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

DOI： 10.1103/physrevd.107.063503

Language：Others  

There exist observational evidence to believe the existence of primordial
magnetic fields generated during inflation. We study primordial gravitational
waves (PGWs) during inflation in the presence of magnetic fields sustained by a
gauge kinetic coupling. In the model, not only gravitons as excitations of
PGWs, but also photons as excitations of electromagnetic fields are highly
squeezed. They become entangled with each other through graviton to photon
conversion and vice versa. We derive the reduced density matrix for the
gravitons and calculate their entanglement entropy. It turns out that the state
of the gravitons is not a squeezed state but a mixed state.

DOI： 10.1103/PhysRevD.107.063503

Language：Others   Publisher：American Physical Society (APS)  

It is believed that relic gravitons are squeezed during inflation. If so, quantum noise induced by them can be significantly enhanced in current interferometers. However, decoherence of the gravitons during cosmic history may change the degree of squeezing. As a first step for analyzing the decoherence of the gravitons, we assume the presence of a sizable magnetic field at the beginning of inflation and study conversion processes of the squeezed gravitons into photons during inflation in the case of minimal coupling between gravitons and photons. We solve the dynamical evolution of a coupled system of gravitons and photons perturbatively and obtain squeezing parameters for the system numerically and analytically. It turns out that the gravitons are robust against the decoherence caused by the cosmological magnetic fields. We also find that the conversion rate of gravitons into photons is at a few percent at most.

DOI： 10.1103/physrevd.106.083508

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.106.083508/fulltext

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

We compute the leading order non-Gaussianity, i.e., the bispectrum, of the tensor perturbation in the general α-vacuum on de Sitter space in general relativity. In addition to the well-known Bunch-Davies (BD) vacuum, there exits an infinite number of de Sitter invariant vacua represented by a real parameter α and a phase ϕ, with α = 0 being the BD vacuum. They are called α-vacua. In the standard slow-roll inflation, as de Sitter invariance no longer applies, the α-vacua lose its relevance in the rigorous sense. Nevertheless, if we assume that the parameter α is only weakly dependent on the wavenumber with an appropriate UV cutoff, we may consider pseudo-α-vacua. In the case of false vacuum inflation where the background spacetime is pure de Sitter, a non-trivial (non-BD) α-vacuum could indeed be realized. We find an intriguing result that the bispectrum may be exponentially enhanced to be detectable by observation even if the spectrum is too small to be detected.

DOI： 10.1007/jhep08(2022)210

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Scopus

arXiv

researchmap

Other Link： https://link.springer.com/article/10.1007/JHEP08(2022)210/fulltext.html

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

Quantum coherence is one of the most striking features of quantum mechanics rooted in the superposition principle. Recently, it has been demonstrated that it is possible to harvest the quantum coherence from a coherent scalar field. In order to explore a new method of detecting axion dark matter, we consider a point-like Unruh–DeWitt detector coupled to the axion field and quantify a coherent measure of the detector. We show that the detector can harvest the quantum coherence from the axion dark matter. To be more precise, we consider a two-level electron system in an atom as the detector. In this case, we obtain the coherence measure [Formula: see text] where [Formula: see text] and [Formula: see text] are an observation time and the Lorentz factor. At the same time, the axion mass [Formula: see text] we can probe is determined by the energy gap of the detector.

DOI： 10.1142/s0217732322500286

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

Particle production during cosmic expansion can be interpreted as a two-mode
squeezing process of quantum states. The two-mode squeezed states consist of an
infinite number of entangled particles and then enhance the nonclassicality of
particles. There are two methods for estimating the degree of squeezing. One is
to employ instantaneous vacuum states, and the other is to adopt adiabatic
vacuum states. We analytically study the squeezing process of gravitons and
axions by using the two methods. We first consider a cosmological model of
inflation followed by instantaneous reheating leading to a radiation dominated
era. In the case of gravitons, we find no qualitatively difference in the
squeezing between the two methods. However, for the axions, it turns out that
the squeezing in the instantaneous vacuum increases as the mass increases while
the squeezing decreases in the adiabatic vacuum as the mass increases. We then
study the effect of non-instantaneous reheating on the the squeezing and show
that the squeezing is enhanced compared with the instantaneous reheating. We
also illustrate how the squeezed states enhance the violation of Bell
inequality and quantum noise of gravitons and axions.

DOI： 10.1142/S0218271822500985

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

DOI： 10.1103/physrevd.104.083516

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

DOI： 10.1103/physrevd.103.044017

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

Polarizations of primordial gravitational waves can be relevant when considering an inflationary universe in modified gravity or when matter fields survive during inflation. Such polarizations have been discussed in the Bunch–Davies vacuum. Instead of taking into account the dynamical generation of polarizations of gravitational waves, in this paper, we consider polarized initial states constructed from S U ( 2 ) coherent states. We then evaluate the power spectrums of the primordial gravitational waves in the states.

DOI： 10.3390/sym12040672

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

DOI： 10.1103/physrevd.100.123536

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

DOI： 10.1103/physrevd.99.084010

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

DOI： 10.1103/physrevd.97.103534

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

DOI： 10.1103/physrevd.97.083520

Language：English  

Infinite violation of Bell inequalities in inflation
We study the violation of Bell-Mermin-Klyshko (BMK) inequalities in initial
quantum states of scalar fields in inflation. We show that the Bell inequality
is maximally violated by the Bunch-Davies vacuum which is a two-mode squeezed
state of a scalar field. However, we find that the violation of the BMK
inequalities does not increase with the number of modes to measure. We then
consider a non-Bunch-Davies vacuum expressed by a four-mode squeezed state of
two scalar fields. Remarkably, we find that the violation of the BMK
inequalities increases exponentially with the number of modes to measure. This
indicates that some evidence that our universe has a quantum mechanical origin
may survive in CMB data even if quantum entanglement decays exponentially
afterward due to decoherence.

DOI： 10.1103/PhysRevD.96.083501

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

DOI： 10.1007/jhep03(2017)068

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

DOI： 10.1103/physrevd.94.125014

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

DOI： 10.1016/j.physletb.2015.10.050

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

DOI： 10.1209/0295-5075/111/60007

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

DOI： 10.1088/1475-7516/2015/03/015

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

DOI： 10.1007/jhep07(2014)072

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

DOI： 10.1088/1475-7516/2014/07/029

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

DOI： 10.1088/1475-7516/2014/04/009

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

DOI： 10.1093/ptep/ptt093

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

DOI： 10.1088/1475-7516/2013/06/034

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

DOI： 10.1088/1475-7516/2012/12/006

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

We present exact Coleman–De Luccia (CDL) instantons, which describe vacuum decay from Anti-de Sitter (AdS) space, de Sitter (dS) space and Minkowski space to AdS space. We systematically obtain these exact solutions by considering deformation of Hawking-Moss (HM) instantons. We analytically calculate the action of instantons and discuss a subtlety in calculation of decay rates.

DOI： 10.1142/s021827181250040x

Language：English  

Destabilizing Tachyonic Vacua at or above the BF Bound
It is well known that tachyonic vacua in an asymptotically Anti-de Sitter
(AdS) space-time are classically stable if the mass squared is at or above the
Breitenlohner and Freedman (BF) bound. We study the quantum stability of these
tachyonic vacua in terms of instantons. We find a series of exact instanton
solutions destabilizing tachyonic state at or above the BF bound in
asymptotically AdS space. We also give an analytic formula for the decay rate
and show that it is finite. Comparing our result with the well-known algebraic
condition for the stability, we discuss stability conditions of tachyonic vacua
at or above the BF bound.

DOI： 10.1143/PTP.128.213

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

DOI： 10.1088/0264-9381/29/7/075010

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

DOI： 10.1016/j.nuclphysb.2011.10.020

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

DOI： 10.1088/0264-9381/28/12/127001

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

Abstract

We study the imprints of the anisotropic inflation on the CMB temperature fluctuations and polarizations. The statistical anisotropy stems not only from the direction dependence of curvature and tensor perturbations, but also from the cross-correlation between curvature and tensor perturbations, and the linear polarization of tensor perturbations. We show that off-diagonal TB and EB spectra as well as on- and off-diagonal TT, EE, BB and TE spectra are induced from the anisotropic inflation. We emphasize that the off-diagonal spectra induced by the cross-correlation could be a characteristic signature of the anisotropic inflation.

DOI： 10.1111/j.1745-3933.2011.01010.x

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

DOI： 10.1088/1475-7516/2010/12/024

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

DOI： 10.1007/jhep12(2010)029

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

DOI： 10.1103/physrevd.82.086003

Language：English  

The Nature of Primordial Fluctuations from Anisotropic Inflation
We study the statistical nature of primordial fluctuations from an
anisotropic inflation which is realized by a vector field coupled to an
inflaton. We find a suitable gauge, which we call the canonical gauge, for
anisotropic inflation by generalizing the flat slicing gauge in conventional
isotropic inflation. Using the canonical gauge, we reveal the structure of the
couplings between curvature perturbations, vector waves, and gravitational
waves. We identify two sources of anisotropy, i.e. the anisotropy due to the
anisotropic expansion of the universe and that due to the anisotropic couplings
among variables. It turns out that the latter effect is dominant. Since the
coupling between the curvature perturbations and vector waves is the strongest
one, the statistical anisotropy in the curvature perturbations is larger than
that in gravitational waves. We find the cross correlation between the
curvature perturbations and gravitational waves which never occurs in
conventional inflation. We also find the linear polarization of gravitational
waves. Finally, we discuss cosmological implication of our results.

DOI： 10.1143/PTP.123.1041

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

DOI： 10.1088/1475-7516/2009/12/009

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

DOI： 10.1088/1126-6708/2009/10/010

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

DOI： 10.1103/physrevlett.102.191302

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

DOI： 10.1088/1475-7516/2008/08/034

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

DOI： 10.1103/physrevd.77.063531

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

DOI： 10.1103/physrevd.77.023526

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

DOI： 10.1103/physrevd.76.083524

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

DOI： 10.1103/physrevd.76.063502

Language：English  

Kaluza-Klein braneworld cosmology with static internal dimensions
We investigate the Kaluza-Klein braneworld cosmology from the point of view
of observers on the brane. We first generalize the Shiromizu-Maeda-Sasaki (SMS)
equations to higher dimensions. As an application, we study a (4+n)-dimensional
brane with n dimensions compactified on the brane, in a (5+n)-dimensional bulk.
By assuming that the size of the internal space is static, that the bulk
energy-momentum tensor can be neglected, we determine the effect of the bulk
geometry on the Kaluza-Klein braneworld. Then we derive the effective Friedmann
equation on the brane. It turns out that the Friedmann equation explicitly
depends on the equation of state, in contrast to the braneworld in a
5-dimensional bulk spacetime. In particular, in a radiation-dominated era, the
effective Newton constant depends on the scale factor logarithmically. If we
include a pressureless matter on the brane, this dependence disappears after
the radiation-matter equality. This may be interpreted as stabilization of the
Newton constant by the matter on the brane. Our findings imply that the
Kaluza-Klein braneworld cosmology is quite different from the conventional
Kaluza-Klein cosmology even at low energy.

DOI： 10.1143/PTP.118.701

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

DOI： 10.1103/physrevd.75.104017

Language：Others  

We investigate (2+1)-dimensional black strings in the Kaluza-Klein spacetime.
The system is classically stable as long as the horizon size is much larger
than the size of the compact space. Semiclassically, however, the horizon size
shrinks gradually due to the energy loss through the Hawking radiation.
Eventually, the system will enter into the regime of the Gregory-Laflamme
instability and get destabilized. Subsequently, the spherically symmetric black
hole is formed and evaporated in the usual manner. This standard picture may be
altered by the dynamics of the internal space which induced by the Hawking
radiation. We argue that the black string is excised from the Kaluza-Klein
spacetime before the onset of the Gregory-Laflamme instability and therefore
before the evaporation.

Language：Others  

We discuss the impact of Lorentz violation on the cosmology. Firstly, we show
that the Lorentz violation affects the dynamics of the chaotic inflationary
model and gives rise to an interesting feature. Secondly, we propose the
Lorentz violating DGP brane models where the Lorentz violating terms on the
brane accelerate the current universe. We conjecture that the ghost disappears
in the Lorentz violating DGP models.

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

DOI： 10.1088/1475-7516/2006/11/009

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

DOI： 10.1103/physrevd.74.063505

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

DOI： 10.1103/physrevd.72.104023

Language：Others  

We present a general formalism for black string perturbations in
Randall-Sundrum 1 model (RS1). First, we derive the master equation for the
electric part of the Weyl tensor $E_{mu
u}$. Solving the master equation
using the gradient expansion method, we give the effective Teukolsky equation
on the brane at low energy. It is useful to estimate gravitational waves
emitted by perturbed rotating black strings. We also argue the effect of the
Gregory-Laflamme instability on the brane using our formalism.

DOI： 10.1007/s10714-005-0142-8

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

DOI： 10.1088/1475-7516/2005/08/002

Language：Others  

Homogeneous cosmology in the braneworld can be studied without solving bulk
equations of motion explicitly. The reason is simply because the symmetry of
the spacetime restricts possible corrections in the 4-dimensional effective
equations of motion. It would be great if we could analyze cosmological
perturbations without solving the bulk. For this purpose, we combine the
geometrical approach and the low energy gradient expansion method to derive the
4-dimensional effective action. Given our effective action, the standard
procedure to obtain the cosmological perturbation theory can be utilized and
the temperature anisotropy of the cosmic background radiation can be computed
without solving the bulk equations of motion explicitly.

DOI： 10.1007/s10714-005-0139-3

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

DOI： 10.1103/physrevd.72.024009

Language：Japanese  

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

DOI： 10.1103/physrevd.71.044031

Language：Others  

We present a general formalism for studying an inflationary scenario in the
two-brane system. We explain the gradient expansion method and obtain the
4-dimensional effective action. Based on this effective action, we give a basic
equations for the background homogeneous universe and the cosmological
perturbations. As an application, we propose a born-again braneworld scenario.

Language：Others  

We study a codimension 2 braneworld in the Einstein-Gauss-Bonnet gravity. In
the linear regime, we show the conventional Einstein gravity can be recovered
on the brane. While, in the nonlinear regime, we find corrections due to the
thickness and the bulk geometry.

Language：Others  

There are various different descriptions of Randall-Sundrum (RS) braneworlds.
Here we present a unified view of the braneworld based on the gradient
expansion approach. In the case of the single-brane model, we reveal the
relation between the geometrical and the AdS/CFT approach. It turns out that
the high energy and the Weyl term corrections found in the geometrical approach
merge into the CFT matter correction found in the AdS/CFT approach. We also
clarify the role of the radion in the two-brane system. It is shown that the
radion transforms the Einstein theory with Weyl correction into the conformally
coupled scalar-tensor theory where the radion plays the role of the scalar
field.

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

DOI： 10.1088/1475-7516/2004/07/002

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

DOI： 10.1016/j.physletb.2004.03.043

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

DOI： 10.1023/b:gerg.0000016919.07175.92

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

DOI： 10.1088/0264-9381/21/7/012

Language：Japanese  

Language：Others  

Evaluating Kaluza-Klein (KK) corrections is indispensable to test the
braneworld scenario. In this report, we propose a novel symmetry approach to an
effective 4-dimensional action with KK corrections for the Randall-Sundrum
two-brane system.

Language：Others  

Non-uniform black strings in the two-brane system are investigated using the
effective action approach. It is shown that the radion acts as a non-trivial
hair of black strings. The stability of solutions is demonstrated using the
catastrophe theory. The black strings are shown to be non-uniform.

Language：Others  

We have developed a formalism to study an inflationary scenario driven by a
bulk inflaton in the two-brane system. The 4-dimensional low energy effective
action is obtained using the gradient expansion method. It is also found that
the dark radiation and the dark scalar source are expressed by the radion. In
the single-brane limit, we find these dark components disappear. Therefore, it
turns out that the inflation due to the bulk inflaton successfully takes place.
Kaluza-Klein corrections are also taken into account in this case.

Language：Others  

In the Randall-Sundrum two-brane model (RS1), a Kerr black hole on the brane
can be naturally identified with a section of rotating black string. To
estimate Kaluza-Klein (KK) corrections on gravitational waves emitted by
perturbed rotating black strings, we give the effective Teukolsky equation on
the brane which is separable equation and hence numerically manageable. In this
process, we derive the master equation for the electric part of the Weyl tensor
$E_{mu
u}$ which would be also useful to discuss the transition from black
strings to localized black holes triggered by Gregory-Laflamme instability.

Language：Others  

To estimate Kaluza-Klein (KK) corrections on gravitational waves emitted by
perturbed rotating black strings, we give the effective Teukolsky equation on
the brane which is separable equation and hence numerically manageable.

Language：Others  

Non-uniform black strings in the two-brane system are investigated using the
effective action approach. It is shown that the radion acts as a non-trivial
hair of black strings. The stability of solutions is demonstrated using the
catastrophe theory. The black strings are shown to be non-uniform.

Language：English  

Radionic Non-uniform Black Strings
Non-uniform black strings in the two-brane system are investigated using the
effective action approach. It is shown that the radion acts as a non-trivial
hair of the black strings. From the brane point of view, the black string
appears as the deformed dilatonic black hole which becomes dilatonic black hole
in the single brane limit and reduces to the Reissner-Nordstr"om black hole in
the close limit of two-branes. The stability of solutions is demonstrated using
the catastrophe theory. From the bulk point of view, the black strings are
proved to be non-uniform. Nevertheless, the zeroth law of black hole
thermodynamics still holds.

DOI： 10.1103/PhysRevD.69.024010

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

DOI： 10.1143/ptp.109.357

Language：Others  

We develop the general formalism to study the low energy regime of the brane
world. We apply our formalism to the single brane model where the AdS/CFT
correspondence will take an important role. We also consider the two-brane
system and show the system is described by the quasi-scalar tensor gravity. Our
result provides a basis for predicting CMB fluctuations in the braneworld
models.

Language：English  

Born-Again Braneworld
We propose a cosmological braneworld scenario in which two branes collide and
emerge as reborn branes whose tensions have signs opposite to the original
tensions of the respective branes. In this scenario, gravity on each of the
branes is described by a scalar-tensor-type theory in which the radion plays
the role of the gravitational scalar, and the branes are assumed to be
inflating. However, the whole dynamics is different from those of the usual
inflation, due to the non-trivial dynamics of the radion field. Transforming
the conformal frame to the Einstein frame, this born-again scenario resembles
the pre-big-bang scenario. Thus, our scenario has features of both inflation
and pre-big-bang scenarios. In particular, gravitational waves produced from
vacuum fluctuations may have a very blue spectrum, while the inflaton field
gives rise to a standard scale-invariant spectrum.

DOI： 10.1143/PTP.109.357

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

DOI： 10.1103/physrevd.66.083506

Language：Others  

The low energy effective theory for the Randall-Sundrum two-brane system is
investigated with an emphasis on the role of the non-linear radion in the brane
world. It is shown that the gravity on the brane world is described by a
quasi-scalar-tensor theory with a specific coupling function.

DOI： 10.1023/A:1022556701901

Language：Others  

The cosmological perturbation theory is revisited from the holographic point
of view. In the case of the single brane model, it turns out that the AdS/CFT
correspondence plays an important role. In the case of the two-brane model, it
is shown that the effective equations of motion becomes the quasi-scalar-tensor
gravity. It is also demonstrated that the radion anisotropy gives the CMB
fluctuations through the Sachs-Wolfe effect.

DOI： 10.1023/A:1022519910504

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

DOI： 10.1103/physrevd.66.043526