記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.107.063503

記述言語：その他  

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

記述言語：その他   出版者・発行元：Physical Review D  

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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その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.106.083508/fulltext

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

Abstract

We 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

Web of Science

Scopus

arXiv

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その他リンク： https://link.springer.com/article/10.1007/JHEP08(2022)210/fulltext.html

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.104.083516

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.103.044017

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.100.123536

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.99.084010

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.97.103534

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.97.083520

記述言語：英語  

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 shows that the BMK inequalities are useful to classify the initial quantum state of the Universe.

DOI： 10.1103/PhysRevD.96.083501

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/jhep03(2017)068

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.94.125014

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.physletb.2015.10.050

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1209/0295-5075/111/60007

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2015/03/015

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/jhep07(2014)072

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2014/07/029

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2014/04/009

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1093/ptep/ptt093

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2013/06/034

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2012/12/006

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：英語  

It is well known that tachyonic vacua in an asymptotically Anti-de Sitter (AdS) spacetime 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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/0264-9381/29/7/075010

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.nuclphysb.2011.10.020

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/0264-9381/28/12/127001

記述言語：その他   掲載種別：研究論文（学術雑誌）  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2010/12/024

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1007/jhep12(2010)029

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.82.086003

記述言語：英語  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2009/12/009

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1126-6708/2009/10/010

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevlett.102.191302

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2008/08/034

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.77.063531

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.77.023526

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.76.083524

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.76.063502

記述言語：英語  

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 ignored, 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 logarithmically on the scale factor. 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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.75.104017

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2006/11/009

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.74.063505

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.72.104023

記述言語：その他  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2005/08/002

記述言語：その他  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.72.024009

記述言語：日本語  

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.71.044031

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2004/07/002

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.physletb.2004.03.043

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1023/b:gerg.0000016919.07175.92

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/0264-9381/21/7/012

記述言語：日本語  

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：その他  

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.

記述言語：英語  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1143/ptp.109.357

記述言語：その他  

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.

記述言語：英語  

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 are 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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.66.083506

記述言語：その他  

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

記述言語：その他  

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

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/physrevd.66.043526