Updated on 2021/11/05

IZUMI Keisuke

Organization
Kobayashi-Maskawa Institute for the Origin of Particles and the Universe (KMI) Lecturer

### Degree 3

1. 博士（理学） （ 2009.3   京都大学 ）

2. 修士（理学） （ 2006.3   京都大学 ）

3. 学士（理学） （ 2004.3   京都大学 ）

1. 宇宙論

2. 重力理論

### Education 3

1. Kyoto University

2006.4 - 2009.3

2. Kyoto University

2004.4 - 2006.3

3. Kyoto University   Faculty of Science

2000.4 - 2004.3

### Papers 56

1. Asymptotic behavior of null geodesics near future null infinity: Significance of gravitational waves

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

PHYSICAL REVIEW D   Vol. 104 ( 6 )   2021.9

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2. Area bound for surfaces in generic gravitational field

Izumi Keisuke, Tomikawa Yoshimune, Shiromizu Tetsuya, Yoshino Hirotaka

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   Vol. 2021 ( 8 )   2021.8

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

Abe Yugo, Inami Takeo, Izumi Keisuke

MODERN PHYSICS LETTERS A   Vol. 36 ( 16 )   2021.5

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4. Holographic entanglement entropy of a de Sitter braneworld with Lovelock terms

Kushihara Kouki, Izumi Keisuke, Shiromizu Tetsuya

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   Vol. 2021 ( 4 )   2021.4

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5. Loosely trapped surface and dynamically transversely trapping surface in Einstein-Maxwell systems Reviewed International journal

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

Vol. 2020 ( 10 )   2020.10

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6. Kouki Kushihara, Keisuke Izumi, Tetsuya Shiromizu

2021.2

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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.

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

2021.1

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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.

8. Yugo Abe, Takeo Inami, Keisuke Izumi

2020.12

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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.

9. Formation of dynamically transversely trapping surfaces and the stretched hoop conjecture Reviewed International journal

Hirotaka Yoshino, Keisuke Izumi, Tetsuya Shiromizu, Yoshimune Tomikawa

Prog. Theor. Exp. Phys. (2020) 053E01   Vol. 2020 ( 5 )   2020.5

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A dynamically transversely trapping surface (DTTS) is a new concept of 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 $\mathcal{C}\lesssim 6\pi GM$ describes the condition for the DTTS
formation well, where $\mathcal{C}$ 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.

10. Transversely trapping surfaces: Dynamical version Reviewed International journal

Hirotaka Yoshino, Keisuke Izumi, Tetsuya Shiromizu, Yoshimune Tomikawa

Prog. Theor. Exp. Phys. (2020) 023E02   Vol. 2020 ( 2 )   2020.2

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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, $A_0\le 4\pi (3GM)^2$, under some assumptions. Differences and
connections between a DTTS and the other two concepts proposed by us
previously, a loosely trapped surface [arXiv:1701.00564] and a
static/stationary transversely trapping surface [arXiv:1704.04637], 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.

11. S-matrix Unitarity and Renormalizability in Higher Derivative Theories Reviewed International journal

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

Prog Theor Exp Phys (2019)   Vol. 2019 ( 8 )   2019.8

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We investigate the relation between the $S$-matrix unitarity
($SS^{\dagger}=1$) and the renormalizability, in theories with negative norm
states. The relation has been confirmed in many theories, such as gauge
theories, Einstein gravity and Lifshitz-type non-relativistic theories 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
essentially because the unitarity bound does not hold due to the negative norm
states. In these theories, it is not clear if the $S$-matrix unitarity provides
a nontrivial constraint related to the renormalizability. In this paper we
introduce scalar field models with a higher derivative kinetic term and analyze
the $S$-matrix unitarity. We have positive results of the relation.

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

2019.7

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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.

13. Divergence equations and uniqueness theorem of static black holes Reviewed International journal

Masato Nozawa, Tetsuya Shiromizu, Keisuke Izumi, Sumio Yamada

Classical and Quantum Gravity   Vol. 35 ( 17 )   2018.9

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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 $(n\ge 5)$ dimensional vacuum spacetimes, a central obstruction for
applicability of the current proof is the integration of the
$(n-2)$-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 $(n-1)$-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.

14. Matter scattering in $R_{μν}^2$ gravity and unitarity Reviewed International journal

Yugo Abe, Takeo Inami, Keisuke Izumi, Tomotaka Kitamura

Prog Theor Exp Phys (2018)   Vol. 2018 ( 3 )   2018.3

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

15. On uniqueness of static spacetimes with non-trivial conformal scalar field Reviewed International journal

Yoshimune Tomikawa, Tetsuya Shiromizu, Keisuke Izumi

Classical and Quantum Gravity   Vol. 34 ( 15 )   2017.8

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We discuss the uniqueness of the static spacetimes with non-trivial conformal
scalar field. Then, 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 photon(photon surface). In addition, we see
that multi-photon surfaces having the same scalar field values do not exist.

16. Extension of photon surfaces and their area: Static and stationary spacetimes Reviewed International journal

Hirotaka Yoshino, Keisuke Izumi, Tetsuya Shiromizu, Yoshimune Tomikawa

Progress of Theoretical and Experimental Physics   Vol. 2017 ( 6 )   2017.6

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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 $A_0$ of a TTS is proved to be bounded as
$A_0\le 4\pi(3GM)^2$ 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 [arXiv:1701.00564] is also examined.

17. On uniqueness of static black hole with conformal scalar hair Reviewed International journal

Yoshimune Tomikawa, Tetsuya Shiromizu, Keisuke Izumi

Progress of Theoretical and Experimental Physics   Vol. 2017 ( 3 )   2017.3

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

18. Area bound for a surface in a strong gravity region Reviewed International journal

Tetsuya Shiromizu, Yoshimune Tomikawa, Keisuke Izumi, Hirotaka Yoshino

Progress of Theoretical and Experimental Physics   Vol. 2017 ( 3 )   2017.3

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For asymptotically flat spacetimes, using the inverse mean curvature flow, we
show that any compact $2$-surface, $S_0$, whose mean curvature and its
derivative for outward direction are positive in spacelike hypersurface with
non-negative Ricci scalar satisfies the inequality $A_0 \leq 4 \pi (3Gm)^2$,
where $A_0$ is the area of $S_0$ and $m$ is the total mass. The upper bound is
realized when $S_0$ is the photon sphere in a hypersurface isometric to
$t=$const. slice of the Schwarzschild spacetime.

19. Hydro-elastic Complementarity in Black Branes at large D Reviewed International journal

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

Journal of High Energy Physics   ( 6 )   2016.2

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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.

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

2016.1

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

21. Tree-Level Unitarity and Renormalizability in Lifshitz Scalar Theory Reviewed International journal

Toshiaki Fujimori, Takeo Inami, Keisuke Izumi, Tomotaka Kitamura

Progress of Theoretical and Experimental Physics   ( 1 )   2015.10

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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.

22. Boundary effect of anomaly-induced action (vol 92, 024035, 2015) Reviewed

Che-Min Shen, Keisuke Izumi, Pisin Chen

PHYSICAL REVIEW D   Vol. 92 ( 4 )   2015.8

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

PHYSICAL REVIEW D   Vol. 92 ( 2 )   2015.7

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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.

24. Power-counting and renormalizability in Lifshitz scalar theory Reviewed International journal

Keisuke Izumi

Physical Review D   Vol. 91 ( 12 )   2015.6

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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.

25. Ghosts in the self-accelerating DGP branch with Gauss–Bonnet effect Reviewed International journal

Keisuke Izumi

The European Physical Journal C   Vol. 75 ( 6 )   2015.6

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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.

26. New definition of a wormhole throat Reviewed International journal

Yoshimune Tomikawa, Keisuke Izumi, Tetsuya Shiromizu

PHYSICAL REVIEW D   Vol. 91 ( 10 )   2015.5

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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.

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

Physical Review D     2015.5

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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.

28. Problems of massive gravities Reviewed International journal

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

MODERN PHYSICS LETTERS A   Vol. 30 ( 3-4 )   2015.2

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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.

29. Boundary effect of anomaly-induced action Reviewed International journal

Che-Min Shen, Keisuke Izumi, Pisin Chen

Physical Review D     2015

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30. Remnant Symmetry, Propagation and Evolution in f(T) Gravity Reviewed International journal

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

PHYSICAL REVIEW D   Vol. 91 ( 6 )   2014.12

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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.

31. Wormhole on DGP brane Reviewed International journal

Yoshimune Tomikawa, Tetsuya Shiromizu, Keisuke Izumi

PHYSICAL REVIEW D   Vol. 90 ( 12 )   2014.12

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We analyze a spacetime structure on a new brane configuration constructed
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 10^{10}cm or so.

32. DGP braneworld with a bubble of nothing Reviewed International journal

Keisuke Izumi, Tetsuya Shiromizu

PHYSICAL REVIEW D   Vol. 90 ( 4 )   2014.8

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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.

33. Causal structures in Gauss-Bonnet gravity Reviewed International journal

Keisuke Izumi

Physical Review D     2014.8

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

2013.12

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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.

35. Massive gravity acausality redux Reviewed International journal

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

Physics Letters B   Vol. 726 ( 1-3 ) page： 544 - 548   2013.10

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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.

36. Acausality and Nonunique Evolution in Generalized Teleparallel Gravity Reviewed International journal

Keisuke Izumi, Je-An Gu, Yen Chin Ong

PHYSICAL REVIEW D   Vol. 89 ( 8 )   2013.9

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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.

37. Tensor Perturbations from Brane-World Inflation with Curvature Effects Reviewed International journal

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

PHYSICAL REVIEW D   Vol. 89 ( 6 )   2013.8

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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.

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

Physical Review D   Vol. 88 ( 12 )   2013.4

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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.

39. An Analysis of Characteristics in Non-Linear Massive Gravity Reviewed International journal

Keisuke Izumi, Yen Chin Ong

Classical and Quantum Gravity     2013.3

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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.

40. Problems with Propagation and Time Evolution in f(T) Gravity Reviewed International journal

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

Physical Review D   Vol. 88 ( 2 )   2013.3

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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.

41. Cosmological Perturbation in f(T) Gravity Revisited Reviewed International journal

Keisuke Izumi, Yen Chin Ong

Journal of Cosmology and Astroparticle Physics     2012.12

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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.

42. Spherically symmetric analysis on open FLRW solution in non-linear massive gravity Reviewed International journal

Chien-I Chiang, Keisuke Izumi, Pisin Chen

Journal of Cosmology and Astroparticle Physics   ( 12 )   2012.8

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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.

43. Trispectrum estimation in various models of equilateral type non-Gaussianity Reviewed International journal

Keisuke Izumi, Shuntaro Mizuno, Kazuya Koyama

Physical Review D   Vol. 85 ( 2 )   2011.9

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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.

44. Systematic solution-generation of five-dimensional black holes Reviewed International journal

Hideo Iguchi, Keisuke Izumi, Takashi Mishima

Progress of Theoretical Physics Supplement   ( 189 ) page： 93 - 125   2011.6

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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.

45. Nonlinear superhorizon perturbations in Horava-Lifshitz gravity Reviewed International journal

Keisuke Izumi, Shinji Mukohyama

Physical Review D   Vol. 84 ( 6 )   2011.5

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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

46. Non-Gaussianity from Lifshitz Scalar Reviewed International journal

Keisuke Izumi, Takeshi Kobayashi, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   ( 10 )   2010.8

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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.

47. Trispectrum from Ghost Inflation Reviewed International journal

Keisuke Izumi, Shinji Mukohyama

Journal of Cosmology and Astroparticle Physics   ( 6 )   2010.4

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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.

48. Stellar center is dynamical in Horava-Lifshitz gravity Reviewed International journal

Keisuke Izumi, Shinji Mukohyama

Physical Review D   Vol. 81 ( 4 )   2009.11

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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.

49. Particle production in models with helicity-0 graviton ghost in de Sitter spacetime Reviewed International journal

Keisuke Izumi, Takahiro Tanaka

Progress of Theoretical Physics   Vol. 121 ( 2 ) page： 427 - 436   2008.10

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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.

50. 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

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51. Orthogonal black di-ring solution Reviewed International journal

Keisuke Izumi

Progress of Theoretical Physics   Vol. 119 ( 5 ) page： 757 - 774   2007.12

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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.

52. Bubbles in the self-accelerating universe Reviewed International journal

Keisuke Izumi, Kazuya Koyama, Oriol Pujolàs, Takahiro Tanaka

Physical Review D - Particles, Fields, Gravitation and Cosmology   Vol. 76 ( 10 )   2007.11

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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.

53. No de Sitter invariant vacuum in massive gravity theory with ghost Reviewed International journal

Keisuke Izumi, Takahiro Tanaka

Progress of Theoretical Physics   Vol. 121 ( 2 ) page： 419 - 426   2007.9

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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.

54. Renormalized Newtonian Cosmic Evolution with Primordial Non-Gaussianity Reviewed International journal

Keisuke Izumi, Jiro Soda

Physical Review D   Vol. 76 ( 8 )   2007.6

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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.

55. Unexorcized ghost in DGP brane world Reviewed International journal

Keisuke Izumi, Kazuya Koyama, Takahiro Tanaka

Journal of High Energy Physics   Vol. 2007 ( 4 )   2006.10

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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.

56. 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     page： 135 - 138   2005

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### Presentations 42

1. Area bound for surfaces in generic gravitational field International conference

K. Izumi

Online JGRG Webinar Series

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Event date： 2021.3

Language：English   Presentation type：Oral presentation (general)

Country：Japan

2. 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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Event date： 2021.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

3. Causal structures in Gauss-Bonnet gravity Invited International conference

Keisuke Izumi

Miyazaki Workshop on Particle Physics and Cosmology in 2020

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Event date： 2020.1

Language：English   Presentation type：Oral presentation (general)

Country：Japan

4. Causal structures in Gauss-Bonnet gravity Invited International conference

Keisuke Izumi

4th Korea-Japan bilateral workshop on String axion cosmology

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Event date： 2020.1

Language：English   Presentation type：Oral presentation (general)

Country：Japan

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

Keisuke Izumi

Gravitation and Mathematical Relativity

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Event date： 2019.12

Language：English   Presentation type：Oral presentation (general)

Country：China

6. Causal structures in Gauss-Bonnet gravity Invited International conference

Keisuke Izumi

the APCTP workshop of Gravity, Strings and QM Matter

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Event date： 2019.11

Language：English   Presentation type：Oral presentation (general)

Country：Korea, Republic of

7. Invertible field transformations with derivatives: necessary and suﬃcient conditions Invited International conference

Keisuke Izumi

3rd Korea-Japan bilateral workshop on String axion cosmology

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Event date： 2019.10

Language：English   Presentation type：Oral presentation (general)

Country：Korea, Republic of

8. Penrose inequality for photon surface Invited International conference

Keisuke Izumi

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

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Event date： 2019.9

Language：English   Presentation type：Oral presentation (general)

Country：Viet Nam

9. S-matrix Unitarity and Renormalizability Invited International conference

Keisuke Izumi

1-day Workshop on Quantum Gravity

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Event date： 2019.7

Language：English   Presentation type：Oral presentation (general)

Country：Japan

10. Locality and Causality Invited International conference

Keisuke Izumi

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

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Event date： 2019.1

Language：English   Presentation type：Oral presentation (general)

Country：Japan

11. Loosely trapped surface Invited International conference

Keisuke Izumi

Korea-Japan bilateral workshop on String axion cosmology

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Event date： 2018.11

Language：English   Presentation type：Oral presentation (general)

Country：Korea, Republic of

12. On uniqueness of static black hole in Einstein-conformal scalar International conference

Keisuke Izumi

Korea-Japan bilateral workshop on String axion cosmology

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Event date： 2018.11

Language：English   Presentation type：Oral presentation (general)

Country：Korea, Republic of

13. S-matrix Unitarity and Renormalizability International conference

Keisuke Izumi

Essential next steps for gravity and cosmology

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Event date： 2018.6

Language：English   Presentation type：Oral presentation (general)

Country：Japan

14. Optical theorem and renormalizability International conference

Keisuke Izumi

Gravitational Dynamics and Black Holes

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Event date： 2017.9

Language：English   Presentation type：Oral presentation (general)

Country：Japan

15. 因果構造解析 Invited

泉　圭介

相対論宇宙論 東北研究会 2017

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Event date： 2017.8

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

16. 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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Event date： 2017.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

17. Theoretical constraints on modified theories of gravity International conference

Keisuke Izumi

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

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Event date： 2017.1

Language：English   Presentation type：Oral presentation (general)

Country：Japan

18. Causal structures in Gauss-Bonnet gravity International conference

K. Izumi

General relativity in higher dimensions -recent progress and future perspective

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Event date： 2016.11

Language：English   Presentation type：Oral presentation (general)

Country：Japan

19. Tree-Unitarity and renormalizability in Lifshitz-scaling theory -- as a toy model of Horava's gravity theory International conference

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

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Event date： 2015.12

Language：English   Presentation type：Oral presentation (general)

Country：Taiwan, Province of China

20. Renormalization and Unitarity in Lifshitz-scaling theory International conference

Keisuke Izumi

the 2nd mini-workshop on gravity and cosmology

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Event date： 2015.10

Language：English   Presentation type：Oral presentation (general)

Country：France

21. Causal structures in Massive gravity and Gauss-Bonnet gravity Invited International conference

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

Superluminality in Effective Field Theories for Cosmology

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Event date： 2015.4

Language：English   Presentation type：Oral presentation (general)

22. Causality in modified gravity theory International conference

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

19th International Symposium on Particles, Strings and Cosmology

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Event date： 2013.11

Language：English   Presentation type：Oral presentation (general)

Country：Taiwan, Province of China

23. Is there acausality in Massive Gravity? International conference

K. Izumi, Y. C. Ong

Massive gravity and its cosmological implications

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Event date： 2013.4

Language：English   Presentation type：Oral presentation (general)

Country：Japan

24. Cosmological Perturbation in f(T) gravity Revisited and Importance of Nonlinearity International conference

K. Izumi, Y. C. Ong

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

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Event date： 2013.1 - 2013.2

Language：English   Presentation type：Oral presentation (general)

Country：Taiwan, Province of China

25. Trispectrum estimation in various models of equilateral type non-Gaussianity International conference

K. Izumi, S. Mizuno, K. Koyama

2012 Asia Pacific School/Workshop on Cosmology and Gravitation

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Event date： 2012.3

Language：English   Presentation type：Oral presentation (general)

Country：Japan

26. Nonlinear analysis in Hořava gravity International conference

Gravity, Lorentz Violation & Superluminality

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Event date： 2011.7

Language：English   Presentation type：Oral presentation (general)

Country：Russian Federation

27. 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

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Event date： 2010.9

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

28. Trispectrum in ghost inflation International conference

K. Izumi, S. Mukohyama

The non-Gaussian universe

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Event date： 2010.3

Language：English   Presentation type：Oral presentation (general)

Country：Japan

29. 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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Event date： 2010.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

30. 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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Event date： 2009.12

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

31. No static star solution in Horava Gravity International conference

K. Izumi, S. Mukohyama

The 19th Workshop on General Relativity and Gravitation in Japan

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Event date： 2009.11 - 2009.12

Language：English   Presentation type：Oral presentation (general)

Country：Japan

32. 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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Event date： 2009.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

33. About the ghost in DGP brane-world International conference

The 18th Workshop on General Relativity and Gravitation in Japan

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Event date： 2008.11

Language：English   Presentation type：Oral presentation (general)

Country：Japan

34. Orthogonal black di-ring solution

泉　圭介

日本物理学会２００８年春季大会

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Event date： 2008.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

35. Renormalized Newtonian Cosmic Evolution International conference

The Theoretical Modeling of Cosmic Structure Formation and its Recent Progress

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Event date： 2008.3

Language：English   Presentation type：Oral presentation (general)

Country：Japan

36. Renormalized Newtonian Cosmic Evolution with Primordial Non-Gaussianity International conference

K. Izumi and J. Soda

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Event date： 2007.12

Language：English   Presentation type：Oral presentation (general)

Country：Japan

37. 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

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Event date： 2007.9

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

38. ABOUT THE GHOST IN THE MASSIVE GRAVITY THEORY International conference

K. Izumi and T. Tanaka

The 18th International Conference on General Relativity and Gravitation

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Event date： 2007.7

Language：English   Presentation type：Oral presentation (general)

Country：Australia

39. 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

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Event date： 2007.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

40. Unexorcized ghost in DGP brane world International conference

K. Izumi, K. Koyama, T. Tanaka

Brane-World Gravity

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Event date： 2006.9

Language：English   Presentation type：Oral presentation (general)

Country：United Kingdom

41. 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

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Event date： 2006.3

Language：Japanese   Presentation type：Oral presentation (general)

Country：Japan

42. Avoidance of ghost problem in DGP braneworld model International conference

K. Izumi, T. Tanaka

The Fifteenth Workshop on General Relativity and Gravitation in Japan

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Event date： 2005.11 - 2005.12

Language：English   Presentation type：Oral presentation (general)

Country：Japan

### KAKENHI (Grants-in-Aid for Scientific Research) 5

1. 極限宇宙の物理法則を創る－量子情報で拓く時空と物質の新しいパラダイム

Grant number：21H05182  2021.9 - 2026.3

日本学術振興会  学術変革領域研究（Ａ）総括班

高柳　匡

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Authorship：Coinvestigator(s)  Grant type：Competitive

2. 量子情報を用いた量子宇宙の数理とその応用

Grant number：21H05189  2021.9 - 2026.3

学術変革領域研究（Ａ）分担班

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Authorship：Coinvestigator(s)  Grant type：Competitive

3. Grant number：20H01902  2020.4 - 2024.3

Japan Society for the Promotion of Science  Grants-in-Aid for Scientific Research  Grant-in-Aid for Scientific Research (B)

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Authorship：Coinvestigator(s)  Grant type：Competitive

Grant amount：\678000 （ Direct Cost: \600000 、 Indirect Cost：\78000 ）

4. Grant number：17H01091  2017.4 - 2022.3

Grants-in-Aid for Scientific Research  Grant-in-Aid for Scientific Research (A)

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Authorship：Coinvestigator(s)  Grant type：Competitive

Grant amount：\1690000 （ Direct Cost: \1300000 、 Indirect Cost：\390000 ）

5. Analysis of the superluminal modes by gravitational quantum effect

Grant number：17K14281  2017.4 - 2020.3

Izumi Keisuke

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Authorship：Principal investigator  Grant type：Competitive

Grant amount：\4160000 （ Direct Cost: \3200000 、 Indirect Cost：\960000 ）

The purpose of this project is to understand the quantum gravity by investigating the classical causality and its quantum object unitarity.
In the classical research, the concept of photon surface, that is defined only in static spacetimes, is extended to generic spacetimes. It has been shown that the area of the extended photon surface has an upper bound, and the hoop conjecture for it is established in some examples.
In the quantum research, the conjecture between unitarity and renormalizability is investigated. Although theories with higher derivatives are its counter examples, we have introduced a new concept S-matrix unitarity, and posited a new conjecture between S-matrix unitarity and renormalizability. We have seen our conjecture is established even in theories with higher derivatives.

### Teaching Experience (On-campus) 7

1. Exercise in Mathematics 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

### Social Contribution 6

1. 都城高専　物理特別講演会 (外部講師講話)

Role(s)：Lecturer

2019.1

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Audience： High school students

2. NHK カルチャー　名古屋教室　「数学散策」

Role(s)：Lecturer

2018.2

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Audience： General

3. 2017 年度名古屋大学数学公開講座

Role(s)：Lecturer

2017.10

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Audience： High school students

4. 平成 29 年度名古屋大学数学アゴラ

Role(s)：Lecturer

2017.8

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Audience： High school students

5. 第５８回名大祭　研究公開セミナー

Role(s)：Lecturer

2017.6

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Audience： High school students,　College students,　General

6. NHK カルチャー　名古屋教室　「数学散策」

Role(s)：Lecturer

2017.5

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Audience： General