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

Abstract

Since the discovery of the accelerated expansion of the present Universe, significant theoretical developments have been made in the area of modified gravity. In the meantime, cosmological observations have been providing more high-quality data, allowing us to explore gravity on cosmological scales. To bridge the recent theoretical developments and observations, we present an overview of a variety of modified theories of gravity and the cosmological observables in the cosmic microwave background and large-scale structure, supplemented with a summary of predictions for cosmological observables derived from cosmological perturbations and sophisticated numerical studies. We specifically consider scalar-tensor theories in the Horndeski and DHOST family, massive gravity/bigravity, vector-tensor theories, metric-affine gravity, and cuscuton/minimally-modified gravity, and discuss the current status of those theories with emphasis on their physical motivations, validity, appealing features, the level of maturity, and calculability. We conclude that the Horndeski theory is one of the most well-developed theories of modified gravity, although several remaining issues are left for future observations. The paper aims to help to develop strategies for testing gravity with ongoing and forthcoming cosmological observations.

DOI： 10.1093/ptep/ptad052

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Recently, a generalization of invertible disformal transformations containing
higher-order derivatives of a scalar field has been proposed in the context of
scalar-tensor theories of gravity. By applying this generalized disformal
transformation to the Horndeski theory, one can obtain the so-called
generalized disformal Horndeski theories which are more general healthy
scalar-tensor theories than ever. However, it is unclear whether or not the
generalized disformal Horndeski theories can be coupled consistently to matter
fields because introducing a matter field could break the degeneracy conditions
of higher-order scalar-tensor theories and hence yield the unwanted
Ostrogradsky ghost. We investigate this issue and explore the conditions under
which a minimal coupling to a matter field is consistent in the generalized
disformal Horndeski theories without relying on any particular gauge such as
the unitary gauge. We find that all the higher derivative terms in the
generalized disformal transformation are prohibited to avoid the appearance of
the Ostrogradsky ghost, leading to the conclusion that only the theories that
are related to the Horndeski theory through a conventional disformal
transformation remain ghost-free in the presence of minimally coupled matter
fields.

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Other Link： http://arxiv.org/pdf/2302.03418v1

We study the homogeneous and anisotropic dynamics of pseudoscalar inflation
coupled to an SU($N$) gauge field. To see how the initially anisotropic
universe is isotropized in such an inflation model, we derive the equations to
obtain axisymmetric SU($N$) gauge field configurations in Bianchi type-I
geometry and discuss a method to identify their isotropic subsets which are the
candidates of their late-time attractor. Each isotropic solution is
characterized by the corresponding SU(2) subalgebra of the SU($N$) algebra. It
is shown numerically that the isotropic universe is a universal late-time
attractor in the case of the SU(3) gauge field. Interestingly, we find that a
transition between the two distinct gauge-field configurations characterized by
different SU(2) subalgebras can occur during inflation. We clarify the
conditions for this to occur. This transition could leave an observable imprint
on the CMB and the primordial gravitational wave background.

DOI： 10.1103/PhysRevD.107.043508

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Other Link： http://arxiv.org/pdf/2211.09489v2

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We provide a cosmological test of modified gravity with two tensorial degrees of freedom and no extra propagating scalar mode. The theory of gravity we consider admits a cosmological model that is indistinguishable from the ΛCDM model at the level of the background evolution. The model has a single modified-gravity parameterβ, the effect of which can be seen in linear perturbations, though no extra scalar mode is propagating. Using the Boltzmann code modified to incorporate the present model, we derive the constraints -0.047 <β< -0.028 at 68% confidence from Planck CMB data. Since our modified gravity model can hardly be constrained by the Solar System tests and gravitational-wave propagation, our result offers the first observational test on the model.

DOI： 10.1088/1475-7516/2022/07/040

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Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2022/07/040/pdf

We consider spatially covariant modified gravity in which the would-be scalar
degree of freedom is made non-dynamical and hence there are just two tensorial
degrees of freedom, i.e., the same number of dynamical degrees of freedom as in
general relativity. Focusing on a class of such modified gravity theories
characterized by three functions of time, we discuss how modified gravity with
two tensorial degrees of freedom can be distinguished observationally or
phenomenologically from general relativity. It is checked that the theory gives
the same predictions as general relativity for weak gravitational fields and
the propagation speed of gravitational waves. We also find that there is no
modification to asymptotically flat black hole solutions. Due to a large degree
of freedom to choose the time-dependent functions in the theory, the
homogeneous and isotropic cosmological dynamics can be made close to or even
identical to that of the $\Lambda$CDM model. We investigate the behavior of
cosmological perturbations in the long and short wavelength limits and show
that in both limits the effects of modified gravity appear only through the
modification of the background evolution. Finally, it is remarked that in the
presence of a galileon field in the matter sector, the scalar degree of freedom
is revived, ruining the essential feature of the theory.

DOI： 10.1103/PhysRevD.104.124020

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Other Link： http://arxiv.org/pdf/2109.10615v1

We investigate static and spherically symmetric stars disformally coupled to
a scalar field. The scalar field is assumed to be shift symmetric, and hence
the conformal and disformal factors of the metric coupled to matter are
dependent only on the kinetic term of the scalar field. Assuming that the
scalar field is linearly dependent on time, we consider a general
shift-symmetric scalar-tensor theory and a general form of the matter
energy-momentum tensor that allows for the anisotropic pressure and the heat
flux in the radial direction. This is a natural starting point in light of how
the gravitational field equations and the energy-momentum tensor transform
under a disformal transformation. By inspecting the structure of the
hydrostatic equilibrium equation in the presence of the derivative-dependent
conformal and disformal factors, we show that the energy density and tangential
pressure must vanish at the surface of a star. This fact is used to prove the
disformal invariance of the surface of a star, which was previously subtle and
unclear. We then focus on the shift-symmetric k-essence disformally coupled to
matter, and study the interior and exterior metric functions and scalar-field
profile in more detail. It is found that there are two branches of the solution
depending on the velocity of the scalar field. The disformally-related metric
functions of the exterior spacetime are also discussed.

DOI： 10.1103/PhysRevD.104.104009

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Other Link： http://arxiv.org/pdf/2107.13804v1

We study the dynamics of a homogeneous, isotropic, and positively curved
universe in the presence of a SU(2) gauge field or a triplet of mutually
orthogonal vector fields. In the SU(2) case we use the previously known ansatz
for the gauge-field configuration, but the case without non-abelian symmetries
is more nontrivial and we develop a new ansatz. We in particular consider
axion-SU(2) inflation and inflation with vector fields having
U(1)$\times$U(1)$\times$U(1) symmetry, and analyze their dynamics in detail
numerically. Novel effects of the spatial curvature come into play through
vector fields, which causes unconventional pre-inflationary dynamics. It is
found that the closed universe with vector fields is slightly more stable
against collapse than that filled solely with an inflaton field.

DOI： 10.1103/PhysRevD.104.083514

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Other Link： http://arxiv.org/pdf/2107.07199v2

SU(2) gauge fields coupled to an axion field can acquire an isotropic
background solution during inflation. We study homogeneous but anisotropic
inflationary solutions in the presence of such (massless) gauge fields. A gauge
field in the cosmological background may pose a threat to spatial isotropy. We
show, however, that such models $\textit{generally}$ isotropize in Bianchi
type-I geometry, and the isotropic solution is the attractor. Restricting the
setup by adding an axial symmetry, we revisited the numerical analysis
presented in Wolfson et.al (2020). We find that the reported numerical
breakdown in the previous analysis is an artifact of parametrization
singularity. We use a new parametrization that is well-defined all over the
phase space. We show that the system respects the cosmic no-hair conjecture and
the anisotropies always dilute away within a few e-folds.

DOI： 10.1088/1475-7516/2021/09/031

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Other Link： http://arxiv.org/pdf/2105.06259v3

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

DOI： 10.1093/ptep/ptab042

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DOI： 10.1103/PhysRevD.103.083503

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We study odd parity perturbations of spherically symmetric black holes with
time-dependent scalar hair in shift-symmetric higher-order scalar-tensor
theories. The analysis is performed in a general way without assuming the
degeneracy conditions. Nevertheless, we end up with second-order equations for
a single master variable, similarly to cosmological tensor modes. We thus
identify the general form of the quadratic Lagrangian for the odd parity
perturbations, leading to a generalization of the Regge-Wheeler equation. We
also investigate the structure of the effective metric for the master variable
and refine the stability conditions. As an application of our generalized
Regge-Wheeler equation, we compute the quasi-normal modes of a certain
nontrivial black hole solution. Finally, our result is extended to include the
matter energy-momentum tensor as a source term.

DOI： 10.1103/PhysRevD.103.084041

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Other Link： http://arxiv.org/pdf/2101.03790v1

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevd.102.103505

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.102.103505/fulltext

We study nanohertz gravitational waves relevant to pulsar timing array
experiments from quantum fluctuations in the early universe with null energy
condition (NEC) violation. The NEC violation admits accelerated expansion with
the scale factor $a\propto (-t)^{-p}$ ($p>0$), which gives the tensor spectral
index $n_t=2/(p+1)>0$. To evade the constraint from Big Bang nucleosynthesis
(BBN), we connect the NEC-violating phase to a subsequent short slow-roll
inflationary phase which ends with standard reheating, and thereby reduce the
high frequency part of the spectrum. An explicit model is constructed within
the cubic Horndeski theory which allows for stable violation of the NEC. We
present numerical examples of the background evolution having the different
maximal Hubble parameters (which determine the peak amplitude of gravitational
waves), the different inflationary Hubble parameters (which determine the
amplitudes of high frequency gravitational waves), and different durations of
the inflationary phase (which essentially determine the peak frequency of the
spectrum). We display the spectra with $n_t=0.8$, $0.9$, and $0.95$ for
$f\lesssim 1/{\rm yr}$, which are consistent with the recent NANOGrav result.
We also check that they do not contradict the BBN constraint. We discuss how
the nearly scale-invariant spectrum of curvature perturbations is produced in
the NEC-violating phase.

DOI： 10.1103/PhysRevD.102.123533

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Other Link： http://arxiv.org/pdf/2011.01605v2

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

It has been found that the primordial non-Gaussianity of the curvature
perturbation in the case of non-Bunch-Davies initial states can be enhanced
compared with those in the case of the Bunch-Davies one due to the interactions
among the perturbations on subhorizon scales. The purpose of the present paper
is to investigate whether tensor non-Gaussianities can also be enhanced or not
by the same mechanism. We consider general gravity theory in the presence of an
inflaton, and evaluate the tensor auto-bispectrum and the cross-bispectrum
involving one tensor and two scalar modes with the non-Bunch-Davies initial
states for tensor modes. The crucial difference from the case of the scalar
auto-bispectrum is that the tensor three-point function vanishes at the
flattened momentum triangles. We point out that the cross-bispectrum can
potentially be enhanced at non-trivial triangle shapes due to the
non-Bunch-Davies initial states.

DOI： 10.1103/physrevd.102.023513

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.102.023513/fulltext

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Late-time cosmology in the extended cuscuton theory is studied, in which
gravity is modified while one still has no extra dynamical degrees of freedom
other than two tensor modes. We present a simple example admitting analytic
solutions for the cosmological background evolution that mimics $\Lambda$CDM
cosmology. We argue that the extended cuscuton as dark energy can be
constrained, like usual scalar-tensor theories, by the growth history of matter
density perturbations and the time variation of Newton's constant.

DOI： 10.1088/1475-7516/2020/07/004

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Other Link： https://iopscience.iop.org/article/10.1088/1475-7516/2020/07/004

We construct a model of higher dimensional cosmology in which extra
dimensions are frozen by virtue of the cubic-order Lovelock gravity throughout
the cosmic history from inflation to the present with radiation and
matter-dominated regimes in between.

DOI： 10.1016/j.physletb.2020.135857

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Other Link： http://arxiv.org/pdf/2005.14481v1

We reformulate the recently proposed regularized version of Lovelock gravity
in four dimensions as a scalar-tensor theory. By promoting the warp factor of
the internal space to a scalar degree of freedom by means of Kaluza-Klein
reduction, we show that regularized Lovelock gravity can be described
effectively by a certain subclass of the Horndeski theory. Cosmological aspects
of this particular scalar-tensor theory are studied. It is found that the
background with a scalar charge is generically allowed. The consequences of
this scalar charge are briefly discussed.

DOI： 10.1088/1475-7516/2020/07/013

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Other Link： http://arxiv.org/pdf/2003.12771v2

Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevd.101.043529

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.101.043529/fulltext

We explore the possibility of a blue-tilted gravitational wave spectrum from
potential-driven slow-roll inflation in the Horndeski theory. In Kamada et al.
(2012), it was claimed that a blue gravitational wave spectrum cannot be
obtained from stable potential-driven slow-roll inflation within the Horndeski
framework. However, it has been demonstrated that the spectrum of primordial
gravitational waves can be blue in inflation with the Gauss-Bonnet term, where
the potential term is dominant and slow-roll conditions as well as the
stability conditions are satisfied. To fill in this gap, we clarify where the
discrepancy is coming from. We extend the formulation of Kamada et al. (2012)
and show that a blue gravitational wave spectrum can certainly be generated
from stable slow-roll inflation if some of the conditions previously imposed on
the form of the free functions in the Lagrangian are relaxed.

DOI： 10.1103/PhysRevD.101.043536

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Other Link： http://arxiv.org/pdf/1911.02143v2

We study relativistic stars in Hordenski theories that evade the
gravitational wave constraints and exhibit the Vainshtein mechanism, focusing
on a model based on the cubic Galileon Lagrangian. We derive the scalar field
profile for static spherically symmetric objects in asymptotically de Sitter
space-time with a linear time dependence. The exterior solution matches to the
black hole solution found in the literature. Due to the Vainshtein mechanism,
the stellar structure is indistinguishable from that of General Relativity with
the same central density as long as the radius of the star is shorter than the
Vainshtein radius. On the other hand, the scalar field is not suppressed beyond
the Vainshtein radius. These solutions have an additional integration constant
in addition to the mass of the star.

DOI： 10.1103/PhysRevD.101.024026

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

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

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We consider a subclass of degenerate higher-order scalar-tensor (DHOST) theories in which gravitational waves propagate at the speed of light and do not decay into scalar fluctuations. The screening mechanism in DHOST theories evading these two gravitational wave constraints operates very differently from that in generic DHOST theories. We derive a spherically symmetric solution in the presence of nonrelativistic matter. General relativity is recovered in the vacuum exterior region provided that functions in the Lagrangian satisfy a certain condition, implying that fine-tuning is required. Gravity in the matter interior exhibits novel features: although the gravitational potentials still obey the standard inverse power law, the effective gravitational constant is different from its exterior value, and the two metric potentials do not coincide. We discuss possible observational constraints on this subclass of DHOST theories, and argue that the tightest bound comes from the Hulse-Taylor pulsar.

DOI： 10.1103/PhysRevD.99.104073

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

We investigate the potential of cosmological observations, such as galaxy surveys, for constraining degenerate higher-order scalar-tensor (DHOST) theories, focusing in particular on the linear growth of the matter density fluctuations. We develop a formalism to describe the evolution of the matter density fluctuations during the matter dominated era and in the early stage of the dark energy dominated era in DHOST theories, and give an approximate expression for the gravitational growth index in terms of several parameters characterizing the theory and the background solution under consideration. By employing the current observational constraints on the growth index, we obtain a new constraint on a parameter space of DHOST theories. Combining our result with other constraints obtained from the Newtonian stellar structure, we show that the degeneracy between the effective parameters of DHOST theories can be broken without using the Hulse-Taylor pulsar constraint.

DOI： 10.1103/PhysRevD.99.104051

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Publishing type：Research paper (scientific journal)  

This article is intended to review the recent developments in the Horndeski theory and its generalization, which provide us with a systematic understanding of scalar-tensor theories of gravity as well as a powerful tool to explore astrophysics and cosmology beyond general relativity. This review covers the generalized Galileons, (the rediscovery of) the Horndeski theory, cosmological perturbations in the Horndeski theory, cosmology with a violation of the null energy condition, degenerate higher-order scalar-tensor theories and their status after GW170817, the Vainshtein mechanism in the Horndeski theory and beyond, and hairy black hole solutions.

DOI： 10.1088/1361-6633/ab2429

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

We show explicitly that the nonminimal coupling between the scalar field and the Ricci scalar in 2D dilaton gravity can be recast in the form of kinetic gravity braiding (KGB). This is as it should be, because KGB is the 2D version of the Horndeski theory. We also determine all the static solutions with a linearly time-dependent scalar configuration in the shift-symmetric KGB theories in 2D.

DOI： 10.1088/1361-6382/ab1355

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Publishing type：Research paper (scientific journal)  

Our Universe is nearly spatially flat, but this does not mean that it is exactly spatially flat. In this paper we derive general quadratic actions for cosmological perturbations in non-flat models from the Horndeski theory. This allows us to study how the spatial curvature influences the behavior of cosmological perturbations in the early universe described by some general scalar-tensor theory. We show that a tiny spatial curvature at the onset of inflation is unlikely to yield large (or ${\cal O}(1)$) effects on the primordial spectra even if one modifies gravity. We also argue that non-singular cosmological solutions in the Horndeski theory are unstable in spatially open cases as well as in flat cases.

DOI： 10.1103/PhysRevD.99.043522

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Among single-field scalar-tensor theories, there is a special class called "cuscuton," which is represented as some limiting case of k-essence in general relativity. This theory has a remarkable feature that the number of propagating degrees of freedom is only two in the unitary gauge in contrast to ordinary scalar-tensor theories with three degrees of freedom. We specify a general class of theories with the same property as the cuscuton in the context of the beyond Horndeski theory, which we dub as the extended cuscuton. We also study cosmological perturbations in the presence of matter in these extended cuscuton theories.

DOI： 10.1088/1475-7516/2018/12/002

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Publishing type：Research paper (scientific journal)  

We consider gravitational waves (GWs) in generic parity-violating gravity including recently proposed ghost-free theories with parity violation as well as Chern-Simons (CS) modified gravity, and study the implications of observational constraints from GW170817/GRB 170817A. Whereas GWs propagate at the speed of light, c, in CS gravity, we point out that this is specific to CS gravity and the GW propagation speed deviates from c, in general, in parity-violating gravity. Therefore, contrary to the previous literature in which only CS gravity is studied as a concrete example, we show that GW170817/GRB 170817A can, in fact, be used to limit gravitational parity violation. Our argument implies that the constraint on the propagation speed of GWs can pin down the parity-violating sector, if any, to CS gravity.

DOI： 10.1103/PhysRevD.98.124018

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Publishing type：Research paper (scientific journal)  

As opposed to Wald's cosmic no-hair theorem in general relativity, it is shown that the Horndeski theory (and its generalization) admits anisotropic inflationary attractors if the Lagrangian depends cubically on the second derivatives of the scalar field. We dub such a solution as a self-anisotropizing inflationary universe because anisotropic inflation can occur without introducing any anisotropic matter fields such as a vector field. As a concrete example of self-anisotropization we present the dynamics of a Bianchi type-I universe in the Horndeski theory.

DOI： 10.1088/1475-7516/2018/07/058

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

DOI： 10.1103/PhysRevD.97.104012

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The Vainshtein mechanism is known as an efficient way of screening the fifth force around a matter source in modified gravity. This has been verified mainly in highly symmetric matter configurations. To study how the Vainshtein mechanism works in a less symmetric setup, we numerically solve the scalar field equation around a disk with a hole at its center in the cubic Galileon theory. We find, surprisingly, that the Galileon force is enhanced, rather than suppressed, in the vicinity of the hole. This anti-screening effect is larger for a thinner, less massive disk with a smaller hole. At this stage our setup is only of academic interest and its astrophysical consequences are unclear, but this result implies that the Vainshtein screening mechanism around less symmetric matter configurations is quite nontrivial.

DOI： 10.1142/S0217732319500135

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

DOI： 10.1103/PhysRevD.97.103517

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DOI： 10.1103/PhysRevD.96.109903

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DOI： 10.1088/1475-7516/2017/11/038

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DOI： 10.1088/1475-7516/2017/07/009

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DOI： 10.1103/PhysRevD.95.084053

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DOI： 10.1103/PhysRevD.95.064011

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Publishing type：Research paper (international conference proceedings)  

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DOI： 10.1103/PhysRevD.95.064001

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DOI： 10.1103/PhysRevD.94.043511

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DOI： 10.1103/PhysRevD.94.103515

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DOI： 10.1088/1475-7516/2016/04/018

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_470

CiNii Article

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DOI： 10.1103/PhysRevD.93.043519

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DOI： 10.1103/PhysRevD.93.064068

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

DOI： 10.1103/PhysRevD.92.103503

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DOI： 10.1103/PhysRevD.93.064078

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We investigate perturbations of a class of spherically symmetric solutions in massive gravity and bi-gravity. The background equations of motion for the particular class of solutions we are interested in reduce to a set of the Einstein equations with a cosmological constant. Thus, the solutions in this class include all the spherically symmetric solutions in general relativity, such as the Friedmann-Lema\^{i}tre-Robertson-Walker solution and the Schwarzschild (-de Sitter) solution, though the one-parameter family of two parameters of the theory admits such a class of solutions. We find that the equations of motion for the perturbations of this class of solutions also reduce to the perturbed Einstein equations at first and second order. Therefore, the stability of the solutions coincides with that of the corresponding solutions in general relativity. In particular, these solutions do not suffer from non-linear instabilities which often appear in the other cosmological solutions in massive gravity and bi-gravity.

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

DOI： 10.1103/PhysRevD.92.103503

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DOI： 10.1142/S021773231650067X

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The Horndeski theory is known as the most general scalar-tensor theory with second-order field equations. In this paper, we explore the bi-scalar extension of the Horndeski theory. Following Horndeski's approach, we determine all the possible terms appearing in the second-order field equations of the bi-scalar-tensor theory. We compare the field equations with those of the generalized multi-Galileons, and confirm that our theory contains new terms that are not included in the latter theory. We also discuss the construction of the Lagrangian leading to our most general field equations.

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DOI： 10.1088/1475-7516/2015/07/017

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

DOI： 10.1088/1475-7516/2015/03/057

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Kyoto University, Yukawa Institute for Theoretical Physics  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：World Scientific  

DOI： 10.1142/9789814623995_0152

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

DOI： 10.1103/PhysRevD.90.124073

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DOI： 10.1103/PhysRevD.91.064013

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DOI： 10.1103/PhysRevD.90.124073

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

DOI： 10.1103/PhysRevD.89.165424

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

DOI： 10.1103/PhysRevB.89.165427

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

DOI： 10.1103/PhysRevD.89.084042

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DOI： 10.1093/ptep/ptu096

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DOI： 10.1103/PhysRevD.89.084042

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DOI： 10.1088/1475-7516/2014/03/008

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：University of Tokyo, Kavli Institute for Physics and Mathematics of the Universe, KIPMU  

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DOI： 10.1016/j.nuclphysbps.2013.10.068

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

DOI： 10.1103/PhysRevD.88.123518

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DOI： 10.1103/PhysRevD.88.083504

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DOI： 10.1103/PhysRevD.88.123518

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DOI： 10.1103/PhysRevD.88.083504

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DOI： 10.1103/PhysRevD.87.124006

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DOI： 10.1093/ptep/ptt033

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DOI： 10.1093/PTEP/PTT031

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DOI： 10.1103/PhysRevD.86.061505

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DOI： 10.1103/PhysRevD.86.023504

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DOI： 10.1103/PhysRevD.85.084025

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DOI： 10.1103/PhysRevD.85.024023

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DOI： 10.1103/PhysRevD.85.123503

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

DOI： 10.1016/j.physletb.2011.11.028

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

DOI： 10.1103/PhysRevLett.107.211301

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

DOI： 10.1143/PTP.126.511

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

DOI： 10.1103/PhysRevD.83.103524

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

DOI： 10.1103/PhysRevD.84.064030

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

DOI： 10.1103/PhysRevD.83.083515

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

DOI： 10.1103/PhysRevD.84.084051

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

DOI： 10.1103/PhysRevD.83.103524

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Publishing type：Research paper (scientific journal)  

We study the impact of modifying the vector sector of gravity on the CMB polarization. We employ the Einstein-aether theory as a concrete example. The Einstein-aether theory admits dynamical vector perturbations generated during inflation, leaving imprints on the CMB polarization. We derive the perturbation equations of the aether vector field in covariant formalism and compute the CMB B-mode polarization using the modified CAMB code. It is found that the amplitude of the B-mode signal from the aether field can surpass the one from the inflationary gravitational waves. The shape of the spectrum is clearly understood in an analytic way using the tight coupling approximation.

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

DOI： 10.1103/PhysRevD.83.083515

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

DOI： 10.1103/PhysRevLett.105.231302

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

DOI： 10.1103/PhysRevD.82.064024

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

DOI： 10.1103/PhysRevD.81.103533

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

DOI： 10.1088/1475-7516/2010/04/025

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

DOI： 10.1103/PhysRevD.81.063513

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

DOI： 10.1088/1475-7516/2009/11/015

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

DOI： 10.1103/PhysRevD.80.063514

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

DOI： 10.1088/1475-7516/2010/07/027

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

DOI： 10.1111/j.1365-2966.2009.15168.x

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

DOI： 10.1088/1475-7516/2009/05/004

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

DOI： 10.1103/PhysRevD.79.024009

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

DOI： 10.1103/PhysRevD.79.024009

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

DOI： 10.1103/PhysRevD.78.064019

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

DOI： 10.1103/PhysRevD.78.064006

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

DOI： 10.1103/PhysRevD.78.064019

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

DOI： 10.1103/PhysRevD.78.084018

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

DOI： 10.1088/0264-9381/26/13/135014

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

DOI： 10.1103/PhysRevD.78.064006

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

DOI： 10.1103/PhysRevD.77.044022

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

DOI： 10.1103/PhysRevD.77.124012

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

DOI： 10.1103/PhysRevD.77.044022

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

DOI： 10.1088/1475-7516/2007/12/006

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

DOI： 10.1103/PhysRevD.76.104052

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

DOI： 10.1088/0264-9381/25/1/015007

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

DOI： 10.1088/1475-7516/2007/07/016

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

DOI： 10.1103/PhysRevD.75.104013

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

DOI： 10.1103/PhysRevD.75.103501

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

DOI： 10.1088/1475-7516/2006/12/008

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

DOI： 10.1103/PhysRevD.74.104031

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

DOI： 10.1088/0264-9381/23/15/005

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

DOI： 10.1103/PhysRevD.74.064027

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

DOI： 10.1103/PhysRevD.73.124031

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

DOI： 10.1103/PhysRevD.73.044005

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

DOI： 10.1103/PhysRevD.73.044005

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

DOI： 10.1103/PhysRevD.71.124028

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

DOI： 10.1103/PhysRevD.71.124028

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

DOI： 10.1007/s10714-005-0192-y

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Publishing type：Research paper (scientific journal)  

Tensor perturbations in an expanding braneworld of the Randall Sundrum type are investigated. We consider a model composed of a slow-roll inflation phase and the succeeding radiation phase. The effect of the presence of an extra dimension through the transition to the radiation phase is studied, giving an analytic formula for leading order corrections.

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Publishing type：Research paper (scientific journal)  

We consider black-string-type solutions in five-dimensional Einstein-Gauss-Bonnet gravity. Numerically constructed solutions under static, axially symmetric and translationally invariant metric ansatz are presented. The solutions are specified by two asymptotic charges: mass of a black string and a scalar charge associated with the radion part of the metric. Regular black string solutions are found if and only if the two charges satisfy a fine-tuned relation, and otherwise the spacetime develops a singular event horizon or a naked singularity. We can also generate bubble solutions from the black strings by using a double Wick rotation.

DOI： 10.1103/PhysRevD.71.084005

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

DOI： 10.1088/1475-7516/2004/10/015

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

We introduce a $(5+m)$-dimensional vacuum description of five-dimensional bulk inflaton models with exponential potentials that makes analysis of cosmological perturbations simple and transparent. We show that various solutions, including the power-law inflation model recently discovered by Koyama and Takahashi, are generated from known $(5+m)$-dimensional vacuum solutions of pure gravity. We derive master equations for all types of perturbations, and each of them becomes a second order differential equation for one master variable supplemented by simple boundary conditions on the brane. One exception is the case for massive modes of scalar perturbations. In this case, there are two independent degrees of freedom, and in general it is difficult to disentangle them into two separate sectors.

DOI： 10.1103/PhysRevD.69.064037

CiNii Article

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

DOI： 10.1103/PhysRevD.68.044025

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

DOI： 10.1103/PhysRevD.68.044025

Scopus

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