所属 理工学術院 先進理工学部 職名 助教

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• 2018年04月
-
2021年03月

早稲田大学   大学院先進理工学研究科   物理学及応用物理学専攻

後期博士課程

### 経歴 【 表示 ／ 非表示 】

• 2021年04月
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継続中

早稲田大学   先進理工学部   助教   助教

• 2019年04月
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2021年03月

早稲田大学   先進理工学部   助手   助手

### 研究分野 【 表示 ／ 非表示 】

• 素粒子、原子核、宇宙線、宇宙物理にする理論   宇宙論

• 中性子星

• ヒッグス場

• インフレーション

• 重力

• 宇宙論

### Misc 【 表示 ／ 非表示 】

• Ryotaro Kase, Rampei Kimura, Seiga Sato, Shinji Tsujikawa

2020年07月

概要を見る

We study the stability of relativistic stars in scalar-tensor theories with a
nonminimal coupling of the form $F(\phi)R$, where $F$ depends on a scalar field
$\phi$ and $R$ is the Ricci scalar. On a spherically symmetric and static
background, we incorporate a perfect fluid minimally coupled to gravity as a
form of the Schutz-Sorkin action. The odd-parity perturbation for the
multipoles $l \geq 2$ is ghost-free under the condition $F(\phi)>0$, with the
speed of gravity equivalent to that of light. For even-parity perturbations
with $l \geq 2$, there are three propagating degrees of freedom arising from
the perfect-fluid, scalar-field, and gravity sectors. For $l=0, 1$, the
dynamical degrees of freedom reduce to two modes. We derive no-ghost conditions
and the propagation speeds of these perturbations and apply them to concrete
theories of hairy relativistic stars with $F(\phi)>0$. As long as the perfect
fluid satisfies a weak energy condition with a positive propagation speed
squared $c_m^2$, there are neither ghost nor Laplacian instabilities for
theories of spontaneous scalarization and Brans-Dicke (BD) theories with a BD
parameter $\omega_{\rm BD}>-3/2$ (including $f(R)$ gravity). In these theories,
provided $0<c_m^2 \le 1$, we show that all the propagation speeds of
even-parity perturbations are sub-luminal inside the star, while the speeds of
gravity outside the star are equivalent to that of light.

• Seiga Sato, Kei-ichi Maeda

2020年01月

概要を見る

Hybrid Higgs inflation model is a hybrid type of the so-called Higgs
inflation model with the conventional non-minimal $\xi$ coupling to a scalar
curvature $R$ and new Higgs inflation model with the derivative coupling to the
Einstein curvature tensor $G_{\mu\nu}$. This model can explain all possible
value of the tensor-to-scalar ratio $r$ allowed by the present CMB constraints
with an appropriate choice of coupling parameters. However the derivative
coupling may causes a gradient instability during oscillation phase after
inflation just as in new Higgs inflation model. Analyzing the behaviours of
perturbations during oscillation phase in the hybrid Higgs inflation model, we
show that the unstable scalar modes found in new Higgs inflation model are
stabilized by the non-minimal $\xi$ coupling for some range of the coupling
parameters.

• Mitsuhiro Fukushima, Yosuke Misonoh, Shoichiro Miyashita, Seiga Sato

2018年12月

概要を見る

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.

• Seiga Sato, Kei-ichi Maeda

2017年12月

概要を見る

We propose a hybrid type of the conventional Higgs inflation and new Higgs
inflation models. We perform a disformal transformation into the Einstein frame
and analyze the background dynamics and the cosmological perturbations in the
truncated model, in which we ignore the higher-derivative terms of the Higgs
field. From the observed power spectrum of the density perturbations, we obtain
the constraint on the non-minimal coupling constant $\xi$ and the mass
parameter $M$ in the derivative coupling. Although the primordial tilt $n_s$ in
the hybrid model barely changes, the tensor-to-scalar ratio $r$ moves from the
value in new Higgs inflationary model to that in the conventional Higgs
inflationary model as $|\xi|$ increases. We confirm our results by numerical
analysis by ADM formalism of the full theory in the Jordan frame.

### 特定課題研究 【 表示 ／ 非表示 】

• 2019年   前田恵一

概要を見る

曲がった時空上で場の量子化を行うと重力との非最小結合が導入される。本研究では量子効果が影響を与える高エネルギーの初期宇宙を用いてこの非最小結合の妥当性を検証した。宇宙背景放射の観測結果から宇宙初期に加速膨張(インフレーション)の存在が期待されることから、この膨張が非最小結合によって引き起こされるか考えた。膨張を引き起こす場はHiggs場を仮定した。本研究では特に加速膨張後の再加熱期に注目し、現在の標準理論であるビッグバン宇宙論との接続が可能か検証した。重力との非最小結合には不安定性を引き起こすものがある。本研究ではこの不安定性が他の非最小結合によって回避できることを示した。