general relativity. This theory has a remarkable feature that the number of propagating degrees of fr...
Atsushi Nishizawa, Tsutomu Kobayashi
Phys. Rev. D 98, 124018 (2018)      Sep 2018
We consider gravitational waves (GWs) in generic parity-violating gravity including recently proposed ghost-free theories with parity violation as well as Chern-Simons (CS) modified gravity, and study the implications of observational constraints ...
Hiroaki W. H. Tahara, Sakine Nishi, Tsutomu Kobayashi, Jun'ichi Yokoyama
JCAP07(2018)058      May 2018
As opposed to Wald's cosmic no-hair theorem in general relativity, it is shown that the Horndeski theory (and its generalization) admits anisotropic inflationary attractors if the Lagrangian depends cubically on the second derivatives of the scala...
Tsutomu Kobayashi, Takashi Hiramatsu
Phys. Rev. D 97, 104012 (2018)      Mar 2018
We study relativistic stars in degenerate higher-order scalar-tensor theories that evade the constraint on the speed of gravitational waves imposed by GW170817. It is shown that the exterior metric is given by the usual Schwarzschild solution if t...
Hiromu Ogawa, Takashi Hiramatsu, Tsutomu Kobayashi
Feb 2018
The Vainshtein mechanism is known as an efficient way of screening the fifth force around a matter source in modified gravity. This has been verified mainly in highly symmetric matter configurations. To study how the Vainshtein mechanism works in ...
Shin'ichi Hirano, Tsutomu Kobayashi, Hiroyuki Tashiro, Shuichiro Yokoyama
Phys. Rev. D 97, 103517 (2018)      Jan 2018
The Horndeski scalar-tensor theory and its recent extensions allow nonlinear derivative interactions of the scalar degree of freedom. We study the matter bispectrum of large scale structure as a probe of these modified gravity theories, focusing i...
Kazufumi Takahashi, Tsutomu Kobayashi
JCAP11(2017)038      Aug 2017
We propose a novel class of degenerate higher-order scalar-tensor theories as an extension of mimetic gravity. By performing a noninvertible conformal transformation on "seed" scalar-tensor theories which may be nondegenerate, we can generate a la...
Shin'ichi Hirano, Sakine Nishi, Tsutomu Kobayashi
Apr 2017
We study the stability of a recently proposed model of scalar-field matter called mimetic dark matter or imperfect dark matter. It has been known that mimetic matter with higher derivative terms suffers from gradient instabilities in scalar pertur...
Kazufumi Takahashi, Hayato Motohashi, Teruaki Suyama, Tsutomu Kobayashi
Phys. Rev. D 95, 084053 (2017)      Feb 2017
An invertible field transformation is such that the old field variables correspond one-to-one to the new variables. As such, one may think that two systems that are related by an invertible transformation are physically equivalent. However, if the...
Shingo Akama, Tsutomu Kobayashi
Phys. Rev. D 95, 064011 (2017)      Jan 2017
It has been pointed out that non-singular cosmological solutions in second-order scalar-tensor theories generically suffer from gradient instabilities. We extend this no-go result to second-order gravitational theories with an arbitrary number of ...
Sakine Nishi, Tsutomu Kobayashi
Phys. Rev. D 95, 064001 (2017)      Nov 2016
We propose a novel branch of the Galilean Genesis scenario as an alternative to inflation, in which the universe starts expanding from Minkowski in the asymptotic past with a gross violation of the null energy condition (NEC). This variant, descri...
Tsutomu Kobayashi
Phys. Rev. D 94, 043511 (2016)      Jun 2016
The null energy condition can be violated stably in generalized Galileon theories, which gives rise to the possibilities of healthy non-singular cosmologies. However, it has been reported that in many cases cosmological solutions are plagued with ...
Shin'ichi Hirano, Tsutomu Kobayashi, Shuichiro Yokoyama
Phys. Rev. D 94, 103515 (2016)      Apr 2016
The conventional slow-roll approximation is broken in the so-called "ultra slow-roll" models of inflation, for which the inflaton potential is exactly (or extremely) flat. The interesting nature of (canonical) ultra slow-roll inflation is that the...
Sakine Nishi, Tsutomu Kobayashi
Jan 2016
Galilean genesis is an alternative to inflation, in which the universe starts expanding from Minkowski with the stable violation of the null energy condition. In this paper, we discuss how the early universe is reheated through the gravitational p...
Yuji Akita, Tsutomu Kobayashi
Phys. Rev. D 93, 043519 (2016)      Dec 2015
We clarify the features of primordial non-Gaussianities of tensor perturbations in Gao's unifying framework of scalar-tensor theories. The general Lagrangian is given in terms of the ADM variables so that the framework maintains spatial covariance...
Kazufumi Takahashi, Teruaki Suyama, Tsutomu Kobayashi
Phys. Rev. D 93, 064068 (2016)      Nov 2015
We analyze spherically symmetric black hole solutions with time-dependent scalar hair in a class of Lovelock-Galileon theories, which are the scalar-tensor theories with second-order field equations in arbitrary dimensions. We first show that know...
Hiromu Ogawa, Tsutomu Kobayashi, Teruaki Suyama
Phys. Rev. D 93, 064078 (2016)      Oct 2015
Recently it was pointed out that in shift-symmetric scalar-tensor theories a black hole can have nontrivial scalar hair which depends linearly on time. We develop black hole perturbation theory for such solutions and compute the quadratic action o...
Tsutomu Kobayashi, Masaru Siino, Masahide Yamaguchi, Daisuke Yoshida
Sep 2015
We investigate perturbations of a class of spherically symmetric solutions in massive gravity and bi-gravity. The background equations of motion for the particular class of solutions we are interested in reduce to a set of the Einstein equations w...
Kohji Yajima, Tsutomu Kobayashi
Phys. Rev. D 92, 103503 (2015)      Aug 2015
We address the question of how one can modify the inflationary tensor spectrum without changing at all the successful predictions on the curvature perturbation. We show that this is indeed possible, and determine the two quadratic curvature correc...
Yuji Akita, Tsutomu Kobayashi
Jul 2015
Recently it was argued that gravity with the squire of the Ricci tensor can be stabilized by adding constraints to the theory. This was so far demonstrated for fluctuations on the Minkowski/de Sitter background. We show that the same scheme works ...
Seiju Ohashi, Norihiro Tanahashi, Tsutomu Kobayashi, Masahide Yamaguchi
JHEP 1507:008,2015      May 2015
The Horndeski theory is known as the most general scalar-tensor theory with second-order field equations. In this paper, we explore the bi-scalar extension of the Horndeski theory. Following Horndeski's approach, we determine all the possible term...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Apr 2015
It has been pointed out that the null energy condition can be violated stably in some non-canonical scalar-field theories. This allows us to consider the Galilean Genesis scenario in which the universe starts expanding from Minkowski spacetime and...
Sakine Nishi, Tsutomu Kobayashi
Jan 2015
The galilean genesis scenario is an alternative to inflation in which the universe starts expanding from Minkowski in the asymptotic past by violating the null energy condition stably. Several concrete models of galilean genesis have been construc...
Tsutomu Kobayashi, Yuki Watanabe, Daisuke Yamauchi
Phys. Rev. D 91, 064013 (2015)      Nov 2014
The Horndeski theory of gravity is known as the most general scalar-tensor theory with second-order field equations. Recently, it was demonstrated by Gleyzes et al. that the Horndeski theory can further be generalized in such a way that although f...
Xian Gao, Tsutomu Kobayashi, Masahide Yamaguchi, Daisuke Yoshida
Phys. Rev. D 90, 124073 (2014)      Sep 2014
The Stückelberg analysis of nonlinear massive gravity in the presence of a general fiducial metric is investigated. We develop a "covariant" formalism for the Stückelberg expansion by working with a local inertial frame, through which helicity...
Tsutomu Kobayashi, Norihiro Tanahashi
Mar 2014
We derive a variety of exact black hole solutions in a subclass of Horndeski's scalar-tensor theory possessing shift symmetry, , and reflection symmetry, . The theory admits two arbitrary functions of $X:=-(\partial\ph... Tsutomu Kobayashi, Hayato Motohashi, Teruaki Suyama Phys. Rev. D 89, 084042 (2014) Feb 2014 We perform a fully relativistic analysis of even-parity linear perturbations around a static and spherically symmetric solution in the most general scalar-tensor theory with second-order field equations. This paper is a sequel to Kobayashi {\em et... Sakine Nishi, Tsutomu Kobayashi, Norihiro Tanahashi, Masahide Yamaguchi Jan 2014 We derive the cosmological matching conditions for the homogeneous and isotropic background and for linear perturbations in Horndeski's most general second-order scalar-tensor theory. In general relativity, the matching is done in such a way that ... Kohei Kamada, Tsutomu Kobayashi, Taro Kunimitsu, Masahide Yamaguchi, Jun'ichi Yokoyama Phys. Rev. D 88, 123518 (2013) Sep 2013 Higgs G-inflation is a Higgs inflation model with a generalized Galileon term added to the standard model Higgs field, which realizes inflation compatible with observations. Recently, it was claimed that the generalized Galileon term induces insta... Tsutomu Kobayashi, Norihiro Tanahashi, Masahide Yamaguchi Phys. Rev. D 88, 083504 (2013) Aug 2013 We propose a multi-field extension of (generalized) G-inflation, based on covariant multi-galileons and their generalization preserving second-order field equations. We compute the quadratic action for cosmological perturbations. By comparing the ... Tatsuya Narikawa, Tsutomu Kobayashi, Daisuke Yamauchi, Ryo Saito Phys.Rev.D87:124006,2013 Feb 2013 We explore the possibility of testing modified gravity exhibiting the Vainshtein mechanism against observations of cluster lensing. We work in the most general scalar-tensor theory with second-order field equations (Horndeski's theory), and derive... Yu-ichi Takamizu, Tsutomu Kobayashi Prog. Theor. Exp. Phys. 2013, 063E03 Jan 2013 We develop a theory of nonlinear cosmological perturbations on superhorizon scales for generic single-field inflation. Our inflaton is described by the Lagrangian of the form with$X=-\partial^{\mu}\phi\partial_{\mu}\...
Xian Gao, Tsutomu Kobayashi, Maresuke Shiraishi, Masahide Yamaguchi, Jun'ichi Yokoyama, Shuichiro Yokoyama
Jul 2012
We compute the full bispectra, namely both auto- and cross- bispectra, of primordial curvature and tensor perturbations in the most general single-field inflation model whose scalar and gravitational equations of motion are of second order. The fo...
Tsutomu Kobayashi, Masaru Siino, Masahide Yamaguchi, Daisuke Yoshida
May 2012
We find new, simple cosmological solutions with flat, open, and closed spatial geometries, contrary to the previous wisdom that only the open model is allowed. The metric and the Stückelberg fields are given explicitly, showing nontrivial conf...
Kohei Kamada, Tsutomu Kobayashi, Tomo Takahashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Mar 2012
We study Higgs inflation in the context of generalized G-inflation, i.e. the most general single-field inflation model with second-order field equations. The four variants of Higgs inflation proposed so far in the literature can be accommodated at...
Tsutomu Kobayashi, Hayato Motohashi, Teruaki Suyama
Phys.Rev. D85 (2012) 084025      Feb 2012
We perform a fully relativistic analysis of odd-type linear perturbations around a static and spherically symmetric solution in the most general scalar-tensor theory with second-order field equations in four-dimensional spacetime. It is shown that...
Rampei Kimura, Tsutomu Kobayashi, Kazuhiro Yamamoto
Phys.Rev.D85:024023,2012      Nov 2011
A generic second-order scalar-tensor theory contains a nonlinear derivative self-interaction of the scalar degree of freedom à la Galileon models, which allows for the Vainshtein screening mechanism. We investigate this effect on subhor...
Rampei Kimura, Tsutomu Kobayashi, Kazuhiro Yamamoto
Phys.Rev.D85:123503,2012      Oct 2011
The cross-correlation between the integrated Sachs-Wolfe (ISW) effect and the large scale structure (LSS) is a powerful tool to constrain dark energy and alternative theories of gravity. In this paper, we obtain observational constraints on kineti...
Antonio De Felice, Tsutomu Kobayashi, Shinji Tsujikawa
Phys.Lett.B706:123-133, 2011      Aug 2011
In the Horndeski's most general scalar-tensor theories the equations of scalar density perturbations are derived in the presence of non-relativistic matter minimally coupled to gravity. Under a quasi-static approximation on sub-horizon scales we o...
Xian Gao, Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Aug 2011
We completely clarify the feature of primordial non-Gaussianities of tensor perturbations in generalized G-inflation, i.e., the most general single-field inflation model with second order field equations. It is shown that the most general cubic ac...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Prog. Theor. Phys. 126 (2011), 511-529      May 2011
We study generalized Galileons as a framework to develop the most general single-field inflation models ever, Generalized G-inflation, containing yet further generalization of G-inflation, as well as previous examples such as k-inflation, extended...
Yosuke Misonoh, Kei-ichi Maeda, Tsutomu Kobayashi
Phys.Rev.D84:064030,2011      Apr 2011
We study a vacuum Bianchi IX universe in the context of Ho\v{r}ava-Lifshitz (HL) gravity. In particular, we focus on the classical dynamics of the universe and analyze how anisotropy changes the history of the universe. For small anisotropy, we fi...
Masahiro Nakashima, Tsutomu Kobayashi
Mar 2011
We study how the dynamical vector degree of freedom in modified gravity affects the CMB B-mode polarization in terms of the Einstein-aether theory. In this theory, vector perturbations can be generated from inflation, which can grow on superhorizo...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D83:103524,2011      Mar 2011
We present a comprehensive study of primordial fluctuations generated from G-inflation, in which the inflaton Lagrangian is of the form with . The Lagrangian still gives rise to second-order g...
Masahiro Nakashima, Tsutomu Kobayashi
Dec 2010
We study the impact of modifying the vector sector of gravity on the CMB polarization. We employ the Einstein-aether theory as a concrete example. The Einstein-aether theory admits dynamical vector perturbations generated during inflation, leaving...
Kohei Kamada, Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D83:083515,2011      Dec 2010
A new class of inflation models within the context of G-inflation is proposed, in which the standard model Higgs boson can act as an inflaton thanks to Galileon-like non-linear derivative interaction. The generated primordial density perturbation ...
Tsutomu Kobayashi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.Lett.105:231302,2010      Aug 2010
We propose a new class of inflation model, G-inflation, which has a Galileon-like nonlinear derivative interaction of the form in the Lagrangian with the resultant equations of motion being of second order. It is ...
Kei-ichi Maeda, Yosuke Misonoh, Tsutomu Kobayashi
Phys.Rev.D82:064024,2010      Jun 2010
We study the dynamics of isotropic and homogeneous universes in the generalized Ho\v{r}ava-Lifshitz gravity, and classify all possible evolutions of vacuum spacetime. In the case without the detailed balance condition, we find a variety of phase s...
Tsutomu Kobayashi
Phys.Rev.D81:103533,2010      Mar 2010
We study models of late-time cosmic acceleration in terms of scalar-tensor theories generalized to include a certain class of non-linear derivative interaction of the scalar field. The non-linear effect suppress the scalar-mediated force at short ...
Tsutomu Kobayashi, Yuko Urakawa, Masahide Yamaguchi
JCAP 1004:025,2010      Feb 2010
In Horava's theory of gravity, Lorentz symmetry is broken in exchange for renormalizability, but the original theory has been argued to be plagued with problems associated with a new scalar mode stemming from the very breaking of Lorentz symmetry....
Tsutomu Kobayashi, Hiroyuki Tashiro, Daichi Suzuki
Phys.Rev.D81:063513,2010      Dec 2009
A scalar-tensor theory of gravity can be made not only to account for the current cosmic acceleration, but also to satisfy solar-system and laboratory constraints, by introducing a non-linear derivative interaction for the scalar field. Such an ad...
Tsutomu Kobayashi, Yuko Urakawa, Masahide Yamaguchi
JCAP 0911:015,2009      Aug 2009
In the non-relativistic theory of gravity recently proposed by Horava, the Hamiltonian constraint is not satisfied locally at each point in space. The absence of the local Hamiltonian constraint allows the system to have an extra dark-matter-like ...
Kazuhiro Yamamoto, Tsutomu Kobayashi, Gen Nakamura
Phys.Rev.D80:063514,2009      Jul 2009
We study the spectral tilt of primordial perturbations in Horava-Lifshitz cosmology. The uniform approximation, which is a generalization of the familiar Wentzel-Kramers-Brillouin (WKB) method, is employed to compute the spectral index both numeri...
Yuko Urakawa, Tsutomu Kobayashi
JCAP 1007:027,2010      Jul 2009
We explore the possibility that the dark matter and dark energy are mimicked by a single fluid or by a single -essence-like scalar field. The so called Chaplygin gas unified dark matter models can reproduce the observed matter power spectrum by...
Tsutomu Kobayashi, Hiroyuki Tashiro
Mar 2009
We investigate the effect of modified gravity on cluster abundance and the Sunyaev-Zel'dovich angular power spectrum. Our modified gravity is based on a phenomenological extension of the Dvali-Gabadadze-Porrati model which includes two free parame...
Tsutomu Kobayashi, Shuichiro Yokoyama
JCAP 0905:004,2009      Mar 2009
Recently it was shown that an inflationary background can be realized by any -form field non-minimally coupled to gravity. In this paper, we study gravitational waves generated during p-form inflation. Even though the background evolution is id...
Tsutomu Kobayashi, Kei-ichi Maeda
Phys.Rev.D79:024009,2009      Oct 2008
Although modified gravity models can be made to satisfy solar system and cosmological constraints, it has been shown that they have the serious drawback of the nonexistence of stars with strong gravitational fields. In this paper, we discus...
Tsutomu Kobayashi, Kei-ichi Maeda
Phys.Rev.D78:064019,2008      Jul 2008
Several f(R) modified gravity models have been proposed which realize the correct cosmological evolution and satisfy solar system and laboratory tests. Although nonrelativistic stellar configurations can be constructed, we argue that relativistic ...
Tsutomu Kobayashi
Phys.Rev.D78:084018,2008      Jun 2008
In the context of six-dimensional conical braneworlds we consider a simple and explicit model that incorporates long distance modification of gravity and regularization of codimension-2 singularities. To resolve the conical singularities we replac...
Satoshi Maeda, Satoshi Kitagawa, Tsutomu Kobayashi, Tetsuya Shiromizu
Class.Quant.Grav.26:135014,2009      May 2008
We explore the possibility of generating large-scale magnetic fields from second-order cosmological perturbations during the pre-recombination era. The key process for this is Thomson scattering between the photons and the charged particles within...
Masato Nozawa, Tsutomu Kobayashi
Phys.Rev.D78:064006,2008      Mar 2008
We investigate conformal scalar, electromagnetic, and massless Dirac quasinormal modes of a brane-localized black hole. The background solution is the four-dimensional black hole on a 2-brane that has been constructed by Emparan, Horowitz, and Mye...
Tsutomu Kobayashi, Tetsuya Shiromizu, Claudia de Rham
Phys.Rev.D77:124012,2008      Feb 2008
We study the effective gravitational theory on a brane in a six-dimensional Einstein-Maxwell model of flux compactification, regularizing a conical defect as a codimension-one brane. We employ the gradient expansion technique valid at low energies...
Tsutomu Kobayashi, Masato Nozawa, Yu-ichi Takamizu
Phys.Rev.D77:044022,2008      Nov 2007
We study the emission of scalar fields into the bulk from a six-dimensional rotating black hole pierced by a 3-brane. We determine the angular eigenvalues in the presence of finite brane tension by using the continued fraction method. The radial e...
Frederico Arroja, Tsutomu Kobayashi, Kazuya Koyama, Tetsuya Shiromizu
JCAP 0712:006,2007      Oct 2007
We derive the low energy effective theory on a brane in six-dimensional chiral supergravity. The conical 3-brane singularities are resolved by introducing cylindrical codimension one 4-branes whose interiors are capped by a regular spacetime. The ...
Shunsuke Fujii, Tsutomu Kobayashi, Tetsuya Shiromizu
Phys.Rev.D76:104052,2007      Aug 2007
Conical brane singularities in six-dimensional flux compactification models can be resolved by introducing cylindrical codimension-one branes with regular caps instead of 3-branes (a la Kaluza-Klein braneworlds with fluxes). In this paper, we cons...
Tsutomu Kobayashi, Yu-ichi Takamizu
Class.Quant.Grav.25:015007,2007      Jul 2007
We consider a six-dimensional axisymmetric Einstein-Maxwell model of warped braneworlds. The bulk is bounded by two branes, one of which is a conical 3-brane and the other is a 4-brane wrapped around the axis of symmetry. The latter brane is assum...
Tsutomu Kobayashi, Masato Minamitsuji
JCAP 0707:016,2007      May 2007
We study cosmology on a conical brane in the six-dimensional Einstein-Maxwell-dilaton system, where the extra dimensions are compactified by a magnetic flux. We systematically construct exact cosmological solutions using the fact that the system i...
Tsutomu Kobayashi, Masato Minamitsuji
Phys.Rev.D75:104013,2007      Mar 2007
We study linearized gravity in a six-dimensional Einstein-Maxwell model of warped braneworlds, where the extra dimensions are compactified by a magnetic flux. It is difficult to construct a strict codimension two braneworld with matter sources oth...
Tsutomu Kobayashi, Roy Maartens, Tetsuya Shiromizu, Keitaro Takahashi
Phys.Rev.D75:103501,2007      Jan 2007
In the standard cosmological model, magnetic fields and vorticity are generated during the radiation era via second-order density perturbations. In order to clarify the complicated physics of this second-order magnetogenesis, we use a covariant ap...
Tsutomu Kobayashi, Masato Minamitsuji
JCAP0612:008,2006      Oct 2006
We study scalar cosmological perturbations in a braneworld model with a bulk Gauss-Bonnet term. For an anti-de Sitter bulk, the five-dimensional perturbation equations share the same form as in the Randall-Sundrum model, which allows us to obtain ...
Tsutomu Kobayashi, Tetsuya Shiromizu, Nathalie Deruelle
Phys.Rev.D74:104031,2006      Aug 2006
We present effective gravitational equations at low energies in a -symmetric braneworld with the Gauss-Bonnet term. Our derivation is based on the geometrical projection approach, and we solve iteratively the bulk geometry using the gradient ...
Tsutomu Kobayashi
Class.Quant.Grav.23:4847-4851,2006      Jun 2006
The braneworld model of Dvali-Gabadadze-Porrati (DGP) provides an interesting alternative to a positive cosmological constant by modifying gravity at large distances. We investigate the asymptotic behavior of homogeneous and anisotropic cosmologie...
Yukinori Iwashita, Tsutomu Kobayashi, Tetsuya Shiromizu, Hirotaka Yoshino
Phys.Rev. D74 (2006) 064027      Jun 2006
We study the holographic representation of the entanglement entropy, recently proposed by Ryu and Takayanagi, in a braneworld context. The holographic entanglement entropy of a de Sitter brane embedded in an anti-de Sitter (AdS) spacetime is evalu...
Tsutomu Kobayashi
Phys.Rev. D73 (2006) 124031      Feb 2006
We study the spectrum of gravitational waves generated from inflation in the Randall-Sundrum braneworld. Since the inflationary gravitational waves are of quantum-mechanical origin, the initial configuration of perturbations in the bulk includes K...
Tsutomu Kobayashi, Takahiro Tanaka
Phys.Rev. D73 (2006) 044005      Nov 2005
We study the generation and evolution of gravitational waves (tensor perturbations) in the context of Randall-Sundrum braneworld cosmology. We assume that the initial and final stages of the background cosmological model are given by de Sitter and...
Tsutomu Kobayashi, Takahiro Tanaka
Phys.Rev. D71 (2005) 124028      May 2005
We study the quantum-mechanical generation of gravitational waves during inflation on a brane embedded in a five-dimensional anti-de Sitter bulk. To make the problem well-posed, we consider the setup in which both initial and final phases are give...
Tsutomu Kobayashi
Gen.Rel.Grav. 37 (2005) 1869-1876      Apr 2005
We consider a Vaidya-type radiating spacetime in Einstein gravity with the Gauss-Bonnet combination of quadratic curvature terms. Simply generalizing the known static black hole solutions in Einstein-Gauss-Bonnet gravity, we present an exact solut...
Tsutomu Kobayashi, Takahiro Tanaka
Mar 2005
Tensor perturbations in an expanding braneworld of the Randall Sundrum type are investigated. We consider a model composed of a slow-roll inflation phase and the succeeding radiation phase. The effect of the presence of an extra dimension through ...
Tsutomu Kobayashi, Takahiro Tanaka
Phys.Rev. D71 (2005) 084005      Dec 2004
We consider black-string-type solutions in five-dimensional Einstein-Gauss-Bonnet gravity. Numerically constructed solutions under static, axially symmetric and translationally invariant metric ansatz are presented. The solutions are specified by ...
Tsutomu Kobayashi, Takahiro Tanaka
JCAP 0410 (2004) 015      Aug 2004
Tensor type perturbations in the expanding brane world of the Randall Sundrum type are investigated. We consider a model composed of slow-roll inflation phase and the succeeding radiation phase. The effect of the presence of an extra dimension thr...
Tsutomu Kobayashi, Takahiro Tanaka
Phys.Rev. D69 (2004) 064037      Nov 2003
We introduce a -dimensional vacuum description of five-dimensional bulk inflaton models with exponential potentials that makes analysis of cosmological perturbations simple and transparent. We show that various solutions, including the powe...
Tsutomu Kobayashi, Hideaki Kudoh, Takahiro Tanaka
Phys.Rev. D68 (2003) 044025      May 2003
We study primordial gravitational waves from inflation in Randall-Sundrum braneworld model. The effect of small change of the Hubble parameter during inflation is investigated using a toy model given by connecting two de Sitter branes. We analyze ...