Updated on 2022/05/24

写真a

 
MIYASHITA, Shoichiro
 
Affiliation
Faculty of Science and Engineering, School of Advanced Science and Engineering
Job title
Assistant Professor(without tenure)

Education

  • 2017.04
    -
    2020.03

    Waseda University   Graduate School of Advanced Science and Engineering   Department of Pure and Applied Physics  

  • 2015.04
    -
    2017.03

    Waseda University   Graduate School of Advanced Science and Engineering   Department of Pure and Applied Physics  

  • 2011.04
    -
    2015.03

    Waseda University   School of Advanced Science and Engineering   Department of Applied Physics  

Research Experience

  • 2020.04
    -
    Now

    Waseda University   School of Advanced Science and Engineering   Assistant Professor

Professional Memberships

  • 2015.09
    -
    Now

    日本物理学会

 

Research Areas

  • Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics   general relativity

Research Interests

  • AdS/CFT correspondence

  • Holographic principle

  • Black hole

  • general relativity

  • gravity

Papers

  • Role of Mann Counterterm in Gravitational Energy

    Shoichiro Miyashita

       2020.07

     View Summary

    In 1999, R. B. Mann proposed a counterterm that is some sort of
    generalization of the well-known Holographic counterterm and that can eliminate
    the divergence of the gravitational action of asymptotically AdS and flat
    spacetimes (Phys. Rev. D $\boldsymbol{60}$ (1999) 104047 [1]). I show it is not
    only for eliminating the divergence of such spacetimes but also for setting the
    ground state energy to zero for any $d$-dimensional spacetimes with an $S^{d-2}
    \times \mathbb{R} $ boundary geometry, and speculate it is also true for
    spacetimes with any (suitable) boundary geometry and topology.

  • Quantum Aspects of Gravity in Thermal Equilibrium

    Shoichiro Miyashita

       2020.03

  • Energy spectrum of a quantum spacetime with boundary

    Shoichiro Miyashita

       2019.06  [Refereed]

     View Summary

    In this paper, I revisit the microcanonical partition function, or density of
    states (DOS), of general relativity. By using the minisuperspace path integral
    approximation, I directly calculate the $S^2 \times Disc$ topology sector of
    the DOS of a (quantum) spacetime with an $S^2\times \mathbb{R}$ Lorentzian
    boundary from the microcanonical path integral, in contrast with previous works
    in which DOSs are derived by inverse Laplace transformation from various
    canonical partition functions. Although I found there always exists only one
    saddle point for any given boundary data, it does not always dominate the
    possible integration contours. There is another contribution to the path
    integral other than the saddle point. One of the obtained DOSs has behavior
    similar to that of the previous DOSs; that is, it exhibits exponential
    Bekenstein--Hawking entropy for the limited energy range $ (1-\sqrt{2/3})
    <GE/R_{b}< (1+\sqrt{2/3})$, where energy $E$ is defined by the Brown--York
    quasi-local energy momentum tensor and $R_{b}$ is the radius of the boundary
    $S^2$. In that range, the DOS is dominated by the saddle point. However, for
    sufficiently high energy, where the saddle point no longer dominates, the DOS
    approaches a positive constant, different from the previous ones, which
    approach zero.

    DOI

  • Stable Singularity-free Cosmological Solutions in non-projectable Horava-Lifshitz Gravity

    Mitsuhiro Fukushima, Yosuke Misonoh, Shoichiro Miyashita, Seiga Sato

    PHYSICAL REVIEW D   99 ( 6 )  2018.12

     View Summary

    We find stable singularity-free cosmological solutions in non-flat
    Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) spacetime in the context of
    Ho\v{r}ava-Lifshitz (HL) theory. Although we encounter the negative squared
    effective masses of the scalar perturbations in the original HL theory, the
    behaviors can be remedied by relaxing the projectability condition. In our
    analysis, the effects from the background dynamics are taken into account as
    well as the sign of the coefficients in the quadratic action for perturbations.
    More specifically, we give further classification of the gradient
    stability/instability into five types. These types are defined in terms of the
    effective squared masses of perturbations $\mathcal{M}^2$, the effective
    friction coefficients in perturbation equations $\mathcal{H}$ and these
    magnitude relations $|\mathcal{M}^2|/\mathcal{H}^2$. Furthermore, we indicate
    that oscillating solutions possibly show a kind of resonance especially in open
    FLRW spacetime. We find that the higher order spatial curvature terms with
    Lifshitz scaling $z=3$ are significant to suppress the instabilities due to the
    background dynamics.

    DOI

  • Stability of Singularity-free Cosmological Solutions in Hořava-Lifshitz Gravity

    Yosuke Misonoh, Mitsuhiro Fukushima, Shoichiro Miyashita

    PHYSICAL REVIEW D   95 ( 4 )  2016.12

     View Summary

    We study stability of singularity-free cosmological solutions with positive
    cosmological constant based on projectable Ho\v{r}ava-Lifshitz (HL) theory. In
    HL theory, the isotropic and homogeneous cosmological solutions with bounce can
    be realized if spacial curvature is non-zero. By performing perturbation
    analysis around non-flat Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime,
    we derive a quadratic action and discuss the stability, i.e, ghost and
    tachyon-free conditions. Although the squared effective mass of scalar
    perturbation must be negative in infrared regime, we can avoid tachyon
    instability by considering strong Hubble friction. Additionally, we estimate
    the backreaction from the perturbations on background geometry, especially,
    against anisotropic perturbation in closed FLRW spacetime. It turns out that
    certain types of bouncing solution may be spoiled even if all perturbation
    modes are stable.

    DOI

  • AdS Monopole Black Hole and Phase Transition

    Shoichiro Miyashita, Kei-ichi Maeda

       2016.10

     View Summary

    We study the Einstein-SO(3)Yang-Mills-Higgs system with a negative
    cosmological constant, and find the monopole black hole solutions as well as
    the trivial Reissner-Nordstr\"{o}m black hole. We discuss thermodynamical
    stability of the monopole black hole in an isolated system. We expect a phase
    transition between those two black holes when the mass of a black hole
    increases or decreases. The type of phase transition depends on the
    cosmological constant $\Lambda$ as well as the vacuum expectation value $v$ and
    the coupling constant $\lambda$ of the Higgs field. Fixing $\lambda$ small, we
    find there are two critical values of the cosmological constant $\Lambda_{\rm
    cr (1)}(v)$ and $\Lambda_{\rm cr(2)}(v)$, which depend on $v$. If $\Lambda_{\rm
    cr(1)}(v)<\Lambda (<0)$, we find the first order transition, while if
    $\Lambda_{\rm cr(2)}(v)<\Lambda<\Lambda_{\rm cr(1)}(v)$, the transition becomes
    second order. For the case of $\Lambda_{b}(v)<\Lambda<\Lambda_{\rm (2)}(v)$, we
    again find the first order irreversible transition from the monopole black hole
    to the extreme Reissner-Nordstr\"{o}m black hole. Beyond $\Lambda_{b}(v)$, no
    monopole black hole exists. We also discuss thermodynamical properties of the
    monopole black hole in a thermal bath system.

    DOI

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Specific Research

  • 量子重力の粗視化された性質の探求

    2021  

     View Summary

    重力の量子論については未だ完全な定式化は存在せず、重力系の量子力学的ダイナミクスを追うことは現時点では出来ていない。しかしながらある程度粗視化されたダイナミクスについては統計力学の手法によって理解できるということが知られている。本研究では昨年度に引き続きその手法を用いて正の宇宙項の場合や物質と結合している場合の(熱)平衡状態について考察した。特に、電磁場と結合した場合についても調べ、Hawking-Page相転移が起こることを示した。

  • 重力の量子化とホログラフィー

    2020  

     View Summary

    重力の量子論については未だ完全な定式化は存在せず、重力系の量子力学的ダイナミクスを追うことは現時点では出来ていない。しかしながらある程度粗視化されたダイナミクスについては統計力学の手法によって理解できるということが知られている。本研究ではその手法を用いて正の宇宙項の場合や物質と結合している場合の(熱)平衡状態について考察した。また、重力の経路積分を定義する際に必要な相殺項として普遍的なものはこれまで知られておらず、状況に応じてその都度適当なものを設定する必要があったが、本研究では普遍的な相殺項の候補を提唱した。

 

Syllabus

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