2023/11/12 更新

写真a

タダ ユウイチロウ
多田 祐一郎
TADA Yuichiro
所属
高等研究院 特任助教
大学院理学研究科 特任助教
職名
特任助教
連絡先
メールアドレス
外部リンク

学位 1

  1. 博士(理学) ( 2017年3月   東京大学 ) 

研究キーワード 4

  1. 初期宇宙

  2. インフレーション

  3. 原始ブラックホール

  4. 重力波

研究分野 1

  1. 自然科学一般 / 素粒子、原子核、宇宙線、宇宙物理にする理論

経歴 7

  1. 高エネルギー加速器研究機構   素粒子原子核研究所   協力研究員

    2022年4月 - 2023年3月

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    国名:日本国

  2. 名古屋大学   高等研究院   特任助教

    2021年4月 - 現在

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    国名:日本国

  3. 大同大学   非常勤講師

    2019年4月 - 2021年3月

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    国名:日本国

    備考:力学1, 2

  4. 名古屋大学   宇宙論研究室   日本学術振興会特別研究員 PD

    2018年4月 - 2021年3月

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    国名:日本国

  5. Institute d'Astrophysique de Paris   Dr. Sébastien Renaux-Petel's Group   PD researcher

    2017年4月 - 2018年3月

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    国名:フランス共和国

  6. 東京大学   カブリ数物連携宇宙研究機構, 宇宙線研究所   日本学術振興会特別研究員 DC2

    2015年4月 - 2017年3月

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    国名:日本国

  7. 東京大学   カブリ数物連携宇宙研究機構, 宇宙線研究所   フォトンサイエンス・リーディング大学院

    2012年10月 - 2017年3月

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    国名:日本国

▼全件表示

学歴 3

  1. 東京大学   大学院理学系研究科

    2014年4月 - 2017年3月

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    国名: 日本国

    備考: 博士課程

  2. 東京大学   大学院理学系研究科

    2012年4月 - 2014年3月

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    国名: 日本国

    備考: 修士課程

  3. 東京大学   理学部   物理学科

    2008年4月 - 2012年3月

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    国名: 日本国

所属学協会 1

  1. 日本物理学会

    2013年6月 - 現在

受賞 3

  1. Outstanding Presentation Award Gold Prize

    2020年11月   Online JGRG workshop 2020   Manifestly covariant theory of stochastic inflation

  2. EPL's Highlights 2020

    Conformal inflation in the metric-affine geometry

    Yusuke Mikura, Yuichiro Tada, Shuichiro Yokoyama

  3. 2019-20 Highlights of Classical and Quantum Gravity

    Inflationary stochastic anomalies

    Lucas Pinol, Sébastien Renaux-Petel, Yuichiro Tada

 

論文 34

  1. Squeezed bispectrum and one-loop corrections in transient constant-roll inflation 査読有り

    Motohashi H., Tada Y.

    Journal of Cosmology and Astroparticle Physics   2023 巻 ( 08 ) 頁: 069 - 069   2023年8月

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    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 canonical single-field inflation, the production of primordial black holes (PBH) requires a transient violation of the slow-roll condition. The transient ultra slow-roll inflation is an example of such scenarios, and more generally, one can consider the transient constant-roll inflation. We investigate the squeezed bispectrum in the transient constant-roll inflation and find that Maldacena's consistency relation holds for a sufficiently long-wavelength mode, whereas it is violated for modes around the peak scale for the non-attractor case. We also demonstrate how the one-loop corrections are modified compared to the case of the transient ultra slow-roll inflation, focusing on representative one-loop terms originating from a time derivative of the second slow-roll parameter in the cubic action. We find that the perturbativity requirement on those terms does not rule out the production of PBH from the transient constant-roll inflation. Therefore, it is a simple counterexample of the recently claimed no-go theorem of PBH production from single-field inflation.

    DOI: 10.1088/1475-7516/2023/08/069

    Scopus

    その他リンク: https://iopscience.iop.org/article/10.1088/1475-7516/2023/08/069/pdf

  2. Primordial black hole formation in hybrid inflation 査読有り

    Tada Y., Yamada M.

    Physical Review D   107 巻 ( 12 )   2023年6月

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

    We revisit the scenario of primordial black hole (PBH) formation from large curvature perturbations generated during the waterfall phase transition in hybrid inflation models. In a minimal setup considered in the literature, the mass and abundance of PBHs are correlated and astrophysical size PBHs tend to be overproduced. This is because a longer length scale for curvature perturbations (or a larger PBH mass) requires a longer waterfall regime with a flatter potential, which results in overproduction of curvature perturbations. However, in this paper, we discuss that the higher-dimensional terms for the inflaton potential affect the dynamics during the waterfall phase transition and show that astrophysical size PBHs of the order of 1017-23 g (which can explain the whole dark matter) can form in some parameter space consistently with any existing constraints. The scenario can be tested by observing the induced gravitational waves from scalar perturbations by future gravitational wave experiments, such as LISA.

    DOI: 10.1103/physrevd.107.123539

    Scopus

    その他リンク: http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevD.107.123539/fulltext

  3. Primordial black holes and gravitational waves induced by exponential-tailed perturbations 査読有り

    Abe, KT; Inui, R; Tada, Y; Yokoyama, S

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2023 巻 ( 5 ) 頁: 044 - 044   2023年5月

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    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

    Web of Science

    Scopus

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

  4. Hybrid metric-Palatini Higgs inflation 査読有り

    He, MX; Mikura, Y; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2023 巻 ( 5 ) 頁: 047 - 047   2023年5月

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    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

    Web of Science

    Scopus

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

  5. Stochastic formalism for U(1) gauge fields in axion inflation 査読有り

    Fujita, T; Mukaida, K; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2022 巻 ( 12 ) 頁: 026 - 026   2022年12月

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    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

    Web of Science

    Scopus

  6. Effective Inspiral Spin Distribution of Primordial Black Hole Binaries 査読有り

    Koga, Y; Harada, T; Tada, Y; Yokoyama, S; Yoo, CM

    ASTROPHYSICAL JOURNAL   939 巻 ( 2 )   2022年11月

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    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

    Web of Science

    Scopus

  7. Effective treatment of U(1) gauge field and charged particles in axion inflation 査読有り

    Fujita, T; Kume, J; Mukaida, K; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2022 巻 ( 9 ) 頁: 023 - 023   2022年9月

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    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

    Web of Science

    Scopus

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

  8. On UV-completion of Palatini-Higgs inflation 査読有り

    Mikura, Y; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2022 巻 ( 5 ) 頁: 035 - 035   2022年5月

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    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

    Web of Science

    Scopus

  9. Statistics of coarse-grained cosmological fields in stochastic inflation 査読有り

    Tada, Y; Vennin, V

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2022 巻 ( 2 ) 頁: 021 - 021   2022年2月

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    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

    Web of Science

    Scopus

    arXiv

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

  10. Primordial black holes in peak theory with a non-Gaussian tail 査読有り

    Kitajima, N; Tada, Y; Yokoyama, S; Yoo, CM

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2021 巻 ( 10 ) 頁: 053 - 053   2021年10月

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    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

    Web of Science

    Scopus

    arXiv

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

  11. Revisiting non-Gaussianity in non-attractor inflation models in the light of the cosmological soft theorem 査読有り

    Suyama T., Tada Y., Yamaguchi M.

    Progress of Theoretical and Experimental Physics   2021 巻 ( 7 )   2021年7月

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    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

  12. Induced gravitational waves as a cosmological probe of the sound speed during the QCD phase transition 査読有り

    Abe, KT; Tada, Y; Ueda, I

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2021 巻 ( 6 ) 頁: 048 - 048   2021年6月

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    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

    Web of Science

    Scopus

    arXiv

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

  13. Minimal k -inflation in light of the conformal metric-affine geometry 査読有り

    Mikura Y., Tada Y., Yokoyama S.

    Physical Review D   103 巻 ( 10 )   2021年5月

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    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

  14. A manifestly covariant theory of multifield stochastic inflation in phase space: solving the discretisation ambiguity in stochastic inflation 査読有り

    Pinol, L; Renaux-Petel, S; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   2021 巻 ( 4 ) 頁: 048 - 048   2021年4月

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    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

    Web of Science

    Scopus

    arXiv

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

  15. Conformal inflation in the metric-affine geometry 査読有り

    Mikura, Y; Tada, Y; Yokoyama, S

    EPL   132 巻 ( 3 ) 頁: 39001 - 39001   2020年11月

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    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

    Web of Science

    Scopus

    arXiv

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

  16. Local observer effect on the cosmological soft theorem 査読有り

    Suyama, T; Tada, Y; Yamaguchi, M

    PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS   2020 巻 ( 11 )   2020年11月

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    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

    Web of Science

    Scopus

    arXiv

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

  17. Escape from the swampland with a spectator field 査読有り

    Kogai, K; Tada, Y

    PHYSICAL REVIEW D   101 巻 ( 10 )   2020年5月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

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

  18. Stochastic inflation with an extremely large number of <i>e</i>-folds 査読有り

    Kitajima, N; Tada, Y; Takahashi, F

    PHYSICS LETTERS B   800 巻   2020年1月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

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

  19. Primordial black hole tower: Dark matter, earth-mass, and LIGO black holes 査読有り

    Tada, Y; Yokoyama, S

    PHYSICAL REVIEW D   100 巻 ( 2 )   2019年7月

     詳細を見る

    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

    Web of Science

    arXiv

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

  20. Inflationary stochastic anomalies 査読有り

    Pinol, L; Renaux-Petel, S; Tada, Y

    CLASSICAL AND QUANTUM GRAVITY   36 巻 ( 7 )   2019年4月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

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

  21. Primordial black hole tower: Dark matter, earth-mass, and LIGO black holes

    多田 祐一郎

    arXiv   1904.10298 巻   頁: 10298   2019年

     詳細を見る

  22. Does the detection of primordial gravitational waves exclude low energy inflation? 査読有り

    Fujita, T; Namba, R; Tada, Y

    PHYSICS LETTERS B   778 巻   頁: 17 - 21   2018年3月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  23. <i>O</i>(10)<i>M</i><sub>⊙</sub> primordial black holes and string axion dark matter 査読有り

    Inomata, K; Kawasaki, M; Mukaida, K; Tada, Y; Yanagida, TT

    PHYSICAL REVIEW D   96 巻 ( 12 ) 頁: 123527 - 123527   2017年12月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  24. Inflationary primordial black holes as all dark matter 査読有り

    Inomata, K; Kawasaki, M; Mukaida, K; Tada, Y; Yanagida, TT

    PHYSICAL REVIEW D   96 巻 ( 4 ) 頁: 043504 - 043504   2017年8月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  25. Inflationary primordial black holes for the LIGO gravitational wave events and pulsar timing array experiments 査読有り

    Inomata, K; Kawasaki, M; Mukaida, K; Tada, Y; Yanagida, TT

    PHYSICAL REVIEW D   95 巻 ( 12 ) 頁: 123510 - 123510   2017年6月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  26. Squeezed bispectrum in the <i>δN</i> formalism: local observer effect in field space 査読有り

    Tada, Y; Vennin, V

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   1702 巻 ( 2 ) 頁: 021 - 021   2017年2月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  27. Primordial black holes as dark matter in supergravity inflation models 査読有り

    Kawasaki, M; Kusenko, A; Tada, Y; Yanagida, TT

    PHYSICAL REVIEW D   94 巻 ( 8 ) 頁: 083523 - 083523   2016年10月

     詳細を見る

    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

    Web of Science

    arXiv

  28. Revisiting constraints on small scale perturbations from big-bang nucleosynthesis 査読有り

    Inomata, K; Kawasaki, M; Tada, Y

    PHYSICAL REVIEW D   94 巻 ( 4 ) 頁: 043527 - 043527   2016年8月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  29. Can massive primordial black holes be produced in mild waterfall hybrid inflation? 査読有り

    Kawasaki, M; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   1608 巻 ( 8 ) 頁: 041 - 041   2016年8月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  30. Primordial black holes as biased tracers 査読有り

    Tada, Y; Yokoyama, S

    PHYSICAL REVIEW D   91 巻 ( 12 ) 頁: 123534 - 123534   2015年6月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  31. Consistent generation of magnetic fields in axion inflation models 査読有り

    Fujita, T; Namba, R; Tada, Y; Takeda, N; Tashiro, H

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   1505 巻 ( 5 ) 頁: 054 - 054   2015年5月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  32. Anisotropic CMB distortions from non-Gaussian isocurvature perturbations 査読有り

    Ota, A; Sekiguchi, T; Tada, Y; Yokoyama, S

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   1503 巻 ( 3 ) 頁: 013 - 013   2015年3月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  33. Non-perturbative approach for curvature perturbations in stochastic-δ<i>N</i> formalism 査読有り

    Fujita, T; Kawasaki, M; Tada, Y

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   1410 巻 ( 10 ) 頁: 030 - 030   2014年10月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

  34. A new algorithm for calculating the curvature perturbations in stochastic inflation 査読有り

    Fujita, T; Kawasaki, M; Tada, Y; Takesako, T

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS   1312 巻 ( 12 ) 頁: 036 - 036   2013年12月

     詳細を見る

    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

    Web of Science

    Scopus

    arXiv

▼全件表示

MISC 4

  1. Cancellation of quantum corrections on the soft curvature perturbations

    Yuichiro Tada, Takahiro Terada, Junsei Tokuda  

        2023年8月

     詳細を見る

    We study the cancellation of quantum corrections on the superhorizon
    curvature perturbations from subhorizon physics beyond the single-clock
    inflation from the viewpoint of the cosmological soft theorem. As an example,
    we focus on the transient ultra-slow-roll inflation scenario and compute the
    one-loop quantum corrections to the power spectrum of curvature perturbations
    taking into account nontrivial surface terms in the action. We find that
    Maldacena's consistency relation is satisfied and guarantees the cancellation
    of contributions from the short-scale modes. As a corollary, primordial black
    hole production in single-field inflation scenarios is not excluded by
    perturbativity breakdown even for the sharp transition case in contrast to some
    recent claims in the literature. We also comment on the relation between the
    tadpole diagram in the in-in formalism and the shift of the elapsed time in the
    stochastic-$\delta N$ formalism. We find our argument is not directly
    generalisable to the tensor perturbations.

    arXiv

    その他リンク: http://arxiv.org/pdf/2308.04732v1

  2. Translating nano-Hertz gravitational wave background into primordial perturbations taking account of the cosmological QCD phase transition

    Katsuya T. Abe, Yuichiro Tada  

        2023年7月

     詳細を見る

    The evidence of the nano-Hertz stochastic gravitational wave (GW) background
    is reported by multiple pulsar timing array collaborations. While a prominent
    candidate of the origin is astrophysical from supermassive black hole binaries,
    alternative models involving GWs induced by primordial curvature perturbations
    can explain the inferred GW spectrum. Serendipitously, the nano-Hertz range
    coincides with the Hubble scale during the cosmological quantum chromodynamics
    (QCD) phase transition. The influence of the QCD phase transition can modify
    the spectrum of induced GWs within the nano-Hertz frequency range,
    necessitating careful analysis. We estimate GWs induced by power-law power
    spectra of primordial curvature perturbations taking account of the QCD phase
    transition. Then we translate the implication of the NANOGrav data into the
    constraint on the power spectrum of the primordial curvature perturbation,
    which suggests that one may miss the correct interpretation if neglecting the
    QCD effect.

    arXiv

    その他リンク: http://arxiv.org/pdf/2307.01653v2

  3. Stochastic dynamics of multi-waterfall hybrid inflation and formation of primordial black holes

    Yuichiro Tada, Masaki Yamada  

        2023年6月

     詳細を見る

    We show that a hybrid inflation model with multiple waterfall fields can
    result in the formation of primordial black hole (PBH) with an astrophysical
    size, by using an advanced algorithm to follow the stochastic dynamics of the
    waterfall fields. This is in contrast to the case with a single waterfall
    field, where the wavelength of density perturbations is usually too short to
    form PBHs of the astrophysical scale (or otherwise PBH are overproduced and the
    model is ruled out) unless the inflaton potential is tuned. In particular, we
    demonstrate that PBHs with masses of order $10^{20}\, {\rm g}$ can form after
    hybrid inflation consistently with other cosmological observations if the
    number of waterfall fields is about 5 for the case of instantaneous reheating.
    Observable gravitational waves are produced from the second-order effect of
    large curvature perturbations as well as from the dynamics of texture or global
    defects that form after the waterfall phase transition.

    arXiv

    その他リンク: http://arxiv.org/pdf/2306.07324v1

  4. Primordial Black Holes

    Albert Escrivà, Florian Kuhnel, Yuichiro Tada  

        2022年11月

     詳細を見る

    We review aspect of primordial black holes, i.e., black holes which have been
    formed in the early Universe. Special emphasis is put on their formation, their
    r\^ole as dark matter candidates and their manifold signatures, particularly
    through gravitational waves.

    arXiv

    その他リンク: http://arxiv.org/pdf/2211.05767v1

講演・口頭発表等 25

  1. 曲率ゆらぎへの非摂動的アプローチ 招待有り

    多田祐一郎

    第11回観測的宇宙論ワークショップ  2022年12月13日 

     詳細を見る

    会議種別:口頭発表(招待・特別)  

  2. 星が先かブラックホールが先か?原始ブラックホール研究の最前線 招待有り

    多田祐一郎

    名古屋大学・名古屋市科学館 公開オンラインセミナー  2021年8月22日 

     詳細を見る

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

  3. 宇宙の「インフレ」と「金融工学」 招待有り

    多田祐一郎

    名大発アカデミックフラッシュ 第2報  2021年9月3日 

     詳細を見る

    記述言語:日本語   会議種別:口頭発表(一般)  

  4. 多重滝場ハイブリッドインフレーションと原始ブラックホール

    多田祐一郎, 山田將樹

    日本物理学会春季大会  2023年3月23日 

     詳細を見る

    会議種別:口頭発表(一般)  

  5. 原始ブラックホール研究の近年の進展

    多田祐一郎

    第35回 理論懇シンポジウム  2022年12月21日 

     詳細を見る

    会議種別:ポスター発表  

  6. 粗視化曲率ゆらぎの確率密度関数

    多田祐一郎, Vincent Vennin

    日本物理学会秋季大会  2021年9月14日 

     詳細を見る

    記述言語:日本語   会議種別:口頭発表(一般)  

  7. インフレーションの「現在」と重力波 招待有り

    多田祐一郎

    DECIGO ワークショップ  2021年12月11日 

     詳細を見る

    記述言語:日本語   会議種別:口頭発表(一般)  

  8. Primordial black holes in peak theory with a non-Gaussian tail

    Naoya Kitajima, Yuichiro Tada, Shuichiro Yokoyama, Chul-Moon Yoo

    The KEK-PH + KEK-Cosmo joint workshop on "Primordial Black Holes"  2021年10月19日 

     詳細を見る

    記述言語:日本語   会議種別:口頭発表(一般)  

  9. Primordial black holes in peak theory with a non-Gaussian tail 招待有り

    Naoya Kitajima, Yuichiro Tada, Shuichiro Yokoyama, Chul-Moon. Yoo

    2021 NRF-JSPS Workshop in particle physics, cosmology, and gravitation  2021年7月21日 

     詳細を見る

    記述言語:英語   会議種別:口頭発表(一般)  

  10. Probability density functions of coarse-grained curvature and density perturbations in stochastic inflation

    Yuichiro Tada, Vincent Vennin

    JGRG30  2021年12月9日 

     詳細を見る

    記述言語:英語   会議種別:口頭発表(一般)  

  11. Probability density functions of coarse-grained curvature and density perturbations in stochastic inflation

    Yuichiro Tada, Vincent Vennin

    COSMO'21  2021年8月2日 

     詳細を見る

    記述言語:英語   会議種別:ポスター発表  

  12. Self-introduction, or a biased view of what theoretical cosmologists are recently interested in 招待有り

    Yuichiro Tada

    E-lab seminar, Nagoya U.  2021年5月25日 

     詳細を見る

    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  

  13. Primordial black holes and induced gravitational waves in light of the non-Gaussian tail 招待有り

    FY2021 学術変革領域研究「ダークマター」シンポジウム  2022年3月30日 

     詳細を見る

    会議種別:口頭発表(一般)  

  14. Hybrid metric-Palatini Higgs inflation

    M. He, Y. Mikura, Y. Tada

    JGRG31  2022年10月25日 

     詳細を見る

    会議種別:口頭発表(一般)  

  15. Inflation theory — D'où venons-nous? Que sommes-nous? Où allons-nous? — 招待有り

    Y. Tada

    19th Rencontres du Vietnam "Theory meeting experiments: particle astrophysics and cosmology"  2023年1月8日 

     詳細を見る

    会議種別:口頭発表(一般)  

  16. On the soft theorem in the transient ultra-slow or constant roll inflation 招待有り

    H. Motohashi, Y. Tada

    Domestic Molecule-type Workshop "Non-linear Nature of Cosmological Perturbations and its Observational Consequences"  2023年3月28日 

     詳細を見る

    会議種別:口頭発表(一般)  

  17. Recent progress on stochastic inflation 招待有り

    多田祐一郎

    Workshop on Gravitation & Cosmology  2022年11月29日 

     詳細を見る

    会議種別:公開講演,セミナー,チュートリアル,講習,講義等  

  18. Revisiting the formation of massive primordial black holes in hybrid inflation

    Y. Tada, M. Yamada

    i-LINK workshop on observational cosmology  2023年6月7日 

     詳細を見る

    会議種別:口頭発表(一般)  

  19. Squeezed bispectrum and one-loop corrections in transient constant-roll inflation

    H. Motohashi, Y. Tada, T. Terada, J. Tokuda

    COSMO'23  2023年9月15日 

     詳細を見る

    会議種別:口頭発表(一般)  

  20. Statistics of coarse-grained cosmological fields in stochastic inflation

    Y. Tada, V. Vennin

    New Horizons in Primordial Black Hole Physics  2023年6月19日 

     詳細を見る

    会議種別:口頭発表(一般)  

  21. Stochastic approach to cosmic inflation

    Y. Tada

    TSQS 2022  2022年11月8日 

     詳細を見る

    会議種別:ポスター発表  

  22. Stochastic approach to the inflationary universe 招待有り

    Y. Tada

    YITP International Molecule-type Workshop "Revisiting cosmological non-linearities in the era of precision surveys"  2023年7月27日 

     詳細を見る

    会議種別:口頭発表(一般)  

  23. アクシオンインフレーションにおける U(1) ゲージ場の確率形式

    藤田智弘, 向田享平, 多田祐一郎

    日本物理学会秋季大会  2022年9月6日 

     詳細を見る

    会議種別:口頭発表(一般)  

  24. 原始ブラックホールのピーク理論と非ガウス尾

    A. Escrivà, 北嶋直弥, 多田祐一郎, 横山修一郎, 柳哲文

    日本物理学会第77回年次大会  2022年3月17日 

     詳細を見る

    会議種別:口頭発表(一般)  

  25. 原始ブラックホールは自然科学なのか 招待有り

    多田祐一郎

    基研研究会 素粒子物理学の進展2023 (PPP2023)  2023年9月1日 

     詳細を見る

    会議種別:口頭発表(招待・特別)  

▼全件表示

科研費 4

  1. 確率解析・原始ブラックホール・重力波観測から迫るインフレーション

    研究課題/研究課題番号:21K13918  2021年4月 - 2024年3月

    日本学術振興会  科学研究費助成事業 若手研究  若手研究

    多田 祐一郎

      詳細を見る

    担当区分:研究代表者 

    配分額:4680000円 ( 直接経費:3600000円 、 間接経費:1080000円 )

    本研究の目的を踏まえ具体的研究課題は次のように3つに分けられる. 1つ目はインフレーションの確率形式を理論的に整備するとともに, 数値計算プログラムを開発・公開し, さらに具体的なインフレーション理論の解析を行うことである. また本形式を応用し原始ブラックホール形成理論を探るとともに, 正確な原始ブラックホール量の計算法を確立する. そして原始ブラックホールに関連した初期重力波を通じてインフレーションに対して得られる情報を模索する.
    昨年度は4本の論文を発表した。特に "Primordial black holes in peak theory with a non-Gaussian tail" (JCAP 10, 053 (2021)) および "Simulation of Primordial Black Holes with large negative non-Gaussianity" (arXiv:2202.01028 [astro-ph.CO]) では非ガウス性がある時の原始ブラックホール形成条件や、生成量を見積もる「ピーク理論」の整備を行い、初期ゆらぎの統計性を仮定した時の原始ブラックホール存在量見積もりの定式化を推し進めた。また "Statistics of coarse-grained cosmological fields in stochastic inflation" (JCAP 02, no.02, 021 (2022)) では確率形式において任意スケールで粗視化した曲率ゆらぎ、密度ゆらぎ、および圧縮関数の確率密度関数を計算する手法を定式化した。これによりインフレーション模型から原始ブラックホール量等の観測量を確率形式でつなぐ最後のピースが完成したと言える。これらの研究成果について昨年度中5件の国際会議と3件の国内会議、4件のセミナーにおいて発表を行った。うち「名古屋大学・名古屋市科学館公開オンラインセミナー」および「名大発アカデミックフラッシュ 第2報」の2件においてはアウトリーチ活動として一般講演を行なった。
    本研究は確率解析、原始ブラックホール、重力波を3本柱とインフレーション理論や暗黒物質の正体に迫ることを目的としており、そのうちの2つである確率解析と原始ブラックホールに関して上述したように重要な定式化を行う論文を発表することができた。また近年注目を集めてきている初期ゆらぎ確率密度関数の非ガウス尾について、会議やセミナーでの発表を通じ周知することができた。以上より本研究課題はおおむね順調に進展していると評価できる。
    初年度では確率解析と原始ブラックホール分野において重要な定式化を行う論文を発表した。そこで次年度は重力波に関し、特に初期ゆらぎに非ガウス尾がある場合の計算法を確立する。当該研究は現在所属研究室の学生らも交え進めているところであるが、そこで重力波スペクトル形から逆に初期ゆらぎの非ガウス性を読み解く可能性も見出した。この可能性を精査し、重力波スペクトルを観測量とし原始ブラックホール量や初期ゆらぎの統計性、さらにはインフレーション理論に対し示唆を与えることができるかを議論する。

  2. ストカスティック形式で迫る重力と量子論

    研究課題/研究課題番号:19K14707  2019年4月 - 2021年3月

    日本学術振興会  科学研究費助成事業 若手研究  若手研究

    多田 祐一郎

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    担当区分:研究代表者 

    配分額:1560000円 ( 直接経費:1200000円 、 間接経費:360000円 )

    量子論と重力理論の統合は現代物理学の課題の1つであるが、両方に関係した重要な現象として重力による量子ゆらぎの古典化がある。特に宇宙初期の急激な膨張期 (インフレーション) ではこの古典化により、現在の宇宙における銀河や星などの構造の元となる密度ゆらぎを生み出したと考えられている。このように古典化したゆらぎを扱う方法としてストカスティック形式が知られており、我々はこの形式と純粋な量子論的計算を比較することで古典化の謎に迫る。同時に、ストカスティック形式では量子ゆらぎを古典ゆらぎとして簡略的に扱うことで逆に計算の幅が広がるので、これをインフレーション模型の包括的解析に応用していく。
    研究期間中に計13本の論文を発表。特に1908.08694では確率形式を用いて非常に長く続くインフレーション理論を提唱・検証し、2008.07497では確率形式をインフラトン場に対し共変な形で再定式化、既存の不定性を解決した。2111.15280では確率形式において任意に粗視化した宇宙論的ゆらぎの確率密度関数を計算する手法を確立し、インフレーションから天体形成までの最後のピースを埋めた。研究成果は24件の国内外会議および11件のセミナーで発表し、Online JGRG Workshop 2020ではOutstanding Presentation Award Gold Prizeを受賞した。
    本研究によってインフレーションの確率形式が不定性なく定式化され、またインフレーションから天体形成までの全ての過程を確率形式において計算することができるようになった。これによりあらゆるインフレーション理論に対し結果的な観測量を計算する準備が整ったと言える。例えば現在暗黒物質の候補として原始ブラックホールが再注目され、その副産物としての重力波などについても、私自身のいくつかの研究を含め世界的に研究が進められているが、これらを一貫した手法で解析するために本研究成果が役立つであろう。

  3. インフレーション宇宙における曲率ゆらぎと原始ブラックホール形成

    研究課題/研究課題番号:18J01992  2018年4月 - 2021年3月

    日本学術振興会  科学研究費助成事業 特別研究員奨励費  特別研究員奨励費

    多田 祐一郎

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    前年度に引き続きインフレーションに対するストカスティック形式を主題の1つとして研究を行った。ストカスティック形式ではインフレーション中のスカラー場のゆらぎをブラウン運動として取り入れるが、ブラウン運動の微分不可な特性からスカラー場間の共変性が崩れることに気づき、前年度所属のパリ IAP に再出張し論文にまとめ、CQG Letter に採択された[1]。その後 NCTS の北本氏により、量子論とストカスティック形式の対応に際し、タドポール図を正しく取り入れることで共変性を回復できることが、スローロール近似の下で示された[1811.01830]。現在これを近似を使わない形式に拡張すべく研究中である。
    またストカスティック形式を応用し、インフレーションによる曲率ゆらぎを非摂動的に解析する手法を数値計算ツールとして実装し学会発表した[国内 3]。現在これを公開コードとして発表する準備を行っている。
    もう1つの課題である原始ブラックホール (PBH) に関しては、同じく名古屋大学所属の横山助教とともに研究を行い、学会でも発表した[国内 4, 国外 2]。PBH の動機として我々は、暗黒物質、LIGO の重力波検出、OGLE による自由惑星の重力レンズ現象の発見の3つに注目し、インフレーションを複数段階に分けることでこれらを同時に説明する PBH 質量スペクトルを実現できることを示した。こうした複数段階インフレーションは、最近発表された string swampland conjecture [1810.05506] からも支持されることを指摘している。これらの結果を論文として発表する準備の途中である。
    最後に同研究室学生の阿部氏とは、振動するスカラー場の重力崩壊現象について研究を行っている。

  4. 小スケール曲率ゆらぎの観測とインフレーション

    研究課題/研究課題番号:15J10829  2015年4月 - 2017年3月

    日本学術振興会  科学研究費助成事業 特別研究員奨励費  特別研究員奨励費

    多田 祐一郎

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    昨年度は5本の論文 (arXiv:1605.04646, 1606.07631, 1609.08876, 1611.06130, 1701.02544) を提出した。1つ目の論文では小スケールにもし大きな曲率ゆらぎが生じていた場合、それがビッグバン元素合成に与える影響について再考察し、既存の結果が定性的レベルで修正されることを示した。また3つ目の論文では、曲率ゆらぎのスクィーズド極限バイスペクトルに関し、近年指摘されているゲージ依存効果についての新たな理解を示し、簡単な計算アルゴリズムを提唱した。これは今後、実際に何らかの観測 (宇宙背景放射や銀河バイアス等) によってバイスペクトルを測定し、それを宇宙初期の物理と対応させるときに非常に重要になってくる。残りの論文では原始ブラックホールについて議論している。昨年 LIGO グループが初の重力波直接検出を発表し話題になったが、この重力波は重いブラックホール連星の合体から来ており、こうしたブラックホールが原始ブラックホールである可能性も注目されてきている。また伴って、原始ブラックホールが暗黒物質である可能性も再注目されてきた。そこで我々は実際に具体的なインフレーション模型を構築し、原始ブラックホールが LIGO の重力波イベントや暗黒物質を説明しうるか、また他に検出方法があるかどうかなどを検証した。
    これら研究成果に関し、国際会議で2度, 国内会議で1度口頭発表を行った. また国内外合わせて4つの研究機関でセミナー発表を行った.

 

担当経験のある科目 (本学) 2

  1. 電磁気学・統計物理学Ⅰ演習

    2023

  2. 電磁気学・統計物理学Ⅰ演習

    2022

担当経験のある科目 (本学以外) 1

  1. 力学1, 2

    2019年4月 - 2021年3月 大同大学)