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：日本語   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   出版者・発行元：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.75.1.0_543

CiNii Research

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

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   出版者・発行元：一般社団法人 日本物理学会  

<p>ビッグバン元素合成が起きる時期には、ホライズン内の10^4^Mpc^-1^<k<10^5^Mpc^-1^の小スケールゆらぎはまだシルク減衰によって均されていない。我々は、まだ均されていない小スケールゆらぎがある中での陽子中性子比の決定を数値計算で評価した。特に、ホライズンに入った直後のゆらぎの振る舞いに注意して計算を行った。さらにその結果を、陽子中性子比と密接に関係する^4^Heの存在量の観測結果と比較することで小スケールゆらぎに対して制限をかけることに成功した。</p>

DOI： 10.11316/jpsgaiyo.71.2.0_328

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：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）