研究者詳細 - 泉　圭介

論文 - 泉　圭介

分割表示 89 件中 61 - 80 件目／全件表示 >>

Tensor perturbations from brane-world inflation with curvature effects 査読有り国際誌 Open Access

Bouhmadi-López, M; Liu, YW; Izumi, K; Chen, PS

PHYSICAL REVIEW D 89 巻 ( 6 ) 2014年3月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D - Particles, Fields, Gravitation and Cosmology

The brane-world scenario provides an intriguing possibility to explore the phenomenological cosmology implied by string/M theory. In this paper, we consider a modified Randall-Sundrum single brane model with two natural generalizations: a Gauss-Bonnet term in the five-dimensional bulk action as well as an induced gravity term in the four-dimensional brane action, which are the leading-order corrections to the Randall-Sundrum model. We study the influence of these combined effects on the evolution of the primordial gravitational waves generated during an extreme slow-roll inflation on the brane. The background, for the early inflationary era, is then modeled through a de Sitter brane embedded within an anti-de Sitter bulk. In this framework, we show that both effects tend to suppress the Randall-Sundrum enhancement of the amplitude of the tensor perturbations at relatively high energies. Moreover, the Gauss-Bonnet effect, relative to standard general relativity, will abruptly enhance the tensor-to-scalar ratio and break the standard consistency relation at high energies, which cannot be evaded by invoking the induced gravity effect, even though the induced gravity strength would mildly counterbalance these significant changes at high energies. We note that the brane-world model with or without the induced gravity effect fulfills the consistency relation. Finally, we discuss some implications of the observational constraints. © 2014 American Physical Society.

DOI： 10.1103/PhysRevD.89.063501

その他リンク： http://arxiv.org/pdf/1308.5765v2

The relation between tree unitarity and renormalizability in lifshitz scalar theory

Kitamura T., Inami T., Izumi K.

JGRG 2014 - Proceedings of the 24th Workshop on General Relativity and Gravitation in Japan 2 巻頁： 342 - 350 2014年

出版者・発行元：JGRG 2014 - Proceedings of the 24th Workshop on General Relativity and Gravitation in Japan

checking the power of the high energy limit in Lifshitz scalar 1. the power is independent on the way to take the high-energy limit (e.g) we can take the high energy limit ω,k1,k2,and,k3,or,the combination of them O.K. 2. We almostly confirmed the equivalence between renormalizability and tree unitarity in Lifshitz scalar theory We can use the equivalence for checking renormalizability of Horava gravity!!.

Natural emergence of cosmological constant and dark radiation from the Stephenson-Kilmister-Yang- Camenzind theory of gravity 査読有り国際誌 Open Access

Chen, P; Izumi, K; Tung, NE

PHYSICAL REVIEW D 88 巻 ( 12 ) 2013年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D - Particles, Fields, Gravitation and Cosmology

We show that the Stephenson-Kilmister-Yang (SKY) equation combined with Camenzind's matter current term naturally provides the cosmological constant and dark radiation as integration constants of the Stephenson-Kilmister-Yang- Camenzind (SKYC) field equation. To characterize the property of the dark radiation, we develop a method to separate it from the ordinary radiation. We find a special property of Camenzind's matter current, namely that the solution space for radiation in fact belongs to that of the vacuum solution of the SKY equation. We also find that this matter current does not obey the conservation condition suggested by Kilmister. Finally, we discuss the possible role of dark radiation emergent from the SKYC theory in recent cosmic microwave background observations and its implications to the inflation scenario. © 2013 American Physical Society.

DOI： 10.1103/PhysRevD.88.123006

その他リンク： http://arxiv.org/pdf/1304.6334v1

Superluminal Propagation and Acausality of Nonlinear Massive Gravity 査読有り

S. Deser, K. Izumi, Y. C. Ong, A. Waldron

2013年12月

Massive gravity acausality redux 査読有り国際誌 Open Access

Deser, S; Izumi, K; Ong, YC; Waldron, A

PHYSICS LETTERS B 726 巻 ( 1-3 ) 頁： 544 - 548 2013年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics

Massive gravity (mGR) is a 5(= 2. s+. 1) degree of freedom, finite range extension of GR. However, amongst other problems, it is plagued by superluminal propagation, first uncovered via a second order shock analysis. First order mGR shock structures have also been studied, but the existence of superluminal propagation in that context was left open. We present here a concordance of these methods, by an explicit (first order) characteristic matrix computation, which confirms mGR's superluminal propagation as well as acausality. © 2013 Elsevier B.V.

DOI： 10.1016/j.physletb.2013.09.001

An analysis of characteristics in nonlinear massive gravity 査読有り国際誌 Open Access

Izumi, K; Ong, YC

CLASSICAL AND QUANTUM GRAVITY 30 巻 ( 18 ) 2013年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Classical and Quantum Gravity

We study the Cauchy problem in a special case of nonlinear massive gravity. Despite being ghost free, it has recently been argued that the theory is inherently problematic due to the existence of superluminal shock waves. Furthermore, it is claimed that an acausal characteristic can arise for any choice of background. In order to further understand the causal structure of the theory, we carefully perform a detailed analysis of the characteristic equations and show that the theory does admit a well-posed Cauchy problem, i.e., there exists hypersurfaces that are not a characteristic hypersurface. Puzzles remain regarding the existence of a superluminal propagating mode in both the minimal ghost-free theory that we analyzed, as well as in the full nonlinear massive gravity. That is, our result should not be taken as any indication of the healthiness of the theory. We also give a detailed review of Cauchy-Kovalevskaya theorem and its application in the appendix, which should be useful for investigating causal structures of other theories of gravity. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0264-9381/30/18/184008

その他リンク： http://arxiv.org/pdf/1304.0211v2

Problems with propagation and time evolution in <i>f</i>(<i>T</i>) gravity 査読有り国際誌 Open Access

Ong, YC; Izumi, K; Nester, JM; Chen, PS

PHYSICAL REVIEW D 88 巻 ( 2 ) 2013年7月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D - Particles, Fields, Gravitation and Cosmology

Teleparallel theories of gravity have a long history. They include a special case referred to as the teleparallel equivalent of general relativity (TEGR, also known as GRâ̂¥). Recently this theory has been generalized to f(T) gravity. Tight constraints from observations suggest that f(T) gravity is not as robust as initially hoped. This might hint at hitherto undiscovered problems at the theoretical level. In this work, we point out that a generic f(T) theory can be expected to have certain problems including superluminal propagating modes, the presence of which can be revealed by using the characteristic equations that govern the dynamics in f(T) gravity and/or the Hamiltonian structure of the theory via Dirac constraint analysis. We use several examples from simpler gauge field theories to explain how such superluminal modes could arise. We also point out problems with the Cauchy development of a constant time hypersurface in Friedmann-Lemaître- Roberson-Walker (FLRW) spacetime in f(T) gravity. The time evolution from a FLRW (and as a special case, Minkowski spacetime) initial condition is not unique. © 2013 American Physical Society.

DOI： 10.1103/PhysRevD.88.024019

その他リンク： http://arxiv.org/pdf/1303.0993v2

Cosmological perturbation in <i>f</i>(<i>T</i>) gravity revisited 査読有り国際誌 Open Access

Izumi, K; Ong, YC

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS 2013 巻 ( 6 ) 2013年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Cosmology and Astroparticle Physics

We perform detailed investigation of cosmological perturbations in f(T) theory of gravity coupled with scalar field. Our work emphasizes on the way to gauge fix the theory and we examine all possible modes of perturbations up to second order. The analysis includes pseudoscalar and pseudovector modes in addition to the usual scalar, vector, and tensor modes. We find no gravitational propagating degree of freedom in the scalar, pseudoscalar, vector, as well as pseudovector modes. In addition, we find that the scalar and tensor perturbations have exactly the same form as their counterparts in usual general relativity with scalar field, except that the factor of reduced Planck mass squared Mpl2 = 1/(8πG) that occurs in the latter has now been replaced by an effective time-dependent gravitational coupling - 2(df/dT)\T=T0, with T0 being the background torsion scalar. The absence of extra degrees of freedom of f(T) gravity at second order linear perturbation indicates that f(T) gravity is highly nonlinear. Consequently one cannot conclusively analyze stability of the theory without performing nonlinear analysis that can reveal the propagation of the extra degrees of freedom. © 2013 IOP Publishing Ltd and Sissa Medialab srl.

DOI： 10.1088/1475-7516/2013/06/029

その他リンク： http://arxiv.org/pdf/1212.5774v2

Spherically symmetric analysis on open FLRW solution in non-linear massive gravity 査読有り国際誌 Open Access

Chiang, CI; Izumi, K; Chen, PS

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS 2012 巻 ( 12 ) 2012年12月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Cosmology and Astroparticle Physics

We study non-linear massive gravity in the spherically symmetric context. Our main motivation is to investigate the effect of helicity-0 mode which remains elusive after analysis of cosmological perturbation around an open Friedmann-Lemaitre-Robertson-Walker (FLRW) universe. The non-linear form of the effective energy-momentum tensor stemming from the mass term is derived for the spherically symmetric case. Only in the special case where the area of the two sphere is not deviated away from the FLRW universe, the effective energy momentum tensor becomes completely the same as that of cosmological constant. This opens a window for discriminating the non-linear massive gravity from general relativity (GR). Indeed, by further solving these spherically symmetric gravitational equations of motion in vacuum to the linear order, we obtain a solution which has an arbitrary time-dependent parameter. In GR, this parameter is a constant and corresponds to the mass of a star. Our result means that Birkhoff's theorem no longer holds in the non-linear massive gravity and suggests that energy can probably be emitted superluminously (with infinite speed) on the self-accelerating background by the helicity-0 mode, which could be a potential plague of this theory. © 2012 IOP Publishing Ltd and Sissa Medialab srl.

DOI： 10.1088/1475-7516/2012/12/025

その他リンク： http://arxiv.org/pdf/1208.1222v2

Trispectrum estimation in various models of equilateral type non-Gaussianity 査読有り国際誌 Open Access

Izumi, K; Mizuno, S; Koyama, K

PHYSICAL REVIEW D 85 巻 ( 2 ) 2012年1月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D - Particles, Fields, Gravitation and Cosmology

We calculate the shape correlations between trispectra in various equilateral non-Gaussian models, including Dirac-Born-Infeld inflation, ghost inflation and Lifshitz scalars, using the full trispectrum as well as the reduced trispectrum. We find that most theoretical models are distinguishable from the shapes of primordial trispectra except for several exceptions where it is difficult to discriminate between the models, such as single field Dirac-Born-Infeld inflation and a Lifshitz scalar model. We introduce an estimator for the amplitude of the trispectrum, gNLequil and relate it to model parameters in various models. Using constraints on gNLequil from WMAP5, we give constraints on the model parameters. © 2012 American Physical Society.

DOI： 10.1103/PhysRevD.85.023521

その他リンク： http://arxiv.org/pdf/1109.3746v3

Nonlinear superhorizon perturbations in Horava-Lifshitz gravity 査読有り国際誌

Izumi, K; Mukohyama, S

PHYSICAL REVIEW D 84 巻 ( 6 ) 2011年9月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D - Particles, Fields, Gravitation and Cosmology

We perform a fully nonlinear analysis of superhorizon perturbation in Hořava-Lifshitz gravity, based on the gradient expansion method. We present a concrete expression for the solution of gravity equations up to the second order in the gradient expansion, and prove that the solution can be extended to any order. The result provides yet another example for analogue of the Vainshtein effect: the nonlinear solution is regular in the limit λ→1 and recovers general relativity coupled to dark matter at low energy. Finally, we propose a definition of nonlinear curvature perturbation R in Hořava-Lifshitz gravity and show that it is conserved up to the first order in the gradient expansion. © 2011 American Physical Society.

DOI： 10.1103/PhysRevD.84.064025

その他リンク： http://arxiv.org/pdf/1105.0246v2

Exact Solutions II: Systematic Solution-Generation of Five-Dimensional Black Holes 査読有り国際誌 Open Access

Iguchi, H; Izumi, K; Mishima, T

PROGRESS OF THEORETICAL PHYSICS SUPPLEMENT ( 189 ) 頁： 93 - 125 2011年

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Progress of Theoretical Physics Supplement

Solitonic solution-generating methods are powerful tools to construct nontrivial black hole solutions of the higher-dimensional Einstein equations systematically. In five dimensions particularly, the solitonic methods can be successfully applied to the construction of asymptotically Minkowski spacetimes with multiple horizons. We review the solitonic methods applicable to higher-dimensional vacuum spacetimes and present some five-dimensional examples derived from the methods.

DOI： 10.1143/PTPS.189.93

その他リンク： http://arxiv.org/pdf/1106.0387v3

Non-gaussianity from lifshitz scalar

Izumi K., Kobayashi T., Mukohyama S.

Proceedings of the 20th Workshop on General Relativity and Gravitation in Japan, JGRG 2010 頁： 193 - 196 2010年12月

出版者・発行元：Proceedings of the 20th Workshop on General Relativity and Gravitation in Japan, JGRG 2010

A Lifshitz scalar with the dynamical critical exponent z =3 obtains scale-invariant, super-horizon field fluctuations without the need of an inflationary era. Since this mechanism is due to the special scaling of the Lifshitz scalar and persists in the presence of unsuppressed self-couplings, the resulting fluctuation spectrum can deviate from a Gaussian distribution. We study the non-Gaussian nature of the Lifshitz scalar's intrinsic field fluctuations, and show that primordial curvature perturbations sourced from such field fluctuations can have large non-Gaussianity of order fNL = O(100), which will be detected by upcoming CMB observations. We compute the bispectrum and trispectrum of the fluctuations, and discuss their configurations in momentum space. In particular, the bispectrum is found to take various shapes, including the local, equilateral, and orthogonal shapes. Intriguingly, all integrals in the in-in formalism can be performed analytically.

Non-Gaussianity from Lifshitz scalar 査読有り国際誌

Izumi, K; Kobayashi, T; Mukohyama, S

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS 2010 巻 ( 10 ) 2010年10月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Cosmology and Astroparticle Physics

A Lifshitz scalar with the dynamical critical exponent z = 3 obtains scale-invariant, super-horizon field fluctuations without the need of an inflationary era. Since this mechanism is due to the special scaling of the Lifshitz scalar and persists in the presence of unsuppressed self-couplings, the resulting fluctuation spectrum can deviate from a Gaussian distribution. We study the non-Gaussian nature of the Lifshitz scalar's intrinsic field fluctuations, and show that primordial curvature perturbations sourced from such field fluctuations can have large non-Gaussianity of order fNL = O(100), which will be detected by upcoming CMB observations. We compute the bispectrum and trispectrum of the fluctuations, and discuss their configurations in momentum space. In particular, the bispectrum is found to take various shapes, including the local, equilateral, and orthogonal shapes. Intriguingly, all integrals in the in-in formalism can be performed analytically. © 2010 IOP Publishing Ltd and SISSA.

DOI： 10.1088/1475-7516/2010/10/031

その他リンク： http://arxiv.org/pdf/1008.1406v2

Trispectrum from ghost inflation 査読有り国際誌 Open Access

Izumi, K; Mukohyama, S

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS 2010 巻 ( 6 ) 2010年6月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of Cosmology and Astroparticle Physics

Ghost inflation predicts almost scale-invariant primordial cosmological perturbations with relatively large non-Gaussianity. The bispectrum is known to have a large contribution at the wavenumbers forming an equilateral triangle and the corresponding nonlinear parameter fNLequil is typically of order O(102). In this paper we calculate trispectrum from ghost inflation and show that the corresponding nonlinear parameter τNL is typically of order O(104). We investigate the shape dependence of the trispectrum and see that it has some features different from DBI inflation. Therefore, our result may be useful as a template to distinguish ghost inflation from other models of inflation by future experiments. © 2010 IOP Publishing Ltd and SISSA.

DOI： 10.1088/1475-7516/2010/06/016

その他リンク： http://arxiv.org/pdf/1004.1776v1

Stellar center is dynamical in Horava-Lifshitz gravity 査読有り国際誌 Open Access

Izumi, K; Mukohyama, S

PHYSICAL REVIEW D 81 巻 ( 4 ) 2010年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D - Particles, Fields, Gravitation and Cosmology

In Hořava-Lifshitz gravity, regularity of a solution requires smoothness of not only the spacetime geometry but also the foliation. As a result, the regularity condition at the center of a star is more restrictive than in general relativity. Assuming that the energy density is a piecewise-continuous, non-negative function of the pressure and that the pressure at the center is positive, we prove that the momentum conservation law is incompatible with the regularity at the center for any spherically symmetric, static configurations. The proof is totally insensitive to the structure of higher spatial curvature terms and, thus, holds for any values of the dynamical critical exponent z. Therefore, we conclude that a spherically symmetric star should include a time-dependent region near the center. This supports the picture that Hořava-Lifshitz gravity does not recover general relativity at low energy but can instead mimic general relativity plus cold dark matter: the cold dark matter accretes toward a star and thus makes the stellar center dynamical. We also comment on the condition under which linear instability of the scalar graviton does not show up. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevD.81.044008

その他リンク： http://arxiv.org/pdf/0911.1814v2

No de Sitter Invariant Vacuum in Massive Gravity Theory with Ghost 査読有り国際誌 Open Access

Izumi, K; Tanaka, T

PROGRESS OF THEORETICAL PHYSICS 121 巻 ( 2 ) 頁： 419 - 426 2009年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Progress of Theoretical Physics

In this paper, we point out that the massive gravity theory with a graviton ghost mode in the de Sitter background cannot possess a de Sitter invariant vacuum state. In order to avoid a negative norm state, we must associate the creation operator of the ghost mode with a negative-energy mode function instead of a positive-energy one as the mode function. Namely, we have to adopt a different procedure of quantization for a ghost. When a theory has a symmetry mixing a ghost mode with ordinary nonghost modes, the choice of a ghost mode is not unique. However, the quantization of a ghost is impossible without specifying a choice of ghost mode, which breaks the symmetry. For this reason, the vacuum state cannot respect the symmetry. In the massive gravity theory with a graviton ghost mode in the de Sitter background, the ghost is the helicity-0 mode of the graviton. This ghost mode is mixed with the other helicity graviton modes under the action of de Sitter symmetry. Therefore, there is no de Sitter invariant vacuum in such models. This leads to an interesting possibility that the non-covariant cutoff of the low-energy effective theory may naturally arise. As a result, the instability due to the pair production of a ghost and normal non-ghost particles gets much milder and that the model may escape from being rejected.

DOI： 10.1143/PTP.121.419

その他リンク： http://arxiv.org/pdf/0709.0199v2

Particle Production in Models with Helicity-0 Graviton Ghost in de Sitter Spacetime 査読有り国際誌 Open Access

Izumi, K; Tanaka, T

PROGRESS OF THEORETICAL PHYSICS 121 巻 ( 2 ) 頁： 427 - 436 2009年2月

記述言語：英語掲載種別：研究論文（学術雑誌）出版者・発行元：Progress of Theoretical Physics

We revisit the problem of the helicity-0 ghost mode of a massive graviton in the de Sitter background. In general, the presence of a ghost particle, which has negative energy, drives the vacuum to be unstable through the pair production of ghost particles and ordinary particles. In the case that the vacuum state preserves the de Sitter invariance, the number density created by the pair production inevitably diverges owing to unsuppressed ultraviolet (UV) contributions. In such cases, one can immediately conclude that the model is not viable. However, in the massive gravity theory, we cannot construct a vacuum state that respects the de Sitter invariance. Therefore, the presence of a ghost does not immediately mean the breakdown of the model. Explicitly estimating the number density and the energy density of particles created by the pair production of two conformal scalar particles and one helicity-0 ghost graviton, we find that these densities both diverge. However, since models with a helicity-0 ghost graviton have no de Sitter invariant vacuum state, it is rather natural to consider a UV cutoff scale in the three-dimensional momentum space. Then, even if we take a cutoff scale as large as the Planck scale, the created number density and energy density are well suppressed. In many models, the cutoff scale is smaller than the Planck scale. In such models, the created number density and energy density are negligible as long as only the physics below the cutoff scale is concerned.

DOI： 10.1143/PTP.121.427

その他リンク： http://arxiv.org/pdf/0810.4811v1

No static star solution in horava gravity Open Access

Izumi K., Mukohyama S.

Proceedings of the 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009 頁： 165 - 168 2009年

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）出版者・発行元：Proceedings of the 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009

In Hořava-Lifshitz gravity, regularity of a solution requires smoothness of not only the spacetime geometry but also the foliation. As a result, the regularity condition at the center of a star is more restrictive than in general relativity. Assuming that the energy density is a piecewise-continuous, non-negative function of the pressure and that the pressure at the center is positive, we prove that the momentum conservation law is incompatible with the regularity at the center for any spherically-symmetric, static configurations. Therefore, we conclude that a spherically-symmetric star should include a time-dependent region near the center. This supports the picture that Hořava-Lifshitz gravity does not recover general relativity at low energy but can instead mimic general relativity plus cold dark matter.

Massive spin-2 ghost in de sitter spacetime Open Access

Izumi K., Koyama K., Pujolas O., Tanaka T.

Proceedings of the 18th Workshop on General Relativity and Gravitation in Japan, JGRG 2008 2008年12月

出版者・発行元：Proceedings of the 18th Workshop on General Relativity and Gravitation in Japan, JGRG 2008

We study the ghost of massive graviton in cle Sitter space. In cle Sitter space with Hubble constant H. if a mass of a spin-2 mode m satisfies m 2 < 2H2. the helicity-() mode becomes ghost. In order to explain the dark energy problem by a modification of gravity theory, the low energy effective theory becomes massive gravity theory. Then, the mass of the graviton naively becomes the present. Hubble parameter and the ghost mode appears. First, we attempt to construct a ghost-free model by a modification of DGP braneworld model. We study the possibility of avoiding the appearance of the ghost by slightly modifying the model via the introduction of a second brane. By changing the separation between the branes. we find that we can erase the spin-2 ghost. However, this can be done only at the expense of the appearance of a spin-0 ghost instead. Secondly, we revisit the ghost problem. In general, the presence of a ghost particle, which has negative energy, drives the vacuum to be unstable through pair production of ghost particles and ordinary particles. Explicitly estimating the number density and the energy density of particles created by the pair production of two eonformal scalar particles and one hclicity-0 ghost graviton, we find that these densities both diverge. However, since1 we show that models with helieity-0 ghost graviton have no de Sitter invariant vacuum state, it is rather natural to consider a UV cutoff scale in the three-dimensional momentum space. Then, even if wv take1 the cutoff scale as large as the Planck scale, the created number density and energy density are well suppressed. In many models the cutoff scale is smaller than the Planck scale. In such models the created number density and the energy density are negligibly small as long as only the physics below the cutoff scale is concerned.

　前のページ - 次のページ　

1
2
3
4
5