Updated on 2022/05/24

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

• 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

• Holographic principle

• Black hole

• general relativity

• gravity

### Papers

• 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.

• Shoichiro Miyashita

2020.03

• 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.

• 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.

• 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.

• 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.

### Specific Research

• 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。

• 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

• School of Fundamental Science and Engineering

2022   spring semester

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year

• School of Advanced Science and Engineering

2022   full year