arXiv subjects

Ui Ri Mun

Publications and source records attributed to Ui Ri Mun.

9 recordsLinked to original sources

arXiv2026

Non-Gaussianity and Strong-Coupling Problem in a Two-Field DHOST Bouncing Model

We recently constructed a two-field Degenerate Higher-Order Scalar-Tensor (DHOST) bouncing model which is fully viable at the linear level [1]. This model is completely free of Belinski-Khalatnikov-Lifshitz (BKL) instability, ghost instability, gradient instability and superluminality. It also predicts the scalar spectral index and tensor-toscalar ratio consistent with observations. The aim of this paper is to extend the viability of the model to the non-linear level. To this end, we first refine the original model such that its prediction on the (local) non-Gaussianity parameter fNL agrees with observations, leaving the viability of the model at the linear level intact. We furthermore demonstrate that the strong-coupling scale is well above the characteristic background energy scale all the time. Our model indeed exemplifies the fully viable two-field DHOST bouncing model, in the sense that it is weakly-coupled, stable and non-superluminal as well as consistent with observations.

hep-th↗

arXiv2025

Fully viable DHOST bounce with extra scalar

In this paper we construct a class of Degenerate Higher-Order Scalar-Tensor (DHOST) theories with an extra scalar field, which admits viable solutions of bouncing universe satisfying the following requirements: (i) absence of Belinski-Khalatnikov-Lifshitz (BKL) instability, ghost and gradient instability, (ii) absence of superluminality, (iii) generation of nearly scale-invariant curvature perturbations and very small tensor-to-scalar ratio, and (iv) conventional asymptotics in the distant past and future, where gravity sector is described by General Relativity and the DHOST scalar has a canonical form of Lagrangian. We also expect our models to have sufficiently small non-Gaussianities of primordial curvature perturbations to be compatible with observations. As such, this work exemplifies for the first time the fully viable two-field DHOST bouncing cosmology, which is free of instability and superluminality problems as well as compatible with observations.

hep-th↗

arXiv2025

Bridging between reheating and late-time observations in quintessential inflation

We propose an idea to build a bridge between reheating and late-time observations in quintessential inflation by backtracking the evolution of the inflaton field from the present time to the end of reheating. This idea is implemented when the potential gradient is negligible compared to the Hubble friction, rendering the inflaton field frozen, till the present time. We find a simple analytic relation between the reheating temperature and the observational parameters for dark energy, and numerically confirm its validity for typical models of quintessential inflation. This relation is universal and can apply to all quintessential inflation models with any reheating mechanism. It also implies that any quintessential inflation model with a successful reheating with the reheating temperature $1\textrm{MeV}\lesssim T_\textrm{re}\lesssim 10^{15}\textrm{GeV}$ predicts the equation of state of dark energy today extremely close to $-1$, i.e. $-1+10^{-60}\lesssim w_0\lesssim -1+10^{-24}$, unless the inflaton field unfreezes before the present time.

hep-th↗

arXiv2022

General Markovian Equation for Scalar Fields in a Slowly Evolving Background

We present a general and model-independent method to obtain an effective Markovian quantum kinetic equation for the expectation value of a slowly evolving scalar field in an adiabatically evolving background from first principles of nonequilibrium quantum field theory. The method requires almost no assumptions about the field's interactions and the composition of the background, except that 1) the coupling constants shall be small enough for perturbation theory to be applicable, 2) there is a clear separation between microphysical time scales and the rate at which bulk properties change, and 3) higher time derivatives of the field remain small. The resulting Markovian equation of motion is expressed in terms of an effective potential and friction coefficients. Motivated by cosmological applications we focus on spatially homogeneous and isotropic systems, but the approach could also be applied to spatial gradients.

hep-ph↗

arXiv2021

Notes on the post-bounce background dynamics in bouncing cosmologies

We investigate the post-bounce background dynamics in a certain class of single bounce scenarios studied in the literature, in which the cosmic bounce is driven by a scalar field with negative exponential potential such as the ekpyrotic potential. We show that those models can actually lead to cyclic evolutions with repeated bounces. These cyclic evolutions, however, do not account for the currently observed late-time accelerated expansion and hence are not cosmologically viable. In this respect we consider a new kind of cyclic model proposed recently and derive some cosmological constraints on this model.

gr-qc↗

arXiv2021

Constant-roll warm inflation and the $β$-function approach

We propose a new approach to constant-roll warm inflation as a generalization of constant-roll inflation. Based on the $β$-function formalism, it is shown that constant-roll warm inflation models with a natural end fall into universality classes defined by three different types of $β$-functions, under the assumption that radiation energy density is quasi-stable. Given that warm inflation is completely specified by the $β$-function and dissipation coefficient ratio, we investigate whether or not the inflation can physically be realized for enough number of e-foldings at the background level for some combinations of the $β$-functions and non-trivial dissipation coefficient ratios.

gr-qc↗

arXiv2017

CMB constraints on the inflaton couplings and reheating temperature in $α$-attractor inflation

We study reheating in $α$-attractor models of inflation in which the inflaton couples to other scalars or fermions. We show that the parameter space contains viable regions in which the inflaton couplings to radiation can be determined from the properties of CMB temperature fluctuations, in particular the spectral index. This may be the only way to measure these fundamental microphysical parameters, which shaped the universe by setting the initial temperature of the hot big bang and contain important information about the embedding of a given model of inflation into a more fundamental theory of physics. The method can be applied to other models of single field inflation.

astro-ph.CO↗

arXiv2016

A new distance law of planets and satellites in the solar system

In the 1960s, it has been substantiated that an equation of Schrodinger type could describe the diffusion phenomena, and the main consequence from this finding has been that there would be wave property in the diffusion processes as well. This theory has been immediately proved through laboratorial experiments. Afterwards the theory was applied to the primordial nebula which was thought to surround the protosun, and has found the consistency of the prediction of the theory with current distance distribution of the planets to be excellent. At the end of 20th century new satellites of planets were discovered. On the basis of the new data, the theory is tested thoroughly and the result allows us to come to the conclusion that the basic process for the distances of the planets from the protosun to be determined has been the diffusion of the primordial nebula consisting of mainly molecular gas.

astro-ph.EP↗

arXiv2015

Apparent Positions of Planets

The apparent positions of planets are determined by means of the fundamental ephemerides, the precession-nutation models of the Earth, the gravitational effects and aberrations et al. Around 2000, many astrometrical conceptions, models and theories had been newly defined and updated:for the fiducial celestial reference system, the ICRS is introduced, the fundamental ephemerides - DE405/LE405 et al.,precession-nutation model - IAU 2000A/IAU 2006 model. Using the traditional algorithm and the updated models, we develop the system of calculating the apparent positions of planets. The results are compared with the Astronomical Almanac and proved in their correctness.

astro-ph.EP↗