Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevD.107.029902

Web of Science

Scopus

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevD.107.029901

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

DOI： 10.1103/PhysRevD.106.084014

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1103/PhysRevD.106.084007

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevD.106.084007

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1007/JHEP10(2022)050

Web of Science

Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevD.106.064028

Web of Science

Scopus

Language：Japanese  

DOI： 10.1103/PhysRevD.105.064074

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1103/PhysRevD.105.044037

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/2111.08237v1

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

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

DOI： 10.1093/ptep/ptab151

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/2109.00912v2

Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1103/PhysRevD.104.104025

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/2108.02588v1

Language：Japanese   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1103/PhysRevD.104.064025

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/2106.03150v3

Language：Japanese   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1093/ptep/ptab107

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/2107.13133v1

Language：Japanese  

DOI： 10.1093/ptep/ptab089

Web of Science

Language：Japanese  

DOI： 10.1142/S0217732321501054

Web of Science

Language：Japanese  

DOI： 10.1093/ptep/ptab038

Web of Science

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.

arXiv

Other Link： http://arxiv.org/pdf/2102.12597v1

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.

arXiv

Other Link： http://arxiv.org/pdf/2101.03860v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：International Press of Boston  

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

DOI： 10.4310/ATMP.2021.v25.n2.a2

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1907.12333v1

Language：Japanese  

Web of Science

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.

arXiv

Other Link： http://arxiv.org/pdf/2012.01717v3

Language：Japanese   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1093/ptep/ptaa133

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/2007.03139v1

Language：Japanese   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1093/ptep/ptaa050

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1911.09893v2

Language：Japanese   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1093/ptep/ptz161

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1909.08420v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

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.

DOI： 10.1093/ptep/ptz084

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1805.00262v1

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1088/1361-6382/aad206

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1805.11385v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

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.

DOI： 10.1093/ptep/pty010

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1712.06305v2

Publisher：Proceedings of the 28th Workshop on General Relativity and Gravitation in Japan, JGRG 2018  

Even Matter scattering in Projectable Horava gravity implication that the relation between renomalizability and tree-level unitarity Projectable Horava gravity Renormalizable → Appropriate gauge fixing ☉ Non-projrctable Horava gravity → Renormalizable? Appropriate gauge.

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

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.

DOI： 10.1088/1361-6382/aa7906

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1702.05682v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS INC  

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.

DOI： 10.1093/ptep/ptx072

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1704.04637v2

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1093/ptep/ptx033

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1612.01228v3

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS INC  

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.

DOI： 10.1093/ptep/ptx022

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1701.00564v2

Publisher：Proceedings of Science  

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

DOI： 10.22323/1.294.0030

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1007/JHEP06(2016)117

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1602.05752v3

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.

DOI： 10.1142/9789813203952_0054

arXiv

Other Link： http://arxiv.org/pdf/1601.06470v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1093/ptep/ptv185

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1510.07237v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

DOI： 10.1103/PhysRevD.92.049902

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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

DOI： 10.1103/PhysRevD.92.024035

Web of Science

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.91.125007

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1502.01820v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

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

DOI： 10.1140/epjc/s10052-015-3463-0

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1405.0850v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a new definition of the wormhole throat including the flare-out condition and the feature corresponding to the traversability for general dynamical spacetimes in terms of null geodesic congruences. We will examine our definition for some examples and see advantages compared to the others.

DOI： 10.1103/PhysRevD.91.104008

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1103/PhysRevD.92.024035

arXiv

Other Link： http://arxiv.org/pdf/1505.00959v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.91.064003

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1412.8383v3

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1142/S0217732315400064

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevD.92.049902, 10.1103/PhysRevD.92.024035

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We analyze a spacetime structure on a new brane configuration constructed
recently in the Dvali-Gabadadze-Porrati braneworld context. The brane, embedded
on a five-dimensional bubble of nothing, has a wormhole structure. It is an
exact solution without any matter fields, and thus the energy conditions for
matter fields are trivially satisfied. We see that, under the traversability
condition, the size of the bubble should be larger than 10^{10}cm or so.

DOI： 10.1103/PhysRevD.90.126001

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1409.6816v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

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.

DOI： 10.1103/PhysRevD.90.046005

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1103/PhysRevD.90.044037

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1406.0677v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.89.084025

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1309.6461v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.89.063501

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1308.5765v2

Publisher：JGRG 2014 - Proceedings of the 24th Workshop on General Relativity and Gravitation in Japan  

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

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.88.123006

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1304.6334v1

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.

DOI： 10.1142/9789814590112_0029

arXiv

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1016/j.physletb.2013.09.001

Web of Science

Scopus

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1088/0264-9381/30/18/184008

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1304.0211v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.88.024019

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1303.0993v2

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1088/1475-7516/2013/06/029

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1212.5774v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1088/1475-7516/2012/12/025

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1208.1222v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.85.023521

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1109.3746v3

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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
order in the gradient expansion.

DOI： 10.1103/PhysRevD.84.064025

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1105.0246v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1143/PTPS.189.93

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1106.0387v3

Publisher：Proceedings of the 20th Workshop on General Relativity and Gravitation in Japan, JGRG 2010  

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

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1088/1475-7516/2010/10/031

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1008.1406v2

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1088/1475-7516/2010/06/016

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/1004.1776v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.81.044008

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0911.1814v2

Publisher：Proceedings of the 19th Workshop on General Relativity and Gravitation in Japan, JGRG 2009  

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

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1143/PTP.121.419

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0709.0199v2

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1143/PTP.121.427

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0810.4811v1

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

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

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1143/PTP.119.757

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0712.0902v3

Language：English  

Publisher：Proceedings of the 17th Workshop on General Relativity and Gravitation in Japan, JGRG 2007  

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

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1103/PhysRevD.76.104041

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0706.1980v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：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.

DOI： 10.1103/PhysRevD.76.083517

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/0706.1604v2

Language：English   Publishing type：Research paper (scientific journal)  

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

DOI： 10.1088/1126-6708/2007/04/053

Web of Science

Scopus

arXiv

Other Link： http://arxiv.org/pdf/hep-th/0610282v3

Publisher：Proceedings of the 16th Workshop on General Relativity and Gravitation in Japan, JGRG 2006  

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

Scopus

Publisher：Proceedings of the 15th Workshop on General Relativity and Gravitation in Japan, JGRG 2005  

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

Scopus

Language：English  

Event date： 2023.6

Language：English  

Venue：institute for basic science   Country：Korea, Republic of  

Event date： 2023.3

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Kobe University   Country：Japan  

Event date： 2023.2

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Nagoya University   Country：Japan  

Event date： 2022.9

Language：English   Presentation type：Oral presentation (invited, special)  

Event date： 2022.9

Language：Japanese  

Event date： 2021.11 - 2021.12

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：LeCosPA   Country：Taiwan, Province of China  

Event date： 2021.8

Language：English   Presentation type：Oral presentation (general)  

Event date： 2021.3

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2021.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2020.1

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2020.1

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2019.12

Language：English   Presentation type：Oral presentation (general)  

Country：China  

Event date： 2019.11

Language：English   Presentation type：Oral presentation (general)  

Country：Korea, Republic of  

Event date： 2019.10

Language：English   Presentation type：Oral presentation (general)  

Country：Korea, Republic of  

Event date： 2019.9

Language：English   Presentation type：Oral presentation (general)  

Country：Viet Nam  

Event date： 2019.7

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2019.1

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2018.11

Language：English   Presentation type：Oral presentation (general)  

Country：Korea, Republic of  

Event date： 2018.11

Language：English   Presentation type：Oral presentation (general)  

Country：Korea, Republic of  

Event date： 2018.6

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2017.9

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2017.8

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2017.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2017.1

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2016.11

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2015.12

Language：English   Presentation type：Oral presentation (general)  

Country：Taiwan, Province of China  

Event date： 2015.10

Language：English   Presentation type：Oral presentation (general)  

Country：France  

Event date： 2015.4

Language：English   Presentation type：Oral presentation (general)  

Country：Canada  

Event date： 2013.11

Language：English   Presentation type：Oral presentation (general)  

Country：Taiwan, Province of China  

Event date： 2013.4

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2013.1 - 2013.2

Language：English   Presentation type：Oral presentation (general)  

Country：Taiwan, Province of China  

Event date： 2012.3

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2011.7

Language：English   Presentation type：Oral presentation (general)  

Country：Russian Federation  

Event date： 2010.9

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2010.3

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2010.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2009.12

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2009.11 - 2009.12

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2009.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2008.11

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2008.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2008.3

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2007.12

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2007.9

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2007.7

Language：English   Presentation type：Oral presentation (general)  

Country：Australia  

Event date： 2007.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2006.9

Language：English   Presentation type：Oral presentation (general)  

Country：United Kingdom  

Event date： 2006.3

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2005.11 - 2005.12

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Language：English   Presentation type：Oral presentation (general)  

Country：India  

Language：English   Presentation type：Oral presentation (invited, special)  

Country：India  

Audience： High school students,　College students,　General

Audience： General