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DOI： 10.1103/PhysRevD.107.063503

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DOI： 10.1103/physrevd.106.083508

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DOI： 10.1103/PhysRevD.107.063524

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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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We compute the leading order non-Gaussianity, i.e., the bispectrum, of the
tensor perturbation in the general $alpha$-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 $alpha$ and a phase $phi$, with $alpha=0$ being the BD
vacuum. They are called $alpha$-vacua. In the standard slow-roll inflation, as
de Sitter invariance no longer applies, the $alpha$-vacua lose its relevance
in the rigorous sense. Nevertheless, if we assume that the parameter $alpha$
is only weakly dependent on the wavenumber with an appropriate UV cutoff, we
may consider pseudo-$alpha$-vacua. In the case of false vacuum inflation where
the background spacetime is pure de Sitter, a non-trivial (non-BD)
$alpha$-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

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

DOI： 10.1142/s0217732322500286

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DOI： 10.1142/S0218271822500985

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DOI： 10.1142/S0218271822500985

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DOI： 10.1103/physrevd.104.083516

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DOI： 10.1103/physrevd.103.044017

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DOI： 10.3390/sym12040672

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DOI： 10.1103/physrevd.100.123536

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DOI： 10.1103/physrevd.99.084010

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DOI： 10.1103/physrevd.97.103534

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DOI： 10.1103/physrevd.97.083520

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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

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DOI： 10.1007/jhep03(2017)068

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DOI： 10.1103/physrevd.94.125014

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DOI： 10.1016/j.physletb.2015.10.050

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DOI： 10.1209/0295-5075/111/60007

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DOI： 10.1088/1475-7516/2015/03/015

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DOI： 10.1007/jhep07(2014)072

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DOI： 10.1088/1475-7516/2014/07/029

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DOI： 10.1088/1475-7516/2014/04/009

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DOI： 10.1093/ptep/ptt093

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DOI： 10.1088/1475-7516/2013/06/034

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DOI： 10.1088/1475-7516/2012/12/006

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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

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DOI： 10.1088/0264-9381/29/7/075010

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DOI： 10.1016/j.nuclphysb.2011.10.020

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DOI： 10.1088/0264-9381/28/12/127001

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DOI： 10.1111/j.1745-3933.2011.01010.x

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DOI： 10.1088/1475-7516/2010/12/024

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DOI： 10.1007/jhep12(2010)029

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DOI： 10.1103/physrevd.82.086003

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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

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DOI： 10.1088/1475-7516/2009/12/009

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DOI： 10.1088/1126-6708/2009/10/010

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DOI： 10.1103/physrevlett.102.191302

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DOI： 10.1088/1475-7516/2008/08/034

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DOI： 10.1103/physrevd.77.063531

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DOI： 10.1103/physrevd.77.023526

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DOI： 10.1103/physrevd.76.083524

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DOI： 10.1103/physrevd.76.063502

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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

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DOI： 10.1103/physrevd.75.104017

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DOI： 10.1088/1475-7516/2006/11/009

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DOI： 10.1103/physrevd.74.063505

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DOI： 10.1103/physrevd.72.104023

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DOI： 10.1007/s10714-005-0142-8

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DOI： 10.1088/1475-7516/2005/08/002

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DOI： 10.1007/s10714-005-0139-3

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DOI： 10.1103/physrevd.72.024009

Language：Japanese  

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DOI： 10.1103/physrevd.71.044031

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DOI： 10.1088/1475-7516/2004/07/002

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DOI： 10.1016/j.physletb.2004.03.043

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DOI： 10.1023/b:gerg.0000016919.07175.92

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DOI： 10.1088/0264-9381/21/7/012

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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

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DOI： 10.1143/ptp.109.357

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DOI： 10.1103/physrevd.66.083506

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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

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DOI： 10.1023/A:1022556701901

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DOI： 10.1023/A:1022519910504

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DOI： 10.1103/physrevd.66.043526