掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

Primordial black holes (PBHs) whose masses are in ∼ [10-15 M ⊙,10-11 M ⊙] have been extensively studied as a candidate of whole dark matter (DM). One of the probes to test such a PBH-DM scenario is scalar-induced stochastic gravitational waves (GWs) accompanied with the enhanced primordial fluctuations to form the PBH with frequency peaked in the mHz band being targeted by the LISA mission. In order to utilize the stochastic GW for checking the PBH-DM scenario, it needs to exactly relate the PBH abundance and the amplitude of the GW spectrum. Recently in Kitajima et al. [1], the impact of the non-Gaussianity of the enhanced primordial curvature perturbations on the PBH abundance has been investigated based on the peak theory, and they found that a specific non-Gaussian feature called the exponential tail significantly increases the PBH abundance compared with the Gaussian case. In this work, we investigate the spectrum of the induced stochastic GW associated with PBH DM in the exponential-tail case. In order to take into account the non-Gaussianity properly, we employ the diagrammatic approach for the calculation of the spectrum. We find that the amplitude of the stochastic GW spectrum is slightly lower than the one for the Gaussian case, but it can still be detectable with the LISA sensitivity. We also find that the non-Gaussian contribution can appear on the high-frequency side through their complicated momentum configurations. Although this feature emerges under the LISA sensitivity, it might be possible to obtain information about the non-Gaussianity from GW observation with a deeper sensitivity such as the DECIGO mission.

DOI： 10.1088/1475-7516/2023/05/044

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Scopus

その他リンク： https://iopscience.iop.org/article/10.1088/1475-7516/2023/05/044/pdf

掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

We propose an extension of the Higgs inflation to the hybrid metric-Palatini gravity, where we introduce non-minimal couplings between Higgs and both the metric-type and the Palatini-type Ricci scalars. We study the inflationary phenomenology of our model and find that slow-roll inflation can be realized in the large-field regime, giving the observationally favored predictions. In particular, the scalar spectral index exhibits an attractor behavior to n s ∼ 0.964, while the tensor-to-scalar ratio can take an arbitrary value depending on the non-minimal coupling parameters, with the metric-Higgs limit r ∼ 10-3 being the maximum. We also investigate the unitarity property of our model. As the ultraviolet (UV) cutoff as a low-energy effective field theory (EFT) of this model is significantly lower than the Planck scale due to a strong curvature of field-space, we consider a possible candidate of UV-extended theories with an additional scalar field introduced so as to flatten the field-space in five-dimension. While the field-space can be flatten completely and this approach can lead to a weakly-coupled EFT, we gain an implication that Planck-scale EFT can be only realized in the limit of metric-Higgs inflation. We also discuss generalizations of the model up to mass-dimension four.

DOI： 10.1088/1475-7516/2023/05/047

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Scopus

その他リンク： https://iopscience.iop.org/article/10.1088/1475-7516/2023/05/047/pdf

掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

We develop the stochastic formalism for U(1) gauge fields that has the Chern-Simons coupling to a rolling pseudo-scalar field during inflation. The Langevin equations for the physical electromagnetic fields are derived and the analytic solutions are studied. Using numerical simulation we demonstrate that the electromagnetic fields averaged over the Hubble scale continuously change their direction and their amplitudes fluctuate around the analytically obtained expectation values. Though the isotropy is spontaneously broken by picking up a particular local Hubble patch, each Hubble patch is understood independent and the isotropy is conserved globally by averaging all the Hubble patches.

DOI： 10.1088/1475-7516/2022/12/026

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Scopus

掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

We investigate the probability distribution of the effective inspiral spin, the mass ratio, and the chirp mass of primordial black hole (PBH) binaries, incorporating the effect of the critical phenomena of gravitational collapse. As a leading order estimation, each binary is assumed to be formed from two PBHs that are randomly chosen according to the probability distribution of single PBHs, and the accretion effects on the mass and spin of each PBH are not considered for simplicity. We find that, although the critical phenomena can lead to large spins on the low-mass tail, the effective inspiral spin of the binary is statistically very small, 〈 χ e f f 2 〉 ∼ 10 − 3 . We also see that there is almost no anticorrelation between the effective inspiral spin and the mass ratio, which can be inferred from observations.

DOI： 10.3847/1538-4357/ac93f1

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Scopus

掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

The axionic inflaton with the Chern-Simons coupling may generate U(1) gauge fields and charged particles simultaneously. In order to incorporate the backreaction from the charged particles on the gauge fields, we develop a procedure to obtain an equilibrium solution for the gauge fields by treating the induced current as effective electric and magnetic conductivities. Introducing mean field approximation, and numerically solving self-consistency equations, we find that the gauge field amplitudes are drastically suppressed. Interestingly, as the production becomes more efficient, the charged particles gain a larger part of the transferred energy from the inflaton and eventually dominate it. Our formalism offers a basis to connect this class of inflationary models to a rich phenomenology such as baryogenesis and magnetogenesis.

DOI： 10.1088/1475-7516/2022/09/023

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Scopus

その他リンク： https://iopscience.iop.org/article/10.1088/1475-7516/2022/09/023/pdf

掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

We investigate the UV-completion of the Higgs inflation in the metric and the Palatini formalisms. It is known that the cutoff scales for the perturbative unitarity of these inflation models become much smaller than the Planck scale to be consistent with observations. Expecting that the low cutoff scales originate in the curvature of a field-space spanned by the Higgs fields, we consider embedding the curved field-space into a higher dimensional flat space and apply this procedure to the metric-Higgs and the Palatini-Higgs scenarios. The new field introduced in this way successfully flattens the field-space and UV-completes the Higgs inflation in the metric formalism. However, in the Palatini formalism, the new field cannot uplift the cutoff up to the Planck scale. We also discuss the unavoidable low cutoff in the Palatini formalism in the context of the local conformal symmetry.

DOI： 10.1088/1475-7516/2022/05/035

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Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

We present a generic framework to compute the one-point statistics of cosmological perturbations, when coarse-grained at an arbitrary scale R, in the presence of quantum diffusion. Making use of the stochastic-δN formalism, we show how it can be related to the statistics of the amount of expansion realised until the scale R crosses out the Hubble radius. This leads us to explicit formulae for the probability density function (PDF) of the curvature perturbation, the comoving density contrast, and the compaction function. We then apply our formalism to the calculation of the mass distribution of primordial black holes produced in a single-field model containing a "quantum well"(i.e. an exactly flat region in the potential). We confirm that the PDFs feature heavy, exponential tails, with an additional cubic suppression in the case of the curvature perturbation. The large-mass end of the mass distribution is shown to be mostly driven by stochastic-contamination effects, which produce black holes more massive than those naively expected. This work bridges the final gap between the stochastic-inflation formalism and the calculation of the mass distribution of astrophysical objects such as primordial black holes, and opens up various prospects that we finally discuss.

DOI： 10.1088/1475-7516/2022/02/021

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arXiv

その他リンク： http://arxiv.org/pdf/2111.15280v2

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

In this paper, we update the peak theory for the estimation of the primordial black hole (PBH) abundance, particularly by implementing the critical behavior in the estimation of the PBH mass and employing the averaged compaction function for the PBH formation criterion to relax the profile dependence. We apply our peak theory to a specific non-Gaussian feature called the exponential tail, which is characteristic in ultra slow-roll models of inflation. With this type of non-Gaussianity, the probability of a large perturbation is not suppressed by the Gaussian factor but decays only exponentially, so the PBH abundance is expected to be much enhanced. Not only do we confirm this enhancement even compared to the case of the corresponding nonlinearity parameter f NL = 5/2, but also we find that the resultant PBH mass spectrum has a characteristic maximal mass which is not seen in the simple Press-Schechter approach.

DOI： 10.1088/1475-7516/2021/10/053

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arXiv

その他リンク： https://iopscience.iop.org/article/10.1088/1475-7516/2021/10/053/pdf

記述言語：日本語   出版者・発行元：Progress of Theoretical and Experimental Physics  

We revisit squeezed-limit non-Gaussianity in single-field non-attractor inflation models from the viewpoint of the cosmological soft theorem. In single-field attractor models, an inflaton's trajectories with different initial conditions effectively converge into a single trajectory in the phase space, and hence there is only one clock degree of freedom (DoF) in the scalar part. Its long-wavelength perturbations can be absorbed into the local coordinate renormalization and lead to the so-called consistency relation between n- and (n+1)-point functions. On the other hand, if the inflaton dynamics deviates from the attractor behavior, its long-wavelength perturbations cannot necessarily be absorbed and the consistency relation is expected not to hold any longer. In this work, we derive a formula for the squeezed bispectrum including the explicit correction to the consistency relation, as a proof of its violation in the non-attractor cases. First one must recall that non-attractor inflation needs to be followed by attractor inflation in a realistic case. Then, even if a specific non-attractor phase is effectively governed by a single DoF of phase space (represented by the exact ultra-slow-roll limit) and followed by a single-DoF attractor phase, its transition phase necessarily involves two DoF in dynamics and hence its long-wavelength perturbations cannot be absorbed into the local coordinate renormalization. Thus, it can affect local physics, even taking account of the so-called local observer effect, as shown by the fact that the bispectrum in the squeezed limit can go beyond the consistency relation. More concretely, the observed squeezed bispectrum does not vanish in general for long-wavelength perturbations exiting the horizon during a non-attractor phase.

DOI： 10.1093/ptep/ptab063

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

The standard model of particle physics is known to be intriguingly successful. However, their rich phenomena represented by the phase transitions (PTs) have not been completely understood yet, including the possibility of the existence of unknown dark sectors. In this paper, we investigate the measurement of the equation of state parameter w and the sound speed cs of the PT plasma with the use of the gravitational waves (GWs) of the universe. Though the propagation of GW is insensitive to cs in itself, the sound speed value affects the dynamics of primordial density (or scalar curvature) perturbations, and the induced GW by their horizon reentry can then be an indirect probe both w and cs. We numerically reveal the concrete spectrum of the predicted induced GW with two simple examples of the scalar perturbation spectrum: The monochromatic and scale-invariant spectra. In the monochromatic case, we see that the resonant amplification and cancellation scales of the induced GW depend on the cs values at different times respectively. The scale-invariant case gives a more realistic spectrum and its specific shape will be compared with observations. In particular, the QCD phase transition corresponds with the frequency range of the pulsar timing array (PTA) observations. If the amplitude of primordial scalar power is in the range of 10-4 ? A? ? 10-2 ,the induced GW is consistent with current observational constraints and detectable in the future observation in Square Kilometer Array. Furthermore?the recent possible detection of stochastic GWs by NANOGrav 12.5 yr analysis [1] can be explained by the induced GW if A?~v 7 × 10-3.

DOI： 10.1088/1475-7516/2021/06/048

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arXiv

その他リンク： https://iopscience.iop.org/article/10.1088/1475-7516/2021/06/048/pdf

記述言語：日本語   出版者・発行元：Physical Review D  

We motivate a minimal realization of slow-roll k-inflation by incorporating the local conformal symmetry and the broken global SO(1,1) symmetry in the metric-affine geometry. With use of the metric-affine geometry where both the metric and the affine connection are treated as independent variables, the local conformal symmetry can be preserved in each term of the Lagrangian and thus higher derivatives of scalar fields can be easily added in a conformally invariant way. Predictions of this minimal slow-roll k-inflation, ns∼0.96, r∼0.005, and cs∼0.03, are not only consistent with current observational data but also have a prospect to be tested by forthcoming observations.

DOI： 10.1103/PhysRevD.103.L101303

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

Stochastic inflation is an effective theory describing the super-Hubble, coarse-grained, scalar fields driving inflation, by a set of Langevin equations. We previously highlighted the difficulty of deriving a theory of stochastic inflation that is invariant under field redefinitions, and the link with the ambiguity of discretisation schemes defining stochastic differential equations. In this paper, we solve the issue of these "inflationary stochastic anomalies"by using the Stratonovich discretisation satisfying general covariance, and identifying that the quantum nature of the fluctuating fields entails the existence of a preferred frame defining independent stochastic noises. Moreover, we derive physically equivalent Itô-Langevin equations that are manifestly covariant and well suited for numerical computations. These equations are formulated in the general context of multifield inflation with curved field space, taking into account the coupling to gravity as well as the full phase space in the Hamiltonian language, but this resolution is also relevant in simpler single-field setups. We also develop a path-integral derivation of these equations, which solves conceptual issues of the heuristic approach made at the level of the classical equations of motion, and allows in principle to compute corrections to the stochastic formalism. Using the Schwinger-Keldysh formalism, we integrate out small-scale fluctuations, derive the influence action that describes their effects on the coarse-grained fields, and show how the resulting coarse-grained effective Hamiltonian action can be interpreted to derive Langevin equations with manifestly real noises. Although the corresponding dynamics is not rigorously Markovian, we show the covariant, phase-space Fokker-Planck equation for the Probability Density Function of fields and momenta when the Markovian approximation is relevant, and we give analytical approximations for the noises' amplitudes in multifield scenarios.

DOI： 10.1088/1475-7516/2021/04/048

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arXiv

その他リンク： https://iopscience.iop.org/article/10.1088/1475-7516/2021/04/048/pdf

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：EPL  

Systematic understanding for classes of inflationary models is investigated from the viewpoint of the local conformal symmetry and the slightly broken global symmetry in the framework of the metric-affine geometry. In the metric-affine geometry, which is a generalisation of the Riemannian one adopted in the ordinary General Relativity, the affine connection is an independent variable of the metric rather than being given, e.g., by the Levi-Civita connection as its function. Thanks to this independency, the metric-affine geometry can preserve the local conformal symmetry in each term of the Lagrangian contrary to the Riemannian geometry, and then the local conformal invariance can be compatible with much more kinds of global symmetries. As simple examples, we consider the two-scalar models with the broken SO(1,1) or O(2), leading to the well-known α-attractor or natural inflation, respectively. The inflaton, can be understood as their pseudo-Nambu-Goldstone boson.

DOI： 10.1209/0295-5075/132/39001

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arXiv

その他リンク： https://iopscience.iop.org/article/10.1209/0295-5075/132/39001/pdf

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Progress of Theoretical and Experimental Physics  

Non-Gaussianities of primordial perturbations in the soft limit provide important information about the light degrees of freedom during inflation. The soft modes of the curvature perturbations, unobservable for a local observer, act to rescale the spatial coordinates. We determine how the trispectrum in the collapsed limit is shifted by the rescaling due to the soft modes. We find that the form of the inequality between the $f\mathrm{NL}$ and $\tau\mathrm{NL}$ parameters is not affected by the rescaling, demonstrating that the role of the inequality as an indicator of the light degrees of freedom remains intact. We also comment on the local observer effect on the consistency relation for ultra-slow-roll inflation.

DOI： 10.1093/ptep/ptaa144

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arXiv

その他リンク： http://academic.oup.com/ptep/article-pdf/2020/11/113E01/34299933/ptaa144.pdf

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review D  

In the context of string theory, several conjectural conditions have been proposed for low energy effective field theories not to be in swampland, the UV-incomplete class. The recent ones represented by the de Sitter and trans-Planckian censorship conjectures in particular seem to conflict with the inflation paradigm of the early universe. We first point out that scenarios where inflation is repeated several times (multiphase inflation) can be easily compatible with these conjectures. In other words, we relax the constraint on the single inflation for the large scale perturbations to only continue at least around 10 e-folds. In this context, we then investigate if a spectator field can be a source of the almost scale-invariant primordial perturbations on the large scale. As a consequence of such an isocurvature contribution, the resultant perturbations exhibit the nonvanishing non-Gaussianity in general. Also the perturbation amplitude on smaller scales can be completely different from that on the large scale due to the multiplicity of inflationary phases. These signatures will be a smoking gun of this scenario by the future observations.

DOI： 10.1103/PhysRevD.101.103514

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arXiv

その他リンク： http://arxiv.org/pdf/2003.06753v2

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics  

We propose a class of single-field, slow-roll inflation models in which a typical number of e-folds can be extremely large. The key point is to introduce a very shallow local minimum near the top of the potential in a hilltop inflation model. In particular, a typical number of e-folds is enhanced if classical behavior dominates around the local minimum such that the inflaton probability distribution is drifted to the local minimum as a whole. After the inflaton escapes from the local minimum due to the stochastic dynamics, the ordinary slow-roll inflation follows and it can generate the primordial density perturbation consistent with observation. Interestingly, our scenario inherits the advantages of the old and new inflation: the typical e-folds can be extremely large as in the old inflation, and slow-roll inflation naturally follows after the stochastic regime as in the new inflation. In our numerical example, the typical number of e-folds can be as large as 101010, which is large enough for various light scalars such the QCD axion to reach the Bunch-Davies distribution.

DOI： 10.1016/j.physletb.2019.135097

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arXiv

その他リンク： http://arxiv.org/pdf/1908.08694v2

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.100.023537

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arXiv

その他リンク： http://arxiv.org/pdf/1904.10298v2

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Classical and Quantum Gravity  

The stochastic approach aims at describing the long-wavelength part of quantum fields during inflation by a classical stochastic theory. It is usually formulated in terms of Langevin equations, giving rise to a Fokker-Planck equation for the probability distribution function of the fields, and possibly their momenta. The link between these two descriptions is ambiguous in general, as it depends on an implicit discretisation procedure, the two prominent ones being the It- and Stratonovich prescriptions. Here we show that the requirement of general covariance under field redefinitions is verified only in the latter case, however at the expense of introducing spurious 'frame' dependences. This stochastic anomaly disappears when there is only one source of stochasticity, like in slow-roll single-field inflation, but manifests itself when taking into account the full phase space, or in the presence of multiple fields. Despite these difficulties, we use physical arguments to write down a covariant Fokker-Planck equation that describes the diffusion of light scalar fields in non-linear sigma models in the overdamped limit. We apply it to test scalar fields in de Sitter space and show that some statistical properties of a class of two-field models with derivative interactions can be reproduced by using a correspondence with a single-field model endowed with an effective potential. We also present explicit results in a simple extension of the single-field λψ 4 theory to a hyperbolic field space geometry. The difficulties we describe seem to be the stochastic counterparts of the notoriously difficult problem of maintaining general covariance in quantum theories, and the related choices of operator ordering and path-integral constructions. Our work thus opens new avenues of research at the crossroad between cosmology, statistical physics, and quantum field theory.

DOI： 10.1088/1361-6382/ab097f

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arXiv

その他リンク： http://arxiv.org/pdf/1806.10126v2

CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1016/j.physletb.2017.12.014

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.96.123527

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.96.043504

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.95.123510

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2017/02/021

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.94.083523

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.94.043527

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2016/08/041

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1103/PhysRevD.91.123534

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of Cosmology and Astroparticle Physics  

There has been a growing evidence for the existence of magnetic fields in the extra-galactic regions, while the attempt to associate their origin with the inflationary epoch alone has been found extremely challenging. We therefore take into account the consistent post-inflationary evolution of the magnetic fields that are originated from vacuum fluctuations during inflation. In the model of our interest, the electromagnetic (EM) field is coupled to a pseudo-scalar inflaton φ through the characteristic term φ F, breaking the conformal invariance. This interaction dynamically breaks the parity and enables a continuous production of only one of the polarization states of the EM field through tachyonic instability. The produced magnetic fields are thus helical. We find that the dominant contribution to the observed magnetic fields in this model comes from the modes that leave the horizon near the end of inflation, further enhanced by the tachyonic instability right after the end of inflation. The EM field is subsequently amplified by parametric resonance during the period of inflaton oscillation. Once the thermal plasma is formed (reheating), the produced helical magnetic fields undergo a turbulent process called inverse cascade, which shifts their peak correlation scales from smaller to larger scales. We consistently take all these effects into account within the regime where the perturbation of φ is negligible and obtain B∼ 10-19 G, indicating the necessity of additional mechanisms to accommodate the observations.

DOI： 10.1088/1475-7516/2015/05/054

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2015/03/013

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2014/10/030

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arXiv

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/1475-7516/2013/12/036

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arXiv

記述言語：日本語   会議種別：口頭発表（一般）  

記述言語：英語   会議種別：口頭発表（一般）  

記述言語：英語   会議種別：口頭発表（一般）  

記述言語：英語   会議種別：ポスター発表  

記述言語：英語   会議種別：公開講演，セミナー，チュートリアル，講習，講義等  

記述言語：日本語   会議種別：口頭発表（一般）  

記述言語：日本語   会議種別：口頭発表（一般）  

会議種別：口頭発表（招待・特別）  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：公開講演，セミナー，チュートリアル，講習，講義等  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：ポスター発表  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）  

会議種別：口頭発表（一般）