arXiv subjects

Hongsu Kim

Publications and source records attributed to Hongsu Kim.

At least 19 recordsLinked to original sources

arXiv2024

Abelian Chern-Simons term as a Kaluza-Klein dimensional reduction of the Gibbons-Hawking surface term

It is suggested that the original, minimal Kaluza-Klein theory should be extended by adding a 5-dimensional version of the Gibbons-Hawking gravitational surface term. It is then demonstrated that the usual dimensional reduction of the newly added surface (boundary) term leads to the emergence of the famous Abelian Chern-Simons term. It is stressed that the advent of this Chern-Simons term is not merely a parametrization artefact but a real thing. Finally, the issue of finite-ranged electromagnetic interaction due to massive photons on a plane has been interpreted in terms of the violation of the local gauge invariance of this extended version of Kaluza-Klein theory.

hep-th↗

arXiv2023

The Bardeen-Petterson Effect as an Observable Support for the Blanford-Payne Process Black Hole Jet Production

Relativistic jets are observed around accreting black holes, from stellar mass to super massive black holes. But Its origin has not been fully understood. Although Blanford-Payne process has been considered as most reliable theory of jet production It has no observational proof or supports. In this problem, We suggested the way to display that Bardeen-Petterson effect can become a supporter as providing observational idea of Blanford-Payne process. And we studied black hole jet production by Blanford-Payne process and Bardeen-Peterson effect. As a result, we could calculate observable timescale of black hole jet preccetion

astro-ph.HE↗

arXiv2023

Inclusion of the effect of radiation pressure on the accretion phenomena in astrophysics

Studying accretion phenomena is a window into understanding most heavenly bodies, from the birth of stars to Active galactic nuclei(AGN). We would adopt the effect of the radiation pressure which reduces accretion rates M on the accretion phenomena. Shakura-Sunyaev$α$-disk model of disk accretion can be regarded as a good candidate theory of ADAF. Reduction of accreting matter's angular velocity leads to the suppression of the disk luminosity and the surface temperature which essentially implies the transition of the standard accretion disk model from CDAF to ADAF.

astro-ph.GA↗

arXiv2021

Thick accretion disk and Its super Eddington luminosity around spinning blackholes

In the general accretion disk model theory, the accretion disk surrounding an astronomical object comprises fluid rings obeying Keplerian motion. However, we should consider relativistic and rotational effects as we close in toward the center of accretion disk surrounding spinning compact massive objects such as a black hole or a neutron star. In this study, we explore the geometry of the inner portion of the accretion disk in the context of Mukhopadhyay's pseudo Newtonian potential approximation for the full general relativity theory. We found that the shape of the accretion disk "puffs up" or becomes thicker and the luminosity of the disk could exceed the Eddington luminosity near the surface of the compact spinning black hole.

astro-ph.HE↗

arXiv2020

Toward practical/realistic Randall-Sundrum Brane world scenario

The Randall- Sundrum brane world scenario is mostly preserved and still secure as we realized that the graviton trap is still safely guaranteed. We would like to empathize here that such necessary but rich elaboration doesn't cost a fortune as the Ricci flatness condition effectively accommodates the realistic structure of our universe on the one hand, while still sustaining the 4-dimensional graviton trap on the other.

gr-qc↗

arXiv2019

Implication of the Superstring theory on Astrophysics and Cosmology

Under the spirit of J. Polchinsky's introduction of D-brane in the string target spacetime (or 'bulk', for short), we consider the presence of both closed and open superstrings and coupling of their low-lying modes in the bulk. As a probe to explore the astrophysical and cosmological implications of the superstring theory at larger length scale or in low-energy dynamics, we construct the Yang-Mills monopole black hole solution and investigate its features. Our results exhibit that as a result of intimate interactions among low-lying modes, some cherished features of classical gravitation and cosmology break down. They are the violation of black hole no-hair theorem and the break down of cosmic censorship hypothesis. We learn from this lesson that these implications of fully developed superstring theory in classical gravitation and cosmology largely depart from our conventional wisdom signaling that the most current version of superstring theory is indeed the best candidate for quantum theory of gravitation.

physics.gen-ph↗

arXiv2019

Spinning closed superstring: as Nature's building block

Ever since its birth, up until its present development, the major role of string theory involves being the best candidate for the theory of quantum gravity and other species of interactions. In the present work, we would like to accomplish this goal by minimally extending its content while greatly simplifying its structure. To be more specific, by endowing the closed superstring with its spin, we successfully achieve this goal. This issue has been addressed in the first part of this work, entitled, "mission 1 of the work". In addition, we would like to make further developments on the selection of compactification manifolds that brings the string theory from the critical 10 dimensions down to four cosmology dimensions. Indeed, this issue has not been fully seriously and extensively explored in the literature. In the present work, therefore, we want to bring serious attention to this missing but non-trivial issue. This issue has been addressed in the second part of this work, entitled, "mission 2 of the work".

physics.gen-ph↗

arXiv2019

Monopole Black Hole in Stringy gravity

The possible transition of large mass magnetic monopole solutions to monopole black hole solutions in Yang-Mills-Higgs theory with spontaneous symmetry breaking (SSB) coupled to the low energy effective theory of superstring with only dilatonic sector is studied. Our main motivation is to explore the effects of the dilaton field on the monopole black hole solutions. Working in the Einstein conformal gauge, it is explicitly shown, in terms of the Hawking evaporation of the monopole black hole, that the presence of the dilaton field appears to introduce a "mass dependent extra attractive interaction" in addition to that of the standard Einstein gravity into the system.

physics.gen-ph↗

arXiv2019

Open string tachyon is the stringy inflaton

Regarding the current status of the superstring theory, despite its fully developed contents of all interactions in nature, it is no doubt that unfortunately it lacks a well established early universe cosmology content, that is, the inflationary universe. In the present work, we would like to address precisely this issue. To be more concrete, we would like to demonstrate that remarkably it is the well established open string tachyon condensation which is essentially equivalent to the stringy inflation that has been missing and hence long-seeking by nearly the entire string community.

physics.gen-ph↗

arXiv2015

Concurrence of the Blandford-Payne Process and the Bardeen-Petterson Effect: Theoretical Prediction and its Observational Evidences

Although the Blandford-Payne process, the standard model for the production of AGN jet outflow, has been fully acknowledged and long-known in both the theoretical Astrophysics and observational Astronomy communities, subsequent research works to gain observational supports have been quite rare. In the present work, therefore, we would like to suggest a likely event and encourage its observation which demonstrates observational supports for the Blandford-Payne process. That is, we propose the coupling of it to the well-known Bardeen-Petterson effect. In order for this set-up to comply with our objective stated above, however, the two coupled processes need to be well-resolved. We, therefore, carefully study and present the condition for this to take place. We also point out that this major concern of our present work allows us to measure the strength of the intra-galactic magnetic field which has been known to be uneasy and unclear even in the galaxy observation Astronomy community for a long time.

astro-ph.GA↗

arXiv2011

Nucleation of vacuum bubbles in Brans-Dicke type theory

In this paper, we explore the nucleation of vacuum bubbles in the Brans-Dicke type theory of gravity. In the Euclidean signature, we evaluate the fields at the vacuum bubbles as solutions of the Euler-Lagrange equations of motion as well as the bubble nucleation probabilities by integrating the Euclidean action. We illustrate three possible ways to obtain vacuum bubbles: true vacuum bubbles for ω>-3/2, false vacuum bubbles for ω<-3/2, and false vacuum bubbles for ω>-3/2 when the vacuum energy of the false vacuum in the potential of the Einstein frame is less than that of the true vacuum. After the bubble is nucleated at the t=0 surface, we can smoothly interpolate the field combinations to some solutions in the Lorentzian signature and consistently continue their subsequent evolutions. Therefore, we conclude that, in general scalar-tensor theories like this Brans-Dicke type theories, which may include and represent certain features of string theory, vacuum bubbles come in false vacuum bubbles as well as in true vacuum bubbles, as long as a special condition is assumed on the potential.

hep-th↗

arXiv2010

Fine structure of the Poynting-Robertson effect for a luminous spinning relativistic star

As a sequel to our recent works challenging toward the systematic inclusion of the effect of radiation on the trajectory of a test particle orbiting around a luminous spinning relativistic star eventually aiming at its application to the accretion flow. We explore in the present work the fine structure of the trajectory of test particle just entering the ``suspension orbit" under the purpose of a detailed investigation of test particle's trajectory in the vicinity of the ``suspension orbit". We end up with a rather puzzling behavior that, contrary to our expectation, the specific angular momentum of the test particle instantly rises instead of decreasing monotonically just before the test particle enters the ``suspension orbit".

astro-ph.HE↗

arXiv2010

Finite size effects on the Poynting-Robertson effect: a fully general relativistic treatment

Ever since the first discovery of Poynting and Robertson, the radiation source has been treated as merely a point. Even in a very few studies where the size of the source has been taken into account, the treatment of the problem remained largely non-relativistic. In the present work, we address the issue of the finite size effects on the Poynting-Robertson effect in a fully relativistic manner for the first time. As a result, the emergence and the characteristic of the critical point/suspension orbit can be studied in a systematic and detailed manner.

astro-ph.HE↗

arXiv2010

Trajectory of test particle around a slowly rotating relativistic star emitting isotropic radiation

We explored the motion of test particles near slowly rotating relativistic star having a uniform luminosity. In order to derive the test particle's equations of motion, we made use of the radiation stress-energy tensor first constructed by Miller and Lamb \cite{ML96}. From the particle's trajectory obtained through the numerical integration of the equations of motion, it is found that for sufficiently high luminosity, "suspension orbit" exists, where the test particle hovers around at uniform angular velocity in the same direction as the star's spin. Interestingly, it turned out that the radial position of the "suspension orbit" was determined by the luminosity and the angular momentum of the star alone and was independent of the initial positions and the specific angular momentum of the particle. Also found is that there exist not only the radiation drag but also "radiation counter-drag" which depends on the stellar radius and the angular momentum and it is this radiation counter-drag that makes the test particle in the "suspension orbit" to hover around at uniform angular velocity which is greater than that induced by the Lense-Thirring effect (i.e., general relativistic dragging of inertial frame).

astro-ph.HE↗

arXiv2007

Can the Brans-Dicke Gravity with $Λ$ Possibly be a Theory of Dark Matter?

The pure Brans-Dicke (BD) gravity with or without the cosmological constant $Λ$ has been taken as a model theory for the dark matter. Indeed, there has been a consensus that unless one modifies either the standard theory of gravity, namely, general relativity, or the standard model for particle physics, or both, one can never achieve a satisfying understanding of the phenomena associated with dark matter and dark energy. Along this line, our dark matter model in this work can be thought of as an attempt to modify the gravity side alone in the simplest fashion to achieve the goal. Among others, it is demonstrated that our model theory can successfully predict the emergence of dark matter halo-like configuration in terms of a self-gravitating spacetime solution to the BD field equations and reproduce the flattened rotation curve in this dark halo-like object in terms of the non-trivial energy density of the BD scalar field, which was absent in the context of general relativity where Newton's constant is strictly a ``constant'' having no dynamics. Our model theory, however, is not entirely without flaw, such as the prediction of relativistic jets in all types of galaxies which actually is not the case.

astro-ph↗

arXiv2006

Relativistic Electrodynamics of Spinning Compact Objects

A theoretical study of some electrodynamic features of a region close to a {\it slowly-rotating} magnetized relativistic star is performed. To be a little more specific, based on the solution-generating method given by Wald, the magnetic fields around both uncharged and (slightly) charged relativistic stars have been obtained. Particularly for a charged relativistic star, again following the argument by Wald, the star was shown to gradually accrete charge until it reached an equilibrium value $\tilde{Q}=2B_{0}J$. This value of the equilibrium charge seems to be generic as a rotating black hole is known to accrete exactly the same amount. Although these results are equally relevant to all species of slowly-rotating relativistic stars, we particularly have the rotating neutron star in mind. As such, it would be of some interest to attempt to make contact with a real pulsar case. Thus, we discuss how many of the theoretical results obtained in the present work can be carried over to a realistic, general relativistic description of a pulsar's magnetosphere.

astro-ph↗

arXiv2005

Brans-Dicke Theory as a Unified Model for Dark Matter - Dark Energy

The Brans-Dicke (BD) theory of gravity is taken as a possible theory of k-essence coupled to gravity. It then has been realized that the BD scalar field does indeed play a role of a k-essence, but in a very unique way which distinguishes it from other k-essence fields studied in the literature. That is, first in the BD scalar field-dominated era when the contribution from this k-essence overwhelms those from other types of matter, the BD theory predicts the emergence of a yet-unknown {\it zero acceleration} epoch which is an intermediate stage acting as a ``crossing bridge'' between the decelerating matter-dominated era and the accelerating phase. Upon realizing this, next, closer study of the effects of this k-essence on the evolutionary behavior of the matter-dominated and the accelerating eras has been performed. The result of the study indicates that the BD scalar field appears to interpolate {\it smoothly} between these two late-time stages by speeding up the expansion rate of the matter-dominated era somewhat while slowing down that of the accelerating phase to some degree. Thus with the newly found BD scalar field-dominated era in between these two, the late-time of the universe evolution appears to be mixed sequence of the three stages.

astro-ph↗

arXiv2005

Dark Matter Haloes and Rotation Curves via Brans-Dicke Theory

In the present work, the Brans-Dicke (BD) theory of gravity is taken as a possible theory of k-essence. Then starting with the (already known) Brans-Dicke-Schwarzschild solution which can represent the gravitationally bound static configurations of the BD scalar k-essence, issues like whether these configurations can reproduce the observed properties of galactic dark matter haloes have been addressed. It has been realized that indeed the BD scalar k-essence can cluster into dark matter halo-like objects with flattened rotation curves while exhibiting a dark energy-like negative pressure on larger scales.

astro-ph↗