Shi Pi
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Secondary gravitational waves in non-local Starobinsky inflation Open
We show how infinite derivative modifications of gravity impact on the stochastic background of Gravitational Waves from early Universe. The generic property of the ghost-free theory fixed on Minkowski space-time is the emergence of an inf…
$δn$ formalism: A new formulation for the probability density of the curvature perturbation Open
$δN$ formalism is a useful method to calculate the curvature perturbation. Contrary to what it is typically done in the literature, we re-formulate the $δN$ formalism by using the $e$-folding number $n$ counted forward in time. For a fixed…
Primordial Black Hole Formation from Power Spectrum with Finite-width Open
Primordial black holes (PBHs) can form from gravitational collapse of large overdensities in the early Universe, giving rise to rich phenomena in astrophysics and cosmology. We develop a novel, general, and systematic method based on theor…
Primordial black holes and induced gravitational waves from logarithmic non-Gaussianity Open
We investigate the formation of primordial black hole (PBH) based on numerical relativity simulations and peak theory as well as the corresponding scalar induced gravitational wave (SIGW) signals in the presence of \emph{logarithmic non-Ga…
Constant roll and non-Gaussian tail in light of logarithmic duality Open
The curvature perturbation in a model of constant-roll (CR) inflation is interpreted in view of the logarithmic duality discovered in Ref. [1] according to the $δN$ formalism. We confirm that the critical value $β:=\ddotφ/(H\dotφ)=-3/2$ de…
Extended $δN$ formalism: Nonspatially Flat Separate Universe Approach Open
The $δN$ formalism is a powerful approach to compute non-linearly the large-scale evolution of the comoving curvature perturbation $ζ$. It assumes a set of FLRW patches that evolve independently, but in doing so, all the gradient terms are…
Induced gravitational wave interpretation of PTA data: a complete study for general equation of state Open
We thoroughly study the induced gravitational wave interpretation of the possible gravitational wave background reported by PTA collaborations, considering the unknown equation of state w of the early universe. We perform a Bayesian analys…
Revisiting the Ultraviolet Tail of the Primordial Gravitational Wave Open
High-frequency primordial gravitational waves (PGWs) with wave numbers larger than the Hubble parameter at the end of inflation are originated from the ultraviolet (UV) modes, which are never stretched out of the horizon. Such a UV tail of…
Non-Gaussianities and Primordial Black Holes Open
The most promising mechanism of generating PBHs is by the enhancement of power spectrum of the primordial curvature perturbation, which is usually accompanied by the the enhancement of non-Gaussianity that crucially changes the abundance o…
Axion-like Universal Gravitational Wave Interpretation of Pulsar Timing Array Data Open
Formation of cosmological solitons is generically accompanied by production of gravitational waves (GWs), with a universal GW background expected at frequency scales below that of non-linear dynamics. Beginning with a general phenomenologi…
Logarithmic Duality of the Curvature Perturbation Open
We study the comoving curvature perturbation $\mathcal{R}$ in the single-field inflation models whose potential can be approximated by a piecewise quadratic potential $V(φ)$ by using the $δN$ formalism. We find a general formula for $\math…
Primordial Black Hole Formation in Starobinsky's Linear Potential Model Open
We study the power spectrum of the comoving curvature perturbation $\cal R$ in the model that glues two linear potentials of different slopes, originally proposed by Starobinsky. We find that the enhanced power spectrum reaches its maximum…
Primordial Black Hole Formation in Non-Minimal Curvaton Scenario Open
In the curvaton scenario, the curvature perturbation is generated after inflation at the curvaton decay, which may have a prominent non-Gaussian effect. For a model with a non-trivial kinetic term, an enhanced curvature perturbation on a s…
Primordial black holes and gravitational waves from resonant amplification during inflation Open
We present a new realization of the resonant production of primordial black holes as well as gravitational waves in a two-stage inflation model consisting of a scalar field \phi with an axion-monodromy-like periodic structure in the potent…
Universal infrared scaling of gravitational wave background spectra Open
We study the general infrared behavior of the power spectrum of a stochastic gravitational wave background produced by stress tensor in the form bilinear in certain dynamical degrees of freedom. We find $\Omega_{\text{GW}}\propto k^3$ for …
Universal upper bound on the inflationary energy scale from the trans-Planckian censorship conjecture Open
We study the constraint on the inflationary energy scale from the recently\nproposed Trans-Planckian Censorship Conjecture (TCC). We find a universal upper\nbound on the inflationary Hubble expansion rate $H_\\text{inf}$ which is solely\nd…
Primordial tensor perturbation in double inflationary scenario with a break Open
We study the primordial tensor perturbation produced from the double\ninflationary scenario with an intermediate break stage. Because of the\ntransitions, the power spectrum deviates from the vacuum one and there will\nappear oscillatory b…
Gravitational Waves Induced by Non-Gaussian Scalar Perturbations Open
We study gravitational waves (GWs) induced by non-Gaussian curvature perturbations. We calculate the density parameter per logarithmic frequency interval, Ω_{GW}(k), given that the power spectrum of the curvature perturbation P_{R}(k) has …
Resonant multiple peaks in the induced gravitational waves Open
We identify analytically a multiple-peak structure in the energy-density spectrum of induced gravitational waves (GWs) generated at second-order from a primordial scalar perturbations also with multiple($n$) peaks at small scales $k_{*i}$.…
Scalaron from <i>R</i><sup>2</sup>-gravity as a heavy field Open
We study a model of inflation in which a scalar field $\chi$ is non-minimally coupled to Starobinsky's $R^2$ gravity. After transforming it to the Einstein frame, a new scalar field, the scalaron $\phi$, will appear and couple to $\chi$ wi…
Power-law modulation of the scalar power spectrum from a heavy field with a monomial potential Open
The effects of heavy fields modulate the scalar power spectrum during inflation. We analytically calculate the modulations of the scalar power spectrum from a heavy field with a separable monomial potential, i.e. V(\phi)~\phi^n. In general…
Oscillating scalar fields in extended quintessence Open
We study a rapidly-oscillating scalar field with potential $V(\\phi) =\nk|\\phi|^n$ nonminally coupled to the Ricci scalar $R$ via a term of the form\n$(1- 8 \\pi G_0 \\xi \\phi^2) R$ in the action. In the weak coupling limit, we\ncalculat…
Strongly coupled quasi-single field inflation Open
We study the power spectrum of quasi-single field inflation where strong coupling is considered. The contribution from the massive propagator can be divided into local and non-local contributions. The local one is the leading contribution …