2022/05/13 更新

イズミ　ケイスケ

IZUMI Keisuke

### 学位 3

1. 博士（理学） （ 2009年3月   京都大学 ）

2. 修士（理学） （ 2006年3月   京都大学 ）

3. 学士（理学） （ 2004年3月   京都大学 ）

1. 宇宙論

2. 重力理論

### 経歴 1

1. 名古屋大学   素粒子宇宙起源研究所   講師

2021年8月 - 現在

### 学歴 3

1. 京都大学   理学研究科 博士課程

2006年4月 - 2009年3月

2. 京都大学   理学研究科 修士課程

2004年4月 - 2006年3月

3. 京都大学   理学部

2000年4月 - 2004年3月

### 論文 63

1. 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

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記述言語：日本語

2. 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

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記述言語：日本語

3. 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

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記述言語：日本語

4. 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

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記述言語：日本語

5. Invertible field transformations with derivatives: necessary and sufficient conditions

Babichev Eugeny, Izumi Keisuke, Tanahashi Norihiro, Yamaguchi Masahide

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

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記述言語：日本語

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

Lee Kangjae, Shiromizu Tetsuya, Yoshino Hirotaka, Izumi Keisuke, Tomikawa Yoshimune

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2020 巻 ( 10 )   2020年10月

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

7. Eugeny Babichev, Keisuke Izumi, Norihiro Tanahashi, Masahide Yamaguchi

Advances in Theoretical and Mathematical Physics   Volume 25 (2021) 巻 ( Number 2 ) 頁： 309   2022年2月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

8. Lee Kangjae, Shiromizu Tetsuya, Izumi Keisuke

PHYSICAL REVIEW D   105 巻 ( 4 )   2022年2月

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記述言語：日本語

9. Babichev Eugeny, Izumi Keisuke, Tanahashi Norihiro, Yamaguchi Masahide

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

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記述言語：日本語

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

2021年10月

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Bearing in mind our previous study on asymptotic behavior of null geodesics
near future null infinity, we analyze the behavior of geometrical quantities
such as a certain extrinsic curvature and Riemann tensor in the Bondi
coordinates. In the sense of asymptotics, the condition for an $r$-constant
hypersurface to be a photon surface is shown to be controlled by a key quantity
that determines the fate of photons initially emitted in angular directions. As
a consequence, in four dimensions, such a non-expanding photon surface can be
realized even near future null infinity in the presence of enormous energy flux
for a short period of time. By contrast, in higher-dimensional cases, no such a
photon surface can exist. This result also implies that the dynamically
transversely trapping surface, which is proposed as an extension of a photon
surface, can have an arbitrarily large radius in four dimensions.

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

PHYSICAL REVIEW D   104 巻 ( 6 )   2021年9月

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記述言語：日本語

12. Shinohara Takeshi, Tomikawa Yoshimune, Izumi Keisuke, Shiromizu Tetsuya

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

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記述言語：日本語

13. Keisuke Izumi, Yoshimune Tomikawa, Tetsuya Shiromizu

2021年8月

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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 $S_p$ associated with the singular point is
unique.

14. Kouki Kushihara, Keisuke Izumi, Tetsuya Shiromizu

2021年2月

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We examine the deSitter entropy in the braneworld model with the
Gauss-Bonnet/Lovelock terms. Then, we can see that the deSitter entropy
computed through the Euclidean action exactly coincides with the holographic
entanglement entropy.

15. Keisuke Izumi, Yoshimune Tomikawa, Tetsuya Shiromizu, Hirotaka Yoshino

2021年1月

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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.

16. Yugo Abe, Takeo Inami, Keisuke Izumi

2020年12月

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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.

17. Yoshino Hirotaka, Izumi Keisuke, Shiromizu Tetsuya, Tomikawa Yoshimune

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

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

18. Yoshino Hirotaka, Izumi Keisuke, Shiromizu Tetsuya, Tomikawa Yoshimune

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

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

19. Abe Yugo, Inami Takeo, Izumi Keisuke, Kitamura Tomotaka, Noumi Toshifumi

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2019 巻 ( 8 )   2019年8月

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記述言語：日本語   掲載種別：研究論文（学術雑誌）

20. Nozawa Masato, Shiromizu Tetsuya, Izumi Keisuke, Yamada Sumio

CLASSICAL AND QUANTUM GRAVITY   35 巻 ( 17 )   2018年9月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

21. Abe Yugo, Inami Takeo, Izumi Keisuke, Kitamura Tomotaka

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2018 巻 ( 3 )   2018年3月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

22. Tomikawa Yoshimune, Shiromizu Tetsuya, Izumi Keisuke

CLASSICAL AND QUANTUM GRAVITY   34 巻 ( 15 )   2017年8月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

23. Yoshino Hirotaka, Izumi Keisuke, Shiromizu Tetsuya, Tomikawa Yoshimune

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2017 巻 ( 6 )   2017年6月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

24. Tomikawa Yoshimune, Shiromizu Tetsuya, Izumi Keisuke

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2017 巻 ( 3 )   2017年3月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

25. Shiromizu Tetsuya, Tomikawa Yoshimune, Izumi Keisuke, Yoshino Hirotaka

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2017 巻 ( 3 )   2017年3月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

26. Roberto Emparan, Keisuke Izumi, Raimon Luna, Ryotaku Suzuki, Kentaro Tanabe

Journal of High Energy Physics   ( 6 )   2016年2月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER

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.

27. Toshiaki Fujimori, Takeo Inami, Keisuke Izumi, Tomotaka Kitamura

2016年1月

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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.

28. Toshiaki Fujimori, Takeo Inami, Keisuke Izumi, Tomotaka Kitamura

Progress of Theoretical and Experimental Physics   ( 1 )   2015年10月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：OXFORD UNIV PRESS INC

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.

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

Che-Min Shen, Keisuke Izumi, Pisin Chen

PHYSICAL REVIEW D   92 巻 ( 4 )   2015年8月

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記述言語：英語   出版者・発行元：AMER PHYSICAL SOC

30. Che-Min Shen, Keisuke Izumi, Pisin Chen

PHYSICAL REVIEW D   92 巻 ( 2 )   2015年7月

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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.

31. Keisuke Izumi

Physical Review D   91 巻 ( 12 )   2015年6月

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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 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 counter terms 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 counter terms to appear. The
symmetry requirements might have important implications for the construction of
matter field sectors in the Horava-Lifshitz gravity.

32. Keisuke Izumi

The European Physical Journal C   75 巻 ( 6 )   2015年6月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：SPRINGER

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
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.

33. Tomikawa Yoshimune, Izumi Keisuke, Shiromizu Tetsuya

PHYSICAL REVIEW D   91 巻 ( 10 )   2015年5月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

34. Che-Min Shen, Keisuke Izumi, Pisin Chen

Physical Review D     2015年5月

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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.

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

MODERN PHYSICS LETTERS A   30 巻 ( 3-4 )   2015年2月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：WORLD SCIENTIFIC PUBL CO PTE LTD

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.

36. 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

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記述言語：英語   掲載種別：研究論文（学術雑誌）

37. Pisin Chen, Keisuke Izumi, James M. Nester, Yen Chin Ong

PHYSICAL REVIEW D   91 巻 ( 6 )   2014年12月

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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

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 non-unique
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.

38. Wormhole on DGP brane 査読有り 国際誌

Tomikawa Yoshimune, Shiromizu Tetsuya, Izumi Keisuke

PHYSICAL REVIEW D   90 巻 ( 12 )   2014年12月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

39. Izumi Keisuke, Shiromizu Tetsuya

PHYSICAL REVIEW D   90 巻 ( 4 )   2014年8月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

40. Keisuke Izumi

Physical Review D     2014年8月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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, $R_{AB}N^AN^B \ge 0$ for any null vector $N^A$, 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.

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

2013年12月

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Massive gravity is an old idea: trading geometry for mass. Much effort has<br />
been expended on establishing a healthy model, culminating in the current<br />
ghost-free version. We summarize here our recent findings -- that it is still<br />
untenable -- because it is locally acausal: CTC solutions can be constructed in<br />
a small neighborhood of any event.

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

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

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：ELSEVIER SCIENCE BV

Massive gravity (mGR) is a 5(= 2s + 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. (C) 2013 Elsevier B.V. All rights reserved.

43. Keisuke Izumi, Je-An Gu, Yen Chin Ong

PHYSICAL REVIEW D   89 巻 ( 8 )   2013年9月

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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 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.

44. Mariam Bouhmadi-Lopez, Yen-Wei Liu, Keisuke Izumi, Pisin Chen

PHYSICAL REVIEW D   89 巻 ( 6 )   2013年8月

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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

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.

45. Pisin Chen, Keisuke Izumi, Nien-En Tung

Physical Review D   88 巻 ( 12 )   2013年4月

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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 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 SKYC field equation. To
characterize the property of the dark radiation, we develop a method to
separate it from the ordinary radiation. We found 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 SKY equation. We also found that
his 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.

46. Keisuke Izumi, Yen Chin Ong

Classical and Quantum Gravity     2013年3月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We study the Cauchy problem in a special case of non-linear massive gravity:
the two-tensor "f-g" theory. 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 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 exist hypersurfaces that are not characteristic
hypersurface. Puzzles remain regarding the existence of a superluminal
propagating mode in both the f-g theory, as well as in the full non-linear
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.

47. Yen Chin Ong, Keisuke Izumi, James M. Nester, Pisin Chen

Physical Review D   88 巻 ( 2 )   2013年3月

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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

Teleparallel theories of gravity have a long history. They include a special
case referred to as the Teleparallel Equivalent of General Relativity (TEGR,
aka GR$_{\|}$). 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 FLRW spacetime in f(T) gravity. The time evolution from a FLRW
(and as a special case, Minkowski spacetime) initial condition is not unique.

48. Keisuke Izumi, Yen Chin Ong

Journal of Cosmology and Astroparticle Physics     2012年12月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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 $M_{\text{pl } }^2 \equiv 1/(8\pi G)$ that occurs in the latter has
now been replaced by an effective time-dependentgravitational coupling $-2 (df/dT)|_{T=T_0}$, with $T_0$ 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.

49. Chien-I Chiang, Keisuke Izumi, Pisin Chen

Journal of Cosmology and Astroparticle Physics   ( 12 )   2012年8月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP PUBLISHING LTD

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.

50. Keisuke Izumi, Shuntaro Mizuno, Kazuya Koyama

Physical Review D   85 巻 ( 2 )   2011年9月

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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 calculate the shape correlations between trispectra in various equilateral
non-Gaussian models, including DBI inflation, ghost inflation and Lifshitz
scalars, using the full trispectrum as well as the reduced trispectum. 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 DBI inflation and a Lifshitz scalar
model. We introduce an estimator for the amplitude of the trispectrum, $g_{\rm NL} ^{equil}$ and relate it to model parameters in various models. Using
constraints on $g_{\rm NL} ^{equil}$ from WMAP5, we give constraints on the
model parameters.

51. Hideo Iguchi, Keisuke Izumi, Takashi Mishima

Progress of Theoretical Physics Supplement   ( 189 ) 頁： 93 - 125   2011年6月

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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 THEORETICAL PHYSICS PUBLICATION OFFICE

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.

52. Keisuke Izumi, Shinji Mukohyama

Physical Review D   84 巻 ( 6 )   2011年5月

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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 perform a fully nonlinear analysis of superhorizon perturbation in
Ho\v{r}ava-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 $\lambda\to 1$ and recovers general relativity coupled to dark matter at low energy.
Finally, we propose a definition of nonlinear curvature perturbation ${\cal R}$
in Ho\v{r}ava-Lifshitz gravity and show that it is conserved up to the first

53. Keisuke Izumi, Takeshi Kobayashi, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   ( 10 )   2010年8月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP PUBLISHING LTD

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 f_NL = 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.

54. Keisuke Izumi, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   ( 6 )   2010年4月

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP PUBLISHING LTD

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 $f_{NL}^{equil}$ is typically of
order $O(10^2)$. In this paper we calculate trispectrum from ghost inflation
and show that the corresponding nonlinear parameter $\tau_{NL}$ is typically of
order $O(10^4)$. 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.

55. Keisuke Izumi, Shinji Mukohyama

Physical Review D   81 巻 ( 4 )   2009年11月

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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

In Horava-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. We also comment on the condition under
which linear instability of the scalar graviton does not show up.

56. Keisuke Izumi, Takahiro Tanaka

Progress of Theoretical Physics   121 巻 ( 2 ) 頁： 427 - 436   2008年10月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We revisit the problem of the helicity-0 ghost mode of 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 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 due to unsuppressed ultra-violet(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 which 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
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 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 negligiblly small as long as only the
physics below the cutoff scale is concerned.

57. 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

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記述言語：英語

58. Keisuke Izumi

Progress of Theoretical Physics   119 巻 ( 5 ) 頁： 757 - 774   2007年12月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

We construct a five dimensional exact solution of the orthogonal black
di-ring which has two black rings whose $S^1$-rotating planes are orthogonal.
This solution has four free parameters which represent radii of and speeds of
$S^1$-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.

59. Keisuke Izumi, Kazuya Koyama, Oriol Pujolàs, Takahiro Tanaka

Physical Review D - Particles, Fields, Gravitation and Cosmology   76 巻 ( 10 )   2007年11月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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.

60. Keisuke Izumi, Takahiro Tanaka

Progress of Theoretical Physics   121 巻 ( 2 ) 頁： 419 - 426   2007年9月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

In this letter we point out that the massive gravity theory with a graviton
ghost mode in 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 non-ghost modes, the choice of a ghost mode is not
unique. However, 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 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
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.

61. Keisuke Izumi, Jiro Soda

Physical Review D   76 巻 ( 8 )   2007年6月

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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 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.

62. Keisuke Izumi, Kazuya Koyama, Takahiro Tanaka

Journal of High Energy Physics   2007 巻 ( 4 )   2006年10月

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記述言語：英語   掲載種別：研究論文（学術雑誌）

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-0 and spin-2 ghosts remains. As a result we find we cannot avoid the
appearance of ghost by two-branes model.

63. 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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記述言語：英語

### 講演・口頭発表等 44

1. Keisuke Izumi

LeCosPA 4th International Symposium Unity of Physics – From Plasma Wakefields to Black Holes  2021年12月3日

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開催年月日： 2021年11月 - 2021年12月

記述言語：英語   会議種別：口頭発表（招待・特別）

開催地：LeCosPA   国名：台湾

2. Area bound for surfaces in general relativity 国際会議

Keisuke Izumi

Strings and Fields 2021  2021年8月23日

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開催年月日： 2021年8月

記述言語：英語   会議種別：口頭発表（一般）

3. Area bound for surfaces in generic gravitational field 国際会議

K. Izumi

Online JGRG Webinar Series

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開催年月日： 2021年3月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

4. S行列ユニタリ性と繰り込み可能性

阿部　裕悟、稲見　武夫、泉　圭介

日本物理学会　第 76 回年次大会

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開催年月日： 2021年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

5. Causal structures in Gauss-Bonnet gravity 招待有り 国際会議

Keisuke Izumi

Miyazaki Workshop on Particle Physics and Cosmology in 2020

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開催年月日： 2020年1月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

6. Causal structures in Gauss-Bonnet gravity 招待有り 国際会議

Keisuke Izumi

4th Korea-Japan bilateral workshop on String axion cosmology

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開催年月日： 2020年1月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

7. Invertible field transformations with derivatives: necessary and sufficient conditions 招待有り 国際会議

Keisuke Izumi

Gravitation and Mathematical Relativity

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開催年月日： 2019年12月

記述言語：英語   会議種別：口頭発表（一般）

国名：中華人民共和国

8. Causal structures in Gauss-Bonnet gravity 招待有り 国際会議

Keisuke Izumi

the APCTP workshop of Gravity, Strings and QM Matter

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開催年月日： 2019年11月

記述言語：英語   会議種別：口頭発表（一般）

国名：大韓民国

9. Invertible field transformations with derivatives: necessary and suﬃcient conditions 招待有り 国際会議

Keisuke Izumi

3rd Korea-Japan bilateral workshop on String axion cosmology

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開催年月日： 2019年10月

記述言語：英語   会議種別：口頭発表（一般）

国名：大韓民国

10. Penrose inequality for photon surface 招待有り 国際会議

Keisuke Izumi

Mini-workshop "Black holes and neutron stars imagination and reality"

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開催年月日： 2019年9月

記述言語：英語   会議種別：口頭発表（一般）

国名：ベトナム社会主義共和国

11. S-matrix Unitarity and Renormalizability 招待有り 国際会議

Keisuke Izumi

1-day Workshop on Quantum Gravity

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開催年月日： 2019年7月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

12. Locality and Causality 招待有り 国際会議

Keisuke Izumi

The 2nd Korea-Japan bilateral workshop on String Axion Cosmology

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開催年月日： 2019年1月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

13. Loosely trapped surface 招待有り 国際会議

Keisuke Izumi

Korea-Japan bilateral workshop on String axion cosmology

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開催年月日： 2018年11月

記述言語：英語   会議種別：口頭発表（一般）

国名：大韓民国

14. On uniqueness of static black hole in Einstein-conformal scalar 国際会議

Keisuke Izumi

Korea-Japan bilateral workshop on String axion cosmology

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開催年月日： 2018年11月

記述言語：英語   会議種別：口頭発表（一般）

国名：大韓民国

15. S-matrix Unitarity and Renormalizability 国際会議

Keisuke Izumi

Essential next steps for gravity and cosmology

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開催年月日： 2018年6月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

16. Optical theorem and renormalizability 国際会議

Keisuke Izumi

Gravitational Dynamics and Black Holes

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開催年月日： 2017年9月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

17. 因果構造解析 招待有り

泉　圭介

相対論宇宙論 東北研究会 2017

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開催年月日： 2017年8月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

18. トレースアノマリー効果を古典作用で表す際の境界項

泉　圭介、 C.-M. Shen、 P. Chen

日本物理学会　第 72 回年次大会

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開催年月日： 2017年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

19. Theoretical constraints on modified theories of gravity 国際会議

Keisuke Izumi

The 3rd KMI International Symposium on "Quest for the Origin of Particles and the Universe"

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開催年月日： 2017年1月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

20. Causal structures in Gauss-Bonnet gravity 国際会議

K. Izumi

General relativity in higher dimensions -recent progress and future perspective

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開催年月日： 2016年11月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

21. Tree-Unitarity and renormalizability in Lifshitz-scaling theory -- as a toy model of Horava's gravity theory 国際会議

T. Fujimori, T. Inami, K. Izumi, T. Kitamura

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開催年月日： 2015年12月

記述言語：英語   会議種別：口頭発表（一般）

国名：台湾

22. Renormalization and Unitarity in Lifshitz-scaling theory 国際会議

Keisuke Izumi

the 2nd mini-workshop on gravity and cosmology

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開催年月日： 2015年10月

記述言語：英語   会議種別：口頭発表（一般）

国名：フランス共和国

23. Causal structures in Massive gravity and Gauss-Bonnet gravity 招待有り 国際会議

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

Superluminality in Effective Field Theories for Cosmology

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開催年月日： 2015年4月

記述言語：英語   会議種別：口頭発表（一般）

国名：カナダ

24. Causality in modified gravity theory 国際会議

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

19th International Symposium on Particles, Strings and Cosmology

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開催年月日： 2013年11月

記述言語：英語   会議種別：口頭発表（一般）

国名：台湾

25. Is there acausality in Massive Gravity? 国際会議

K. Izumi, Y. C. Ong

Massive gravity and its cosmological implications

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開催年月日： 2013年4月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

26. Cosmological Perturbation in f(T) gravity Revisited and Importance of Nonlinearity 国際会議

K. Izumi, Y. C. Ong

The 4th APCosPA Winter School/Workshop on Cosmology and Particle

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開催年月日： 2013年1月 - 2013年2月

記述言語：英語   会議種別：口頭発表（一般）

国名：台湾

27. Trispectrum estimation in various models of equilateral type non-Gaussianity 国際会議

K. Izumi, S. Mizuno, K. Koyama

2012 Asia Pacific School/Workshop on Cosmology and Gravitation

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開催年月日： 2012年3月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

28. Nonlinear analysis in Hořava gravity 国際会議

Gravity, Lorentz Violation & Superluminality

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開催年月日： 2011年7月

記述言語：英語   会議種別：口頭発表（一般）

国名：ロシア連邦

29. Non-Gaussianity from Horava-Lifshitz Cosmology

泉　圭介、小林　洸、向山　信治

日本物理学会２０１０年秋季大会

詳細を見る

開催年月日： 2010年9月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

30. Trispectrum in ghost inflation 国際会議

K. Izumi, S. Mukohyama

The non-Gaussian universe

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開催年月日： 2010年3月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

31. Stellar center is dynamical in Horava-Lifshitz gravity

泉　圭介、向山　信治

日本物理学会第６５回年次大会

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開催年月日： 2010年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

32. 厳密解の系統的構成

泉　圭介

基研研究会 高次元Black Hole研究最前線

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開催年月日： 2009年12月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

33. No static star solution in Horava Gravity 国際会議

K. Izumi, S. Mukohyama

The 19th Workshop on General Relativity and Gravitation in Japan

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開催年月日： 2009年11月 - 2009年12月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

34. de Sitter時空中のスピン2ゴースト

泉　圭介、　田中　貴浩

日本物理学会第６４回年次大会

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開催年月日： 2009年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

35. About the ghost in DGP brane-world 国際会議

The 18th Workshop on General Relativity and Gravitation in Japan

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開催年月日： 2008年11月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

36. Orthogonal black di-ring solution

泉　圭介

日本物理学会２００８年春季大会

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開催年月日： 2008年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

37. Renormalized Newtonian Cosmic Evolution 国際会議

The Theoretical Modeling of Cosmic Structure Formation and its Recent Progress

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開催年月日： 2008年3月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

38. Renormalized Newtonian Cosmic Evolution with Primordial Non-Gaussianity 国際会議

K. Izumi and J. Soda

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開催年月日： 2007年12月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

39. Bubbles in DGP

泉　圭介、小山　和哉、田中　貴浩、Oriol Pujolas

日本物理学会第６２回年次大会

詳細を見る

開催年月日： 2007年9月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

40. ABOUT THE GHOST IN THE MASSIVE GRAVITY THEORY 国際会議

K. Izumi and T. Tanaka

The 18th International Conference on General Relativity and Gravitation

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開催年月日： 2007年7月

記述言語：英語   会議種別：口頭発表（一般）

国名：オーストラリア連邦

41. DGPブレーンワールドのゴーストについて

泉　圭介、小山　和哉、田中　貴浩

日本物理学会２００７年春季大会

詳細を見る

開催年月日： 2007年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

42. Unexorcized ghost in DGP brane world 国際会議

K. Izumi, K. Koyama, T. Tanaka

Brane-World Gravity

詳細を見る

開催年月日： 2006年9月

記述言語：英語   会議種別：口頭発表（一般）

国名：グレートブリテン・北アイルランド連合王国(英国)

43. DGPブレーンワールドにおけるゴースト粒子の評価

泉　圭介、　田中　貴浩

日本物理学会第６１回年次大会

詳細を見る

開催年月日： 2006年3月

記述言語：日本語   会議種別：口頭発表（一般）

国名：日本国

44. Avoidance of ghost problem in DGP braneworld model 国際会議

K. Izumi, T. Tanaka

The Fifteenth Workshop on General Relativity and Gravitation in Japan

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開催年月日： 2005年11月 - 2005年12月

記述言語：英語   会議種別：口頭発表（一般）

国名：日本国

### 科研費 3

1. 研究課題/研究課題番号：20H01902  2020年4月 - 2024年3月

日本学術振興会  科学研究費助成事業  基盤研究(B)

野海 俊文, 早田 次郎, 泉 圭介

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担当区分：研究分担者  資金種別：競争的資金

配分額：678000円 （ 直接経費：600000円 、 間接経費：78000円 ）

量子重力理論の構築は現代物理学の最重要課題の１つであるが、その実験的検証は重要かつ挑戦的な問題である。特に、近年の研究では「量子重力理論に基づく素粒子論・宇宙論模型が満たすべき性質は何か？」という問いを投げかけることでこの難問に挑もうという機運が高まっている。
そのような中で提案された「弱い重力予想」は「すべての相互作用の中で重力が一番弱い」ことを主張し、理論模型のパラメータに様々な上限や下限を与える。本研究では、弱い重力予想の証明に向けた理論検証を行い、またこれと並行して素粒子論・宇宙論への応用を行う。これにより弱い重力予想を鍵とした量子重力理論への現象論的アプローチを開拓する。

2. 研究課題/研究課題番号：17H01091  2017年4月 - 2022年3月

科学研究費助成事業  基盤研究(A)

山田 澄生, 白水 徹也, 儀我 美一, 松本 幸夫, 小野寺 有紹, 三石 史人, 泉 圭介, 野澤 真人

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担当区分：研究分担者  資金種別：競争的資金

配分額：1690000円 （ 直接経費：1300000円 、 間接経費：390000円 ）

研究計画第3年度の本年度は、11月に開催した国際研究集会"Geometric Analysis and General Ralativity"を中心として、アインシュタイン方程式を満たす時空の幾何学的構造に関する研究活動を進めた。
昨年度に引き続いて、研究連携者であるM.Khuri氏およびG.Weinstein氏とともに、５次元、真空、アインシュタイン方程式の定常解の構成を調和写像を用いて行った。このエルンスト形式とよばれる方法論で構成されるアインシュタイン時空には、幾何学的な特異点が内在しており、それら特異性の除去をする方法論について2018年度に引き続いて研究の進展を図った。これらの業績は論文として執筆中である。
東京大学駒場キャンパスにおいて行われた国際研究集会"Geometric Analysis and General Ralativity"においては、海外から7名の講演者に加え、国内から若手の研究者を募り、一般相対性理論と幾何解析に関する分野融合的な情報発信を行った。本研究課題の中心となるアインシュタイン方程式を介して、国外からの参加者からも、学際的な切り口は好評であり、国境および世代を超えた活発な議論の場を提供できたことは、本研究グループにとっても、大きな手応えとなった。
研究代表者は、本研究課題とは独立に距離空間の幾何学に関わってきた。時間的な距離空間というH. BusemannおよびA.D.Alexandrovによって半世紀前に提唱された分野においての研究代表者の近年の研究が、期せずして本研究課題と関連するに至り、本年度には、Timelike Funk and Hilbert Geometriesという論文を発表した。
2018年度から2019年度にかけて、研究代表者は、研究分担者及び連携研究者との共同研究として3本の論文の発表・出版を介して、非自明な事象の地平線を内包する5次元アインシュタイン定常時空の構成法を情報発信した。これらは、幾何解析の観点から、数理物理学の分野に新たな視点を提供したことで、国際的に注目を浴びている。1916年に発表されたヘルマン・ワイルのアインシュタイン方程式の厳密解の変分的な構成法に根ざした調和写像を介した５次元アインシュタイン時空の構成は、既知の厳密解の各論に囚われない統括的な特徴から、現在多くの新しい問題が提起されるに至っており、2020年度を含む3年間の研究期間における具体的な研究課題が定式化されており、申請時に計画した課題の進捗状況は良好であると判断する。
また5次元時空内の時間一定面として現れる4次元リーマン多様体内における曲率流等の偏微分方程式の方法論の定式化を、昨年度に継続して行い、曲率流の特異点と単調性公式の関連性を定式化することを目標とする。

3. 弦理論・量子重力理論に現れる超光速伝搬の解析

研究課題/研究課題番号：17K14281  2017年4月 - 2020年3月

泉 圭介

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担当区分：研究代表者  資金種別：競争的資金

配分額：4160000円 （ 直接経費：3200000円 、 間接経費：960000円 ）

本研究では、量子重力理論の理解に向けて、古典因果構造とその量子的対応物であるユニタリ性を調べた。
古典的研究では、静的時空でのみ定義される光球面の概念を動的時空に拡張し、その面積に上限があることを示した。また、拡張された光球面に対してフープ仮説が成立することを、具体例で見た。
量子的研究では、ユニタリ性と繰り込み可能性の関係性についての研究を行った。この関係性については、高階微分理論がその反例になっていることが知られている。本研究では、S行列ユニタリ性という概念を導入し、それと繰り込み可能性が関連するという新たな仮説を立て、高階微分理論でも新たな仮説であれば成り立つことを具体例で示した。
古典的研究で扱った光球面は、ブラックホールの影の観測で見ている対象である。それを深く理解することで、重力理論の検証を精密に行うことができるようになった。
一方、ユニタリ性や繰り込み可能性は、量子重力理論を構築する際に克服するべき重要な問題点である。本研究は、その間の関係性を見出し、量子重力理論の理解につながる結果を得た。

1. 数学演習Ⅰ

2020

2. 数学演習Ⅰ

2019

3. 数学演習II

2018

4. 数学演習V

2017

5. 数学演習VI

2017

6. 数学演習V

2016

7. 数学演習VI

2016

### 社会貢献活動 7

1. 時空の幾何学

役割：出演,　講師

愛知県立豊田西高等学校  豊西総合大学  2021年11月

2. 都城高専　物理特別講演会 (外部講師講話)

役割：講師

2019年1月

詳細を見る

対象： 高校生

3. NHK カルチャー　名古屋教室　「数学散策」

役割：講師

2018年2月

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対象： 社会人・一般

4. 2017 年度名古屋大学数学公開講座

役割：講師

2017年10月

詳細を見る

対象： 高校生

5. 平成 29 年度名古屋大学数学アゴラ

役割：講師

2017年8月

詳細を見る

対象： 高校生

6. 第５８回名大祭　研究公開セミナー

役割：講師

2017年6月

詳細を見る

対象： 高校生,　大学生,　社会人・一般

7. NHK カルチャー　名古屋教室　「数学散策」

役割：講師

2017年5月

詳細を見る

対象： 社会人・一般