研究者詳細 - 泉　圭介

論文 - 泉　圭介

分割表示 >> ／全件表示 89 件中 1 - 89 件目

Attractive gravity probe surface in the Einstein-Maxwell system Open Access

Lee K., Izumi K., Shiromizu T., Yoshino H., Tomikawa Y.

Physical Review D 110 巻 ( 12 ) 2024年12月

出版者・発行元：Physical Review D

We derive areal inequalities for five types of attractive gravity probe surfaces, which were proposed by us in order to characterize the strength of gravity in different ways including weak gravity region, taking into account of contributions of electric and magnetic charges, angular momentum, gravitational waves, and matters. These inequalities are generalizations of the Riemannian Penrose inequality for minimal surfaces and lead to the concept of extremality for a given surface whose condition is given in terms of the gravitational mass and the electromagnetic charges. This means that the extremality is a characteristic property not only of black hole horizons or minimal surfaces but also of surfaces in weak gravity region. We also derive areal inequalities and extremality conditions for surfaces in asymptotically locally anti-de Sitter spacetimes.

DOI： 10.1103/PhysRevD.110.124051

Loosely Trapped Surface for Slowly Rotating Black Hole Open Access

Izumi K., Shiromizu T., Yoshida D., Tomikawa Y., Yoshino H.

Progress of Theoretical and Experimental Physics 2024 巻 ( 11 ) 2024年11月

出版者・発行元：Progress of Theoretical and Experimental Physics

We construct the marginal loosely trapped surface (marginal LTS) for the Kerr spacetime with a small Kerr parameter perturbatively, where the LTS condition is saturated. An LTS is a surface that specifies the strong gravity region, which is a generalization of the photon sphere in the Schwarzschild spacetime. It turns out that there are an infinite number of marginal LTSs. At the leading order of the small Kerr parameter, all of the marginal LTSs have the same area. However, one can see that the maximal marginal LTS among them is uniquely determined at the higher order.

DOI： 10.1093/ptep/ptae158

Generalization of conformal-disformal transformations of the metric in scalar-tensor theories 査読有り

Babichev E., Izumi K., Noui K., Tanahashi N., Yamaguchi M.

Physical Review D 110 巻 ( 6 ) 頁： 064063 2024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

We study new classes of metric transformations in the context of scalar-tensor theories, which involve both higher derivatives of the scalar field and derivatives of the metric itself. In general, such transformations are not invertible as they involve derivatives of the metric, which typically leads to instability due to Ostrogradsky ghosts. We show, however, that a certain class of this type of transformations is invertible: We construct new examples of invertible conformal (and also disformal) transformations with higher derivatives. Finally, we make use of these new transformations to construct extended mimetic theories of gravity, and we study their properties in the context of cosmology.

DOI： 10.1103/PhysRevD.110.064063

Gedanken experiments to destroy a black hole by a test particle: Multiply charged black hole with higher derivative corrections Open Access

Izumi K., Noumi T., Yoshida D.

Physical Review D 110 巻 ( 4 ) 2024年8月

出版者・発行元：Physical Review D

We investigate a gedanken experiment to destroy an extremally charged black hole by dropping a test particle, provided that there are multiple U(1) gauge fields coupled with each other through higher derivative interactions. It is known that the Coulomb repulsion prevents a test particle that would break the extremal condition from falling into an extremal black hole and therefore the black hole cannot be destroyed. We extend this observation to include higher derivative corrections. Although the extremal condition is modified by the higher derivative interactions, we find that the repulsive force induced by the higher derivative couplings is responsible for preventing a test particle that would break the modified extremal condition to reach the event horizon. Thus, we confirm that the weak cosmic censorship conjecture holds for extremally charged black holes even in the presence of higher derivative corrections, as long as the test particle approximation is justified.

DOI： 10.1103/PhysRevD.110.044008

A generalization of photon sphere based on escape/capture cone Open Access

Amo M., Izumi K., Yoshino H., Tomikawa Y., Shiromizu T.

European Physical Journal C 84 巻 ( 6 ) 2024年6月

出版者・発行元：European Physical Journal C

In general asymptotically flat spacetimes, bearing the null geodesics reaching the future null infinity in mind, we propose new concepts, the “dark horizons” (outer dark horizon and inner dark horizon) as generalizations of the photon sphere. They are defined in terms of the structure of escape/capture cones of photons with respect to a unit timelike vector field to capture the motion of light sources. More specifically, considering a two-sphere that represents a set of emission directions of photons, the dark horizons are located at positions where a hemisphere is marginally included in the capture and escape cones, respectively. In addition, our definition succeeds in incorporating relativistic beaming effect. We show that the dark horizon is absent in the Minkowski spacetime, while they exist in spacetimes with black hole(s) under a certain condition. We derive the general properties of the dark horizons in spherically symmetric spacetimes and explicitly calculate the locations of the dark horizons in the Vaidya spacetime and the Kerr spacetime. In particular, in the Kerr spacetime, the outer dark horizon coincides with the shadow observed from infinity on the rotation axis.

DOI： 10.1140/epjc/s10052-024-12954-9

Attractive gravity probe surface, positivity of quasi-local mass, and Arnowitt-Deser-Misner mass expression Open Access

Shiromizu, T; Izumi, K

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2024 巻 ( 1 ) 2024年1月

Attractive gravity probe surface with positive cosmological constant Open Access

Shiromizu, T; Izumi, K

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2023 巻 ( 10 ) 2023年10月

出版者・発行元：Progress of Theoretical and Experimental Physics

In 4D spacetimes with a positi v e cosmological constant, we introduce a new geometrical object associated with the cosmological horizon and then show the areal inequality. We also e xamine the attracti v e gravity probe surfaces as an indicator for the existence of attractive gravity.

DOI： 10.1093/ptep/ptad119

Boundary conditions for constraint systems in variational principle Open Access

Izumi, K; Shimada, K; Tomonari, K; Yamaguchi, M

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2023 巻 ( 10 ) 2023年10月

出版者・発行元：Progress of Theoretical and Experimental Physics

We show the well-posed variational principle in constraint systems. In a naive procedure of the variational principle with constraints, the proper number of boundary conditions does not match that of physical degrees of freedom, which implies that, even in theories with up to first-order derivatives, the minimal (or extremal) value of the action with the boundary terms is not a solution of the equation of motion in the Dirac procedure of constrained systems. We propose specific and concrete steps to solve this problem. These steps utilize the Hamilton formalism, which allows us to separate the physical degrees of freedom from the constraints. This reveals the physical degrees of freedom that are necessary to be fixed on boundaries, and also enables us to specify the variables to be fixed and the surface terms.

DOI： 10.1093/ptep/ptad122

その他リンク： http://arxiv.org/pdf/2303.03805v2

Asymptotic behavior of null geodesics near future null infinity. IV. Null-access theorem for generic asymptotically flat spacetime Open Access

Amo, M; Izumi, K; Tomikawa, Y; Shiromizu, T; Yoshino, H

PHYSICAL REVIEW D 107 巻 ( 12 ) 2023年6月

出版者・発行元：Physical Review D

In our previous papers [Phys. Rev. D 104, 064025 (2021); PRVDAQ2470-001010.1103/PhysRevD.104.064025105, 064074 (2022); PRVDAQ2470-001010.1103/PhysRevD.105.064074106, 084007 (2022)PRVDAQ2470-001010.1103/PhysRevD.106.084007], we analyzed the asymptotic behavior of future directed null geodesics near future null infinity, and then we showed a proposition on the accessibility of the null geodesics to future null infinity in a specific class of asymptotically flat spacetimes. In this paper, we adopt the retarded time of the Bondi coordinate as the parameter for the null geodesics and then see that one can relax the assumptions imposed in our previous studies. As a consequence, we obtain a new null-access theorem for generic asymptotically flat spacetimes.

DOI： 10.1103/PhysRevD.107.124050

その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.107.124050/fulltext

Attractive gravity probe surfaces in higher dimensions Open Access

Izumi K., Tomikawa Y., Shiromizu T., Yoshino H.

Progress of Theoretical and Experimental Physics 2023 巻 ( 4 ) 2023年4月

出版者・発行元：Progress of Theoretical and Experimental Physics

A generalization of the Riemannian Penrose inequality in n-dimensional space (3 ≤ n < 8) is done. We introduce a parameter α () indicating the strength of the gravitational field, and define a refined attractive gravity probe surface (refined AGPS) with α. Then, we show the area inequality for a refined AGPS,, where A is the area of the refined AGPS, ωn - 1 is the area of the standard unit (n - 1)-sphere, G is Newton's gravitational constant, and m is the Arnowitt-Deser-Misner mass. The obtained inequality is applicable not only to surfaces in strong gravity regions such as a minimal surface (corresponding to the limit α → ∞), but also to those in weak gravity existing near infinity (corresponding to the limit).

DOI： 10.1093/ptep/ptad046

High-energy properties of the graviton scattering in quadratic gravity Open Access

Abe, Y; Inami, T; Izumi, K

JOURNAL OF HIGH ENERGY PHYSICS 2023 巻 ( 3 ) 2023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：Journal of High Energy Physics

We obtain the matter-graviton scattering amplitude in the gravitational theory of quadratic curvature, which has Rμν2 term in the action. Unitarity bound is not satisfied because of the existence of negative norm states, while an analog of unitarity bound for S-matrix unitarity holds due to the cancelation among the positive norm states and negative norm ones in the unitarity summation in the optical theorem. The violation of unitarity bound is a counter example of Llewellyn Smith’s conjecture on the relation between tree-level unitarity and renormalizability. We have recently proposed a new conjecture that an analog of the unitarity bound for S-matrix unitarity gives the equivalent conditions to those for renormalizability. We show that the gravitational theory of quadratic curvature is a nontrivial example consistent with our conjecture.

DOI： 10.1007/JHEP03(2023)213

その他リンク： https://link.springer.com/article/10.1007/JHEP03(2023)213/fulltext.html

Asymptotic behavior of null geodesics near future null infinity. III. Photons towards inward directions (vol 106, 084007, 2022) Open Access

Amo, M; Izumi, K; Tomikawa, Y; Yoshino, H; Shiromizu, T

PHYSICAL REVIEW D 107 巻 ( 2 ) 2023年1月

Asymptotic behavior of null geodesics near future null infinity: Significance of gravitational waves (vol 104, 064025, 2021) Open Access

Amo, M; Izumi, K; Tomikawa, Y; Yoshino, H; Shiromizu, T

PHYSICAL REVIEW D 107 巻 ( 2 ) 2023年1月

Maximum size of black holes in our accelerating Universe Open Access

Shiromizu, T; Izumi, K; Lee, K; Soligon, D

PHYSICAL REVIEW D 106 巻 ( 8 ) 2022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：Physical Review D

In accordance with current models of the accelerating Universe as a spacetime with a positive cosmological constant, new results about a cosmological upper bound for the area of stable marginally outer trapped surfaces are found taking into account angular momentum, gravitational waves, and matter. Compared to previous results which take into account only some of the aforementioned variables, the bound is found to be tighter, giving a concrete limit to the size of black holes especially relevant in the early Universe.

DOI： 10.1103/PhysRevD.106.084014

Asymptotic behavior of null geodesics near future null infinity. III. Photons towards inward directions 査読有り

PHYSICAL REVIEW D 106 巻 2022年10月

Asymptotic behavior of null geodesics near future null infinity. III. Photons towards inward directions

Amo, M; Izumi, K; Tomikawa, Y; Yoshino, H; Shiromizu, T

PHYSICAL REVIEW D 106 巻 ( 8 ) 2022年10月

掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D

A new sufficient condition for photons emitted near future null infinity to reach future null infinity is derived by studying null geodesics in the Bondi coordinates in asymptotically flat spacetimes. In our previous works [M. Amo, Phys. Rev. D 104, 064025 (2021).PRVDAQ2470-001010.1103/PhysRevD.104.064025; M. Amo, Phys. Rev. D 105, 064074 (2022).PRVDAQ2470-001010.1103/PhysRevD.105.064074], such a condition was established for photons emitted in outward or tangential directions to constant radial surfaces. This paper improves our previous result by including photons emitted in inward directions. In four dimensions, imposing the same assumptions on the metric functions as previously, we prove that photons reach future null infinity if their initial values of |dr/du| are smaller than a certain quantity, where r and u are the radial and retarded time coordinates, respectively. This quantity is determined by the asymptotic properties of the metric and is connected to the conjectured maximal luminosity. In higher dimensions, photons emitted with dr/du>-(1-1/3)≈-0.423 are shown to reach future null infinity without the assumptions on the metric functions.

DOI： 10.1103/PhysRevD.106.084007

Brane dynamics of holographic BCFTs Open Access

Izumi, K; Shiromizu, T; Suzuki, K; Takayanagi, T; Tanahashi, N

JOURNAL OF HIGH ENERGY PHYSICS 2022 巻 ( 10 ) 2022年10月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Journal of High Energy Physics

In this paper we study various dynamical aspects of the AdS/BCFT correspondence in higher dimensions. We study properties of holographic stress energy tensor by analyzing the metric perturbation in the gravity dual. We also calculate the stress energy tensor for a locally excited state on a half plane in a free scalar CFT. Both of them satisfy a reflective boundary condition that is expected for any BCFTs. We also study the behavior of the scalar field perturbation in the AdS/BCFT setup and show that they also show complete reflections. Moreover, we find that the entanglement entropy of a BCFT computed from the AdS/BCFT matched with that calculated from the Island formula, which supports the Island/BCFT correspondence in higher dimensions. Finally we show how we can calculate one point functions in a BCFT in our gravity dual.

DOI： 10.1007/JHEP10(2022)050

Four types of attractive gravity probe surfaces Open Access

Lee, KJ; Shiromizu, T; Izumi, K; Yoshino, H; Tomikawa, Y

PHYSICAL REVIEW D 106 巻 ( 6 ) 2022年9月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D

We reexamine the concept of the attractive gravity probe surface recently proposed as an indicator for strength of gravity. Then, we propose three new variant concepts and show refined inequalities for the four types of the attractive gravity probe surfaces by taking account of the angular momentum, gravitational waves, and matter.

DOI： 10.1103/PhysRevD.106.064028

Asymptotic behavior of null geodesics near future null infinity. II. Curvatures, photon surface, and dynamically transversely trapping surface

Amo Masaya, Shiromizu Tetsuya, Izumi Keisuke, Yoshino Hirotaka, Tomikawa Yoshimune

PHYSICAL REVIEW D 105 巻 ( 6 ) 2022年3月

Refined inequalities for a loosely trapped surface and attractive gravity probe surface Open Access

Lee, K; Shiromizu, T; Izumi, K

PHYSICAL REVIEW D 105 巻 ( 4 ) 2022年2月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D

We reexamine a loosely trapped surface proposed as an indicator for strong gravity and an attractive gravity probe surface as that for gravity. Refined inequalities for them are derived by taking account of angular momentum, gravitational waves, and matters.

DOI： 10.1103/PhysRevD.105.044037

その他リンク： http://arxiv.org/pdf/2111.08237v1

Invertibility conditions for field transformations with derivatives: Toward extensions of disformal transformation with higher derivatives Open Access

Babichev, E; Izumi, K; Tanahashi, N; Yamaguchi, M

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2022 巻 ( 1 ) 2022年1月

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

We discuss a field transformation from fields ψa to other fields φi that involves derivatives, φi = φi(ψa, αψa, xμ), and derive conditions for this transformation to be invertible, primarily focusing on the simplest case that the transformation maps between a pair of fields and involves up to their first derivatives. General field transformation of this type changes the number of degrees of freedom; hence, for the transformation to be invertible, it must satisfy certain degeneracy conditions so that additional degrees of freedom do not appear. Our derivation of necessary and sufficient conditions for invertible transformation is based on the method of characteristics, which is used to count the number of independent solutions of a given differential equation. As applications of the invertibility conditions, we show some non-trivial examples of the invertible field transformations with derivatives, and also give a rigorous proof that a simple extension of the disformal transformation involving a second derivative of the scalar field is not invertible.

DOI： 10.1093/ptep/ptab151

その他リンク： http://arxiv.org/pdf/2109.00912v2

Uniqueness of static spacetime with conformal scalar in higher dimensions Open Access

Izumi, K; Tomikawa, Y; Shiromizu, T

PHYSICAL REVIEW D 104 巻 ( 10 ) 2021年11月

掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D

We discuss the uniqueness of asymptotically flat and static spacetimes in the n-dimensional Einstein-conformal scalar system. This theory potentially has a singular point in the field equations where the effective Newton constant diverges. We will show that the static spacetime with the conformal scalar field outside a certain surface Sp associated with the singular point is unique.

DOI： 10.1103/PhysRevD.104.104025

その他リンク： http://arxiv.org/pdf/2108.02588v1

Asymptotic behavior of null geodesics near future null infinity: Significance of gravitational waves Open Access

Amo, M; Izumi, K; Tomikawa, Y; Yoshino, H; Shiromizu, T

PHYSICAL REVIEW D 104 巻 ( 6 ) 2021年9月

記述言語：日本語掲載種別：研究論文（学術雑誌）出版者・発行元：Physical Review D

We investigate the behavior of null geodesics near future null infinity in asymptotically flat spacetimes. In particular, we focus on the asymptotic behavior of null geodesics that correspond to worldlines of photons initially emitted in the directions tangential to the constant radial surfaces in the Bondi coordinates. The analysis is performed for general dimensions, and the difference between the four-dimensional cases and the higher-dimensional cases is stressed. In four dimensions, some assumptions are required to guarantee the null geodesics to reach future null infinity, in addition to the conditions of asymptotic flatness. Without these assumptions, gravitational waves may prevent photons from reaching null infinity. In higher dimensions, by contrast, such assumptions are not necessary, and gravitational waves do not affect the asymptotic behavior of null geodesics.

DOI： 10.1103/PhysRevD.104.064025

その他リンク： http://arxiv.org/pdf/2106.03150v3

Divergence equations and uniqueness theorem of static spacetimes with conformal scalar hair Open Access

Shinohara, T; Tomikawa, Y; Izumi, K; Shiromizu, T

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2021 巻 ( 9 ) 2021年9月

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

We reexamine the Israel-type proof of the uniqueness theorem of the static spacetime outside the photon surface in the Einstein-conformal scalar system. We derive in a systematic fashion a new divergence identity which plays a key role in the proof. Our divergence identity includes three parameters, allowing us to give a new proof of the uniqueness.

DOI： 10.1093/ptep/ptab107

その他リンク： http://arxiv.org/pdf/2107.13133v1

Area bound for surfaces in generic gravitational field

Izumi Keisuke, Tomikawa Yoshimune, Shiromizu Tetsuya, Yoshino Hirotaka

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2021 巻 ( 8 ) 2021年8月

Perturbative S-matrix unitarity (S*S=1) in R-mu nu(2) gravity

Abe Yugo, Inami Takeo, Izumi Keisuke

MODERN PHYSICS LETTERS A 36 巻 ( 16 ) 2021年5月

Holographic entanglement entropy of a de Sitter braneworld with Lovelock terms

Kushihara Kouki, Izumi Keisuke, Shiromizu Tetsuya

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2021 巻 ( 4 ) 2021年4月

Holographic entanglement entropy of deSitter braneworld with Lovelock

Kouki Kushihara, Keisuke Izumi, Tetsuya Shiromizu

2021年2月

Area bound for surfaces in generic gravitational field

Keisuke Izumi, Yoshimune Tomikawa, Tetsuya Shiromizu, Hirotaka Yoshino

2021年1月

We define an attractive gravity probe surface (AGPS) as a compact 2-surface
$S_\alpha$ with positive mean curvature $k$ satisfying $r^a D_a k / k^2 \ge
\alpha$ (for a constant $\alpha>-1/2$) in the local inverse mean curvature
flow, where $r^a D_a k$ is the derivative of $k$ in the outward unit normal
direction. For asymptotically flat spaces, any AGPS is proved to satisfy the
areal inequality $A_\alpha \le 4\pi [ ( 3+4\alpha)/(1+2\alpha) ]^2(Gm)^2$,
where $A_{\alpha}$ is the area of $S_\alpha$ and $m$ is Arnowitt-Deser-Misner
(ADM) mass. Equality is realized when the space is isometric to the
$t=$constant hypersurface of the Schwarzschild spacetime and $S_\alpha$ is an
$r=\mathrm{constant}$ surface with $r^a D_a k / k^2 = \alpha$. We adapt the two
methods, the inverse mean curvature flow and the conformal flow. Therefore, our
result is applicable to the case where $S_\alpha$ has multiple components. For
anti-de Sitter (AdS) spaces, the similar inequality is derived, but the proof
is performed only by using the inverse mean curvature flow. We also discuss the
cases with asymptotically locally AdS spaces.

その他リンク： http://arxiv.org/pdf/2101.03860v1

Invertible field transformations with derivatives: necessary and sufficient conditions 国際共著 Open Access

Babichev E., Izumi K., Tanahashi N., Yamaguchi M.

Advances in Theoretical and Mathematical Physics 25 巻 ( 2 ) 頁： 309 - 325 2021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：Advances in Theoretical and Mathematical Physics

We formulate explicitly the necessary and sufficient conditions for the local invertibility of a field transformation involving derivative terms. Our approach is to apply the method of characteristics of differential equations, by treating such a transformation as differential equations that give new variables in terms of original ones. The obtained results generalise the well-known and widely used inverse function theorem. Taking into account that field transformations are ubiquitous in modern physics and mathematics, our criteria for invertibility will find many useful applications.

DOI： 10.4310/ATMP.2021.v25.n2.a2

DOI： 10.4310/atmp.2021.v25.n2.a2

その他リンク： http://arxiv.org/pdf/1907.12333v1

Invertible field transformations with derivatives: necessary and sufficient conditions

Babichev, E; Izumi, K; Tanahashi, N; Yamaguchi, M

ADVANCES IN THEORETICAL AND MATHEMATICAL PHYSICS 25 巻 ( 2 ) 頁： 309 - 325 2021年

Perturbative $S$-matrix unitarity ($S^{\dagger}S=1$) in $R_{μν} ^2$ gravity

Yugo Abe, Takeo Inami, Keisuke Izumi

2020年12月

We show that in the quadratic curvature theory of gravity, or simply $R_{\mu
\nu} ^2$ gravity, the tree-level unitariy bound (tree unitarity) is violated in
the UV region but an analog for $S$-matrix unitarity ($SS^{\dagger} = 1$) is
satisfied. This theory is renormalizable, and hence the failure of tree
unitarity is a counter example of Llewellyn Smith's conjecture on the relation
between them. We have recently proposed a new conjecture that $S$-matrix
unitarity gives the same conditions as renormalizability. We verify that
$S$-matrix unitarity holds in the matter-graviton scattering at tree level in
the $R_{\mu \nu} ^2$ gravity, demonstrating our new conjecture.

その他リンク： http://arxiv.org/pdf/2012.01717v3

Loosely trapped surface and dynamically transversely trapping surface in Einstein-Maxwell systems 査読有り国際誌

Lee K., Shiromizu T., Yoshino H., Izumi K., Tomikawa Y.

Progress of Theoretical and Experimental Physics 2020 巻 ( 10 ) 2020年10月

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

We study the properties of the loosely trapped surface (LTS) and the dynamically transversely trapping surface (DTTS) in Einstein-Maxwell systems. These concepts of surfaces were proposed by four of the present authors in order to characterize strong gravity regions. We prove the Penrose-like inequalities for the area of LTSs/DTTSs. Interestingly, although the naively expected upper bound for the area is that of the photon sphere of a Reissner-Nordström black hole with the same mass and charge, the obtained inequalities include corrections represented by the energy density or pressure/tension of electromagnetic fields. Due to this correction, the Penrose-like inequality for the area of LTSs is tighter than the naively expected one. We also evaluate the correction term numerically in the Majumdar-Papapetrou two-black-hole spacetimes.

DOI： 10.1093/ptep/ptaa133

その他リンク： http://arxiv.org/pdf/2007.03139v1

Formation of dynamically transversely trapping surfaces and the stretched hoop conjecture 査読有り国際誌 Open Access

Yoshino, H; Izumi, K; Shiromizu, T; Tomikawa, Y

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2020 巻 ( 5 ) 2020年5月

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

A dynamically transversely trapping surface (DTTS) is a new concept for an extension of a photon sphere that appropriately represents a strong gravity region and has close analogy with a trapped surface. We study formation of a marginally DTTS in time-symmetric, conformally flat initial data with two black holes, with a spindle-shaped source, and with a ring-shaped source, and clarify that C ≲ 6π GM describes the condition for the DTTS formation well, where C is the circumference and M is the mass of the system. This indicates that an understanding analogous to the hoop conjecture for the horizon formation is possible. Exploring the ring system further, we find configurations where a marginally DTTS with the torus topology forms inside a marginally DTTS with the spherical topology, without being hidden by an apparent horizon. There also exist configurations where a marginally trapped surface with the torus topology forms inside a marginally trapped surface with the spherical topology, showing a further similarity between DTTSs and trapped surfaces.

DOI： 10.1093/ptep/ptaa050

その他リンク： http://arxiv.org/pdf/1911.09893v2

Transversely trapping surfaces: Dynamical version 査読有り国際誌 Open Access

Yoshino, H; Izumi, K; Shiromizu, T; Tomikawa, Y

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2020 巻 ( 2 ) 2020年2月

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

We propose new concepts, a dynamically transversely trapping surface (DTTS) and a marginally DTTS, as indicators for a strong gravity region. A DTTS is defined as a two-dimensional closed surface on a spacelike hypersurface such that photons emitted from arbitrary points on it in transverse directions are acceleratedly contracted in time, and a marginally DTTS is reduced to the photon sphere in spherically symmetric cases. (Marginally) DTTSs have a close analogy with (marginally) trapped surfaces in many aspects. After preparing the method of solving for a marginally DTTS in the time-symmetric initial data and the momentarily stationary axisymmetric initial data, some examples of marginally DTTSs are numerically constructed for systems of two black holes in the Brill-Lindquist initial data and in the Majumdar-Papapetrou spacetimes. Furthermore, the area of a DTTS is proved to satisfy the Penrose-like inequality A0 ≤ 4π (3GM)2, under some assumptions. Differences and connections between a DTTS and the other two concepts proposed by us previously, a loosely trapped surface [Prog. Theor. Exp. Phys. 2017, 033E01 (2017)] and a static/stationary transversely trapping surface [Prog. Theor. Exp. Phys. 2017, 063E01 (2017)], are also discussed. A (marginally) DTTS provides us with a theoretical tool to significantly advance our understanding of strong gravity fields. Also, since DTTSs are located outside the event horizon, they could possibly be related with future observations of strong gravity regions in dynamical evolutions.

DOI： 10.1093/ptep/ptz161

その他リンク： http://arxiv.org/pdf/1909.08420v2

<i>S</i>-matrix unitarity and renormalizability in higher-derivative theories 査読有り国際誌 Open Access

Abe, Y; Inami, T; Izumi, K; Kitamura, T; Noumi, T

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2019 巻 ( 8 ) 2019年8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 and Experimental Physics

We investigate the relation between S-matrix unitarity (SS†=1) and renormalizability in theories with negative-norm states. The relation has been confirmed in many field theories, including gauge theories and Einstein gravity, by analyzing the unitarity bound, which follows from the S-matrix unitarity and the norm positivity. On the other hand, renormalizable theories with a higher-derivative kinetic term do not necessarily satisfy the unitarity bound because of the negative-norm states. In these theories it is not known whether the S-matrix unitarity provides a nontrivial constraint related to the renormalizability. In this paper, by relaxing the assumption of norm positivity we derive a bound on scattering amplitudes weaker than the unitarity bound, which may be used as a consistency requirement for S-matrix unitarity. We demonstrate in scalar field models with a higher-derivative kinetic term that the weaker bound and the renormalizability imply identical constraints.

DOI： 10.1093/ptep/ptz084

その他リンク： http://arxiv.org/pdf/1805.00262v1

Divergence equations and uniqueness theorem of static black holes 査読有り国際誌 Open Access

Nozawa, M; Shiromizu, T; Izumi, K; Yamada, S

CLASSICAL AND QUANTUM GRAVITY 35 巻 ( 17 ) 2018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

Equations of divergence type in static spacetimes play a significant role in the proof of uniqueness theorems of black holes. We generalize the divergence equation originally discovered by Robinson in four dimensional vacuum spacetimes into several directions. We find that the deviation from spherical symmetry is encoded in a symmetric trace-free tensor H ij on a static timeslice. This tensor is the crux for the construction of the desired divergence equation, which allows us to conclude the uniqueness of the Schwarzschild black hole without using Smarr's integration mass formula. In Einstein-Maxwell(-dilaton) theory, we apply the maximal principle for a number of divergence equations to prove the uniqueness theorem of static black holes. In higher dimensional vacuum spacetimes, a central obstruction for applicability of the current proof is the integration of the -dimensional scalar curvature over the horizon cross-section, which has been evaluated to be a topological constant by the Gauss-Bonnet theorem for n = 4. Nevertheless, it turns out that the -dimensional symmetric and traceless tensor H ij is still instrumental for the modification of the uniqueness proof based upon the positive mass theorem, as well as for the derivation of the Penrose-type inequality.

DOI： 10.1088/1361-6382/aad206

その他リンク： http://arxiv.org/pdf/1805.11385v2

Matter scattering in quadratic gravity and unitarity 査読有り国際誌 Open Access

Abe, Y; Inami, T; Izumi, K; Kitamura, T

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2018 巻 ( 3 ) 2018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：Progress of Theoretical and Experimental Physics

We investigate the ultraviolet (UV) behavior of two-scalar elastic scattering<br />
with graviton exchanges in higher curvature gravity theory. In the Einstein<br />
gravity, matter scattering is shown not to satisfy tree unitarity at high<br />
energy. Among a few possible directions to cure unitarity (i.e. UV completion<br />
of Einstein gravity), string theory, modified gravity, inclusion of<br />
high-mass/high-spin states, we take $R_{\mu\nu}^2$ gravity coupled to matter.<br />
We show that the matter scattering with graviton interactions satisfies the<br />
unitarity bound at high energy, in contrast with the Einstein gravity. The<br />
difference in unitarity property of the two gravity theories is due to that in<br />
the UV behavior of the propagator and is probably connected to that in another<br />
UV property, namely renormalizability property of the two.

DOI： 10.1093/ptep/pty010

その他リンク： http://arxiv.org/pdf/1712.06305v2

Matter scattering and unitarity in Horava-Lifshitz gravity

Kitamura T., Inami T., Izumi K.

Proceedings of the 28th Workshop on General Relativity and Gravitation in Japan, JGRG 2018 2 巻頁： 105 - 116 2018年

On uniqueness of static spacetimes with non-trivial conformal scalar field 査読有り国際誌

Tomikawa, Y; Shiromizu, T; Izumi, K

CLASSICAL AND QUANTUM GRAVITY 34 巻 ( 15 ) 2017年8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 discuss the uniqueness of static spacetimes with non-trivial conformal scalar field. We can show that the spacetime is unique to be the Bocharova-Bronnikov-Melnikov-Bekenstein solution outside the surface composed of the unstable circular orbit of a photon (photon surface). In addition, we see that multiple disconnected photon surfaces having the same values of the scalar field do not exist.

DOI： 10.1088/1361-6382/aa7906

その他リンク： http://arxiv.org/pdf/1702.05682v2

Extension of photon surfaces and their area: Static and stationary spacetimes 査読有り国際誌 Open Access

Yoshino, H; Izumi, K; Shiromizu, T; Tomikawa, Y

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2017 巻 ( 6 ) 2017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：Progress of Theoretical and Experimental Physics

We propose a new concept, the transversely trapping surface (TTS), as an extension of the static photon surface characterizing the strong gravity region of a static/stationary spacetime in terms of photon behavior. The TTS is defined as a static/stationary timelike surface S whose spatial section is a closed two-surface, such that arbitrary photons emitted tangentially to S from arbitrary points on S propagate on or toward the inside of S. We study the properties of TTSs for static spacetimes and axisymmetric stationary spacetimes. In particular, the area A0 of a TTS is proved to be bounded as A0 ≤ 4π(3GM)2 under certain conditions, where G is the Newton constant and M is the total mass. The connection between the TTS and the loosely trapped surface proposed by us [Prog. Theor. Exp. Phys. 2017, 033E01 (2017)] is also examined.

DOI： 10.1093/ptep/ptx072

その他リンク： http://arxiv.org/pdf/1704.04637v2

On the uniqueness of the static black hole with conformal scalar hair 査読有り国際誌 Open Access

Tomikawa, Y; Shiromizu, T; Izumi, K

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2017 巻 ( 3 ) 2017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：Progress of Theoretical and Experimental Physics

We discuss the uniqueness of the static black hole in Einstein gravity with a conformally coupled scalar field. In particular, we prove the uniqueness of the region outside the photon surface, not the event horizon.

DOI： 10.1093/ptep/ptx033

その他リンク： http://arxiv.org/pdf/1612.01228v3

Area bound for a surface in a strong gravity region 査読有り国際誌 Open Access

Shiromizu, T; Tomikawa, Y; Izumi, K; Yoshino, H

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2017 巻 ( 3 ) 2017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：Progress of Theoretical and Experimental Physics

For asymptotically flat spacetimes, using the inverse mean curvature flow we show that any compact 2-surface, S0, whose mean curvature and its derivative for the outward direction are positive in a spacelike hypersurface with non-negative Ricci scalar satisfies the inequality A0 ≤4π(3Gm)2, where A0 is the area of S0 and m is the total mass. The upper bound is realized when S0 is the photon sphere in a hypersurface isometric to a t = const. slice of the Schwarzschild spacetime.

DOI： 10.1093/ptep/ptx022

その他リンク： http://arxiv.org/pdf/1701.00564v2

Theoretical constraints on modified theories of gravity Open Access

Izumi K.

Proceedings of Science 2017-January 巻 2017年

出版者・発行元：Proceedings of Science

We discuss about the theoretical constraints on modified theories of gravity. To explain the dark component of the Universe, many modified theories of gravity have been proposed. Meanwhile, to quantize the gravitational theory, we would need a new theory of gravity in UV physics. In this talk, we focus on three theoretical aspects of gravitational theories; causal structures, positivity of energy and black hole physics. The recent developments of these topics were discussed.

DOI： 10.22323/1.294.0030

Hydro-elastic complementarity in black branes at large <i>D</i> 査読有り国際誌 Open Access

Emparan, R; Izumi, K; Luna, R; Suzuki, R; Tanabe, K

JOURNAL OF HIGH ENERGY PHYSICS 2016 巻 ( 6 ) 2016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 High Energy Physics

Abstract: We obtain the effective theory for the non-linear dynamics of black branes — both neutral and charged, in asymptotically flat or Anti-deSitter spacetimes — to leading order in the inverse-dimensional expansion. We find that black branes evolve as viscous fluids, but when they settle down they are more naturally viewed as solutions of an elastic soap-bubble theory. The two views are complementary: the same variable is regarded in one case as the energy density of the fluid, in the other as the deformation of the elastic membrane. The large-D theory captures finite-wavelength phenomena beyond the conventional reach of hydrodynamics. For asymptotically flat charged black branes (either Reissner-Nordstrom or p-brane-charged black branes) it yields the non-linear evolution of the Gregory-Laflamme instability at large D and its endpoint at stable non-uniform black branes. For Reissner-Nordstrom AdS black branes we find that sound perturbations do not propagate (have purely imaginary frequency) when their wavelength is below a certain charge-dependent value. We also study the polarization of black branes induced by an external electric field.

DOI： 10.1007/JHEP06(2016)117

その他リンク： http://arxiv.org/pdf/1602.05752v3

Tree-Unitarity and renormalizability in Lifshitz-scaling theory -- as a toy model of Hořava's gravity theory 査読有り

Toshiaki Fujimori, Takeo Inami, Keisuke Izumi, Tomotaka Kitamura

2016年1月

We study tree-unitarity and renormalizability in Lifshitz-scaling theory,<br />
which is characterized by an anisotropic scaling between the spacial and time<br />
directions. Due to the lack of the Lorentz symmetry, the conditions for both<br />
unitarity and renormalizability are modified from those in relativistic<br />
theories. For renormalizability, the conventional discussion of the power<br />
counting conditions has to be extended. Because of the dependence of $S$-matrix<br />
elements on the reference frame, unitarity requires stronger conditions than<br />
those in relativistic cases. We show that the conditions for unitarity and<br />
renormalizabilty are identical as in relativistic theories. We discuss the<br />
importance of symmetries for a theory to be renormalizable.

DOI： 10.1142/9789813203952_0054

その他リンク： http://arxiv.org/pdf/1601.06470v1

Tree-level unitarity and renormalizability in Lifshitz scalar theory 査読有り国際誌 Open Access

Fujimori, T; Inami, T; Izumi, K; Kitamura, T

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS 2016 巻 ( 1 ) 2016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：Progress of Theoretical and Experimental Physics

We study unitarity and renormalizability in the Lifshitz scalar field theory, which is characterized by an anisotropic scaling between the space and time directions.Without the Lorentz symmetry, both the unitarity and the renormalizability conditions are modified from those in relativistic theories. We show that for renormalizability, an extended version of the power-counting condition is required in addition to the conventional one. The unitarity bound for S-matrix elements also gives stronger constraints on interaction terms because of the reference frame dependence of scattering amplitudes. We prove that both unitarity and renormalizability require identical conditions as in the case of conventional relativistic theories.

DOI： 10.1093/ptep/ptv185

その他リンク： http://arxiv.org/pdf/1510.07237v1

Boundary effect of anomaly-induced action (vol 92, 024035, 2015) 査読有り Open Access

Shen, CM; Izumi, K; Chen, PS

PHYSICAL REVIEW D 92 巻 ( 4 ) 2015年8月

Boundary effect of anomaly-induced action 査読有り

Che-Min Shen, Keisuke Izumi, Pisin Chen

PHYSICAL REVIEW D 92 巻 ( 2 ) 2015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：AMER PHYSICAL SOC

We discuss the boundary effect of anomaly-induced action in two-dimensional spacetime, which is ignored in previous studies. Anomaly-induced action, which gives the stress tensor with the same trace as the trace anomaly, can be represented in terms of local operators by introducing an auxiliary scalar field. Although the degrees of freedom of the auxiliary field can in principle describe the quantum states of the original field, the principal relation between them was unclear. We show here that, by considering the boundary effect, the solutions of classical auxiliary fields are naturally related to the quantum states of the original field. We demonstrate this conclusion via several examples such as the flat, black hole, and the de Sitter spacetime.

DOI： 10.1103/PhysRevD.92.024035

Power-counting and renormalizability in Lifshitz scalar theory 査読有り国際誌 Open Access

Fujimori, T; Inami, T; Izumi, K; Kitamura, T

PHYSICAL REVIEW D 91 巻 ( 12 ) 2015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：Physical Review D - Particles, Fields, Gravitation and Cosmology

We study the renormalizability in theories of a self-interacting Lifshitz scalar field. We show that although the statement of power-counting is true at one-loop order, in generic cases where the scalar field is dimensionless, an infinite number of counterterms are involved in the renormalization procedure. This problem can be avoided by imposing symmetries, the shift symmetry in the present paper, which allow only a finite number of counterterms to appear. The symmetry requirements might have important implications for the construction of matter field sectors in the Hořava-Lifshitz gravity.

DOI： 10.1103/PhysRevD.91.125007

その他リンク： http://arxiv.org/pdf/1502.01820v1

Ghosts in the self-accelerating DGP branch with Gauss-Bonnet effect 査読有り国際誌 Open Access

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

EUROPEAN PHYSICAL JOURNAL C 75 巻 ( 6 ) 2015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：European Physical Journal C

The Dvali–Gabadadze–Porrati brane-world model provides a possible approach to address the late-time cosmic acceleration. However, it has subsequently been pointed out that a ghost instability will arise on the self-accelerating branch. Here, we carefully investigate whether this ghost problem could be possibly cured by introducing the Gauss–Bonnet term in the five-dimensional bulk action, a natural generalization to the Dvali–Gabadadze–Porrati model. Our analysis is carried out for a background where a de Sitter brane is embedded in an anti–de Sitter bulk. Our result shows that the ghost excitations cannot be avoided even in this modified model.

DOI： 10.1140/epjc/s10052-015-3463-0

その他リンク： http://arxiv.org/pdf/1405.0850v2

New definition of a wormhole throat 査読有り国際誌 Open Access

Tomikawa, Y; Izumi, K; Shiromizu, T

PHYSICAL REVIEW D 91 巻 ( 10 ) 2015年5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 present a new definition of the wormhole throat including the flare-out condition and the feature corresponding to the traversability for general dynamical spacetimes in terms of null geodesic congruences. We will examine our definition for some examples and see advantages compared to the others.

DOI： 10.1103/PhysRevD.91.104008

The boundary effect of anomaly-induced action 査読有り

Che-Min Shen, Keisuke Izumi, Pisin Chen

Physical Review D 2015年5月

記述言語：英語掲載種別：研究論文（学術雑誌）

We discuss the boundary effect of anomaly-induced action in two-dimensional
spacetime, which is ignored in previous studies. Anomaly-induced action, which
gives the stress tensor with the same trace as the trace anomaly, can be
represented in terms of local operators by introducing an auxiliary scalar
field. Although the degrees of freedom of the auxiliary field can in principle
describe the quantum states of the original field, the principal relation
between them was unclear. We show here that, by considering the boundary
effect, the solutions of classical auxiliary fields are naturally related to
the quantum states of the original field. We demonstrate this conclusion via
several examples such as the flat, black hole and the de Sitter spacetime.

DOI： 10.1103/PhysRevD.92.024035

その他リンク： http://arxiv.org/pdf/1505.00959v1

Remnant symmetry, propagation, and evolution in <i>f</i>(<i>T</i>) gravity 査読有り国際誌 Open Access

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

PHYSICAL REVIEW D 91 巻 ( 6 ) 2015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

It was recently argued that f(T) gravity could inherit "remnant symmetry" from the full Lorentz group, despite the fact that the theory is not locally Lorentz invariant. Confusion has arisen regarding the implication of this result for the previous works, which established that f(T) gravity is pathological due to superluminal propagation, local acausality, and nonunique time evolution. We clarify that the existence of the "remnant group" does not rid the theory of these various problems but, instead, strongly supports it.

DOI： 10.1103/PhysRevD.91.064003

その他リンク： http://arxiv.org/pdf/1412.8383v3

Problems of massive gravities 査読有り国際誌 Open Access

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

MODERN PHYSICS LETTERS A 30 巻 ( 3-4 ) 2015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：Modern Physics Letters A

The method of characteristics is a key tool for studying consistency of equations of motion; it allows issues such as predictability, maximal propagation speed, superluminality, unitarity and acausality to be addressed without requiring explicit solutions. We review this method and its application to massive gravity (mGR) theories to show the limitations of these models' physical viability: Among their problems are loss of unique evolution, superluminal signals, matter coupling inconsistencies and micro-acausality (propagation of signals around local closed time-like curves (CTCs)/closed causal curves (CCCs)). We extend previous no-go results to the entire three-parameter range of mGR theories. It is also argued that bimetric models suffer a similar fate.

DOI： 10.1142/S0217732315400064

Boundary effect of anomaly-induced action 査読有り国際誌

Che-Min Shen, Keisuke Izumi, Pisin Chen

Physical Review D 2015年

Wormhole on DGP brane 査読有り国際誌 Open Access

Tomikawa, Y; Shiromizu, T; Izumi, K

PHYSICAL REVIEW D 90 巻 ( 12 ) 2014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 analyze a spacetime structure on a new brane configuration constructed recently in the Dvali-Gabadadze-Porrati braneworld context. The brane, embedded on a five-dimensional bubble of nothing, has a wormhole structure. It is an exact solution without any matter fields, and thus the energy conditions for matter fields are trivially satisfied. We see that, under the traversability condition, the size of the bubble should be larger than 1010cm or so.

DOI： 10.1103/PhysRevD.90.126001

その他リンク： http://arxiv.org/pdf/1409.6816v2

DGP braneworld with a bubble of nothing 査読有り国際誌

Izumi, K; Shiromizu, T

PHYSICAL REVIEW D 90 巻 ( 4 ) 2014年8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 construct exact solutions with the bubble of nothing in the Dvali-Gabadadze-Porrati braneworld model. The configuration with a single brane can be constructed, unlike in the Randall-Sundrum braneworld model. The geometry on the single brane looks like the Einstein-Rosen bridge. We also discuss the junction of multibranes. Surprisingly, even without any artificial matter fields on the branes such as three-dimensional tension of the codimension-two objects, two branes can be connected in certain configurations. We investigate solutions of multibranes too. The presence of solutions may indicate the semiclassical instability of the models. © 2014 American Physical Society.

DOI： 10.1103/PhysRevD.90.046005

Causal structures in Gauss-Bonnet gravity 査読有り国際誌 Open Access

Izumi, K

PHYSICAL REVIEW D 90 巻 ( 4 ) 2014年8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 analyze causal structures in Gauss-Bonnet gravity. It is known that Gauss-Bonnet gravity potentially has superluminal propagation of gravitons due to its noncanonical kinetic terms. In a theory with superluminal modes, an analysis of causality based on null curves makes no sense, and thus, we need to analyze them in a different way. In this paper, using the method of the characteristics, we analyze the causal structure in Gauss-Bonnet gravity. We have the result that, on a Killing horizon, gravitons can propagate in the null direction tangent to the Killing horizon. Therefore, a Killing horizon can be a causal edge as in the case of general relativity; i.e. a Killing horizon is the "event horizon" in the sense of causality. We also analyze causal structures on nonstationary solutions with (D-2)-dimensional maximal symmetry, including spherically symmetric and flat spaces. If the geometrical null energy condition, RABNANB≥0 for any null vector NA, is satisfied, the radial velocity of gravitons must be less than or equal to that of light. However, if the geometrical null energy condition is violated, gravitons can propagate faster than light. Hence, on an evaporating black hole where the geometrical null energy condition is expected not to hold, classical gravitons can escape from the "black hole" defined with null curves. That is, the causal structures become nontrivial. It may be one of the possible solutions for the information loss paradox of evaporating black holes. © 2014 American Physical Society.

DOI： 10.1103/PhysRevD.90.044037

その他リンク： http://arxiv.org/pdf/1406.0677v2

Acausality and nonunique evolution in generalized teleparallel gravity 査読有り国際誌 Open Access

Izumi, K; Gu, JA; Ong, YC

PHYSICAL REVIEW D 89 巻 ( 8 ) 2014年4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 the existence of physical superluminal modes and acausality in the Brans-Dicke type of extension of teleparallel gravity that includes F(T) gravity and teleparallel dark energy as special cases. We derive the characteristic hypersurface for the extra degrees of freedom in the theory, thereby showing that the time evolution is not unique and closed causal curves can appear. Furthermore, we present a concrete disastrous solution in Bianchi type I spacetime, in which the anisotropy in expansion can be any function of time, and thus anisotropy can emerge suddenly, a simple demonstration that the theory is physically problematic. © 2014 American Physical Society.

DOI： 10.1103/PhysRevD.89.084025

その他リンク： http://arxiv.org/pdf/1309.6461v2

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.

Orthogonal black di-ring solution 査読有り国際誌 Open Access

Izumi, K

PROGRESS OF THEORETICAL PHYSICS 119 巻 ( 5 ) 頁： 757 - 774 2008年5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 construct a five dimensional exact solution of the orthogonal black di-ring which has two black rings whose S1-rotating planes are orthogonal. This solution has four free parameters which represent radii of and speeds of 51-rotation of the black rings. We use the inverse scattering method. This method needs the seed metric. We also present a systematic method how to construct a seed metric. Using this method, we can probably construct other solutions having many black rings on the two orthogonal planes with or without a black hole at the center.

DOI： 10.1143/PTP.119.757

その他リンク： http://arxiv.org/pdf/0712.0902v3

Massive spin-2 ghost in de sitter spacetime

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

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

Renormalized newtonian cosmic evolution with primordial non-gaussianity

Izumi K., Soda J.

Proceedings of the 17th Workshop on General Relativity and Gravitation in Japan, JGRG 2007 頁： 153 - 156 2007年12月

出版者・発行元：Proceedings of the 17th Workshop on General Relativity and Gravitation in Japan, JGRG 2007

We study Newtonian cosmological perturbation theory from a field theoretical point of view. We derive a path integral representation for the cosmological evolution of stochastic fluctuations. Our main result is the closed form of the generating functional valid for any initial statistics. Moreover, we extend the renormalization group method proposed by Mataresse and Pietroni to the case of primordial non-Gaussian density and velocity fluctuations. As an application, we calculate the nonlinear propagator and examine how the non-Gaussianity affects the memory of cosmic fields to their initial conditions. It turns out that the non-Gaussianity affect the nonlinear propagator. In the case of positive skewness, the onset of the nonlinearity is advanced with a given comoving wavenumber. On the other hand, the negative skewness gives the opposite result.

Bubbles in the self-accelerating universe 査読有り国際誌 Open Access

Izumi, K; Koyama, K; Pujolàs, O; Tanaka, T

PHYSICAL REVIEW D 76 巻 ( 10 ) 2007年11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 revisit the issue of the stability in the Dvali-Gabadadze-Porrati model by considering the nucleation of bubbles of the conventional branch within the self-accelerating branch. We construct an instanton describing this process in the thin wall approximation. On one side of the bubble wall, the bulk consists of the exterior of the brane, while on the other side it is the interior. The solution requires the presence of a 2-brane (the bubble wall) which induces the transition. However, we show that this instanton cannot be realized as the thin wall limit of any smooth solution. Once the bubble thickness is resolved, the equations of motion do not allow O(4) symmetric solutions joining the two branches. We conclude that the thin wall instanton is unphysical, and that one cannot have processes connecting the two branches, unless negative tension bubble walls are introduced. This also suggests that the self-accelerating branch does not decay into the conventional branch nucleating bubbles. We comment on other kinds of bubbles that could interpolate between the two branches. © 2007 The American Physical Society.

DOI： 10.1103/PhysRevD.76.104041

その他リンク： http://arxiv.org/pdf/0706.1980v1

Renormalized Newtonian cosmic evolution with primordial non-Gaussianity 査読有り国際誌

Izumi, K; Soda, J

PHYSICAL REVIEW D 76 巻 ( 8 ) 2007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：Physical Review D - Particles, Fields, Gravitation and Cosmology

We study Newtonian cosmological perturbation theory from a field theoretical point of view. We derive a path integral representation for the cosmological evolution of stochastic fluctuations. Our main result is the closed form of the generating functional valid for any initial statistics. Moreover, we extend the renormalization group method proposed by Mataresse and Pietroni to the case of primordial non-Gaussian density and velocity fluctuations. As an application, we calculate the nonlinear propagator and examine how the non-Gaussianity affects the memory of cosmic fields to their initial conditions. It turns out that the non-Gaussianity affects the nonlinear propagator. In the case of positive skewness, the onset of the nonlinearity is advanced with a given comoving wave number. On the other hand, the negative skewness gives the opposite result. © 2007 The American Physical Society.

DOI： 10.1103/PhysRevD.76.083517

その他リンク： http://arxiv.org/pdf/0706.1604v2

Unexorcized ghost in DGP brane world 査読有り国際誌 Open Access

Izumi, K; Tanaka, T; Koyama, K

JOURNAL OF HIGH ENERGY PHYSICS 2007 巻 ( 4 ) 2007年4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 High Energy Physics

The brane world model proposed by Dvali-Gabadadze-Porrati realizes self-acceleration of the universe. However, it is known that this cosmological solution contains a spin-2 ghost. We study the possibility of avoiding the appearance of the ghost by slightly modifying the model via the introduction of a second brane. First, we consider a simple model without stabilization of the brane separation. 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. We discuss why these two different types of ghosts are correlated. Then, we examine a model with stabilization of the brane separation. Even in this case, we find that the correlation between spin-0 and spin-2 ghosts remains. As a result we find that we cannot avoid the appearance of a ghost by introducing a second brane in the model. © SISSA 2007.

DOI： 10.1088/1126-6708/2007/04/053

その他リンク： http://arxiv.org/pdf/hep-th/0610282v3

Unexorcized ghost in DGP brane world

Izumi K., Koyama K., Tanaka T.

Proceedings of the 16th Workshop on General Relativity and Gravitation in Japan, JGRG 2006 頁： 267 - 270 2006年12月

出版者・発行元：Proceedings of the 16th Workshop on General Relativity and Gravitation in Japan, JGRG 2006

The braneworld model of Dvali-Gabadadze-Porrati realizes the self-accelerating universe. However, it is known that this cosmological solution contains a spin-2 ghost. We study the possibility of avoiding the appearance of the ghost by slightly modifying the model, introducing the second brane. First we consider a simple model without stabilization of the separation of the brane. By changing the separation between the branes, we find 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. We discuss why these two different types of ghosts are correlated. Then, we examine a model with stabilization of the brane separation. Even in this case, we find that the correlation between spin-0and spin-2 ghosts remains. As a result we find we cannot avoid the appearance of ghost by two-branes model.

The avoidance of the ghost problem in the DGP brane world: Two-branes model

Izumi K., Tanaka T.

Proceedings of the 15th Workshop on General Relativity and Gravitation in Japan, JGRG 2005 頁： 135 - 138 2005年12月

出版者・発行元：Proceedings of the 15th Workshop on General Relativity and Gravitation in Japan, JGRG 2005

We discuss gravitational perturbations of de Sitter brane world in the DGP model. In the original DGP model with a self-Accelerating single brane, a ghost-like field appeal's, which is thought to be problematic. To cure this ghost problem keeping the advantage of the self-Acceleration, we propose a two-branes model, which has another boundary brane in the bulk. When the new boundary brane is sufficiently close to the original brane, there is no ghost in this model. Moreover, when the acceleration of the universe is solely due to the modification of gravity without the aid of dark energy, the model is free from ghost irrespective of the position of the new boundary brane.

The avoidance of the ghost problem in the DGP brane world: Two-branes model

K. Izumi, T. Tanaka

Proceedings of the 15th Workshop on General Relativity and Gravitation in Japan, JGRG 2005 頁： 135 - 138 2005年

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