arXiv subjects

Chul-Moon Yoo

Publications and source records attributed to Chul-Moon Yoo.

At least 73 records · Page 4Linked to original sources

arXiv2018

Chameleon Field in a Spherical Shell System

We investigate the validity of screening mechanism of the fifth force for chameleon field in highly inhomogeneous density profile. For simplicity, we consider a static and spherically symmetric system which is composed of concentric infinitely thin shells. We calculate the fifth force profile by using a numerical method for a relatively large Compton wavelength of the chameleon field. An approximate solution is also derived for the small Compton wavelength limit. Our results show that, if the thin-shell condition for the corresponding smoothed density profile is satisfied, the fifth force is safely screened outside the system irrespective of the configuration of the shells inside the system. In contrast to the screening outside the system, we find that the fifth force can be comparable to the Newtonian gravitational force inside the system. If the system is highly inhomogeneous, the chameleon field cannot trace the potential minimum varying with the density and repeats being kicked, climbing up and rolling down the potential even when the effective mass of the chameleon field is sufficiently large in the system on average. One should not feel complacent about the wellbehavedness of the fifth force field with an averaged density distribution when we consider inhomogeneous objects.

gr-qc↗

arXiv2018

Rigidly Rotating String Sticking in a Kerr Black Hole

We analyze rigidly rotating Nambu--Goto strings in the Kerr spacetime, particularly focusing on the strings sticking in the horizon. From the regularity on the horizon, we find the condition for sticking in the horizon, which is consistent with the second law of the black hole thermodynamics. Energy extraction through the sticking string from a Kerr black hole occurs. We obtain the maximum value of the luminosity of the energy extraction.

gr-qc↗

arXiv2018

Remarks on the maximum luminosity

The quest for fundamental limitations on physical processes is old and venerable. Here, we investigate the maximum possible power, or luminosity, that any event can produce. We show, via full nonlinear simulations of Einstein's equations, that there exist initial conditions which give rise to arbitrarily large luminosities. However, the requirement that there is no past horizon in the spacetime seems to limit the luminosity to below the Planck value, ${{\cal L}_\textrm{P}\!=\!c^5/G}$. Numerical relativity simulations of critical collapse yield the largest luminosities observed to date, ${\approx \! 0.2 {\cal L}_\textrm{P}}$. We also present an analytic solution to the Einstein equations which seems to give an unboundedly large luminosity; this will guide future numerical efforts to investigate super-Planckian luminosities.

gr-qc↗

arXiv2018

Observational Constraint on Spherical Inhomogeneity with CMB and Local Hubble Parameter

We derive an observational constraint on a spherical inhomogeneity of the void centered at our position from the angular power spectrum of the cosmic microwave background(CMB) and local measurements of the Hubble parameter. The late time behaviour of the void is assumed to be well described by the so-called $Λ$-Lemaître-Tolman-Bondi~($Λ$LTB) solution. Then, we restrict the models to the asymptotically homogeneous models each of which is approximated by a flat Friedmann-Lemaître-Robertson-Walker model. The late time $Λ$LTB models are parametrized by four parameters including the value of the cosmological constant and the local Hubble parameter. The other two parameters are used to parametrize the observed distance-redshift relation. Then, the $Λ$LTB models are constructed so that they are compatible with the given distance-redshift relation. Including conventional parameters for the CMB analysis, we characterize our models by seven parameters in total. The local Hubble measurements are reflected in the prior distribution of the local Hubble parameter. As a result of a Markov-Chains-Monte-Carlo analysis for the CMB temperature and polarization anisotropies, we found that the inhomogeneous universe models with vanishing cosmological constant are ruled out as is expected. However, a significant under-density around us is still compatible with the angular power spectrum of CMB and the local Hubble parameter.

astro-ph.CO↗

arXiv2017

Self-gravitating oscillons and new critical behavior

The dynamical evolution of self-interacting scalars is of paramount importance in cosmological settings, and can teach us about the content of Einstein's equations. In flat space, nonlinear scalar field theories can give rise to localized, non-singular, time-dependent, long-lived solutions called {\it oscillons}. Here, we discuss the effects of gravity on the properties and formation of these structures, described by a scalar field with a double well potential. We show that oscillons continue to exist even when gravity is turned on, and we conjecture that there exists a sequence of critical solutions with infinite lifetime. Our results suggest that a new type of critical behavior appears in this theory, characterized by modulations of the lifetime of the oscillon around the scaling law and the modulations of the amplitude of the critical solutions.

gr-qc↗

arXiv2017

3D Simulation of Spindle Gravitational Collapse of a Collisionless Particle System

We simulate the spindle gravitational collapse of a collisionless particle system in a 3D numerical relativity code and compare the qualitative results with the old work done by Shapiro and Teukolsky(ST). The simulation starts from the prolate-shaped distribution of particles and a spindle collapse is observed. The peak value and its spatial position of curvature invariants are monitored during the time evolution. We find that the peak value of the Kretschmann invariant takes a maximum at some moment, when there is no apparent horizon, and its value is greater for a finer resolution, which is consistent with what is reported in ST. We also find a similar tendency for the Weyl curvature invariant. Therefore, our results lend support to the formation of a naked singularity as a result of the axially symmetric spindle collapse of a collisionless particle system in the limit of infinite resolution. However, unlike in ST, our code does not break down then but go well beyond.We find that the peak values of the curvature invariants start to gradually decrease with time for a certain period of time. Another notable difference from ST is that, in our case, the peak position of the Kretschmann curvature invariant is always inside the matter distribution.

gr-qc↗

arXiv2016

Primordial black hole formation in the matter-dominated phase of the Universe

We investigate primordial black hole formation in the matter-dominated phase of the Universe, where nonspherical effects in gravitational collapse play a crucial role. This is in contrast to the black hole formation in a radiation-dominated era. We apply the Zel'dovich approximation, Thorne's hoop conjecture, and Doroshkevich's probability distribution and subsequently derive the production probability $β_{0}$ of primordial black holes. The numerical result obtained is applicable even if the density fluctuation $σ$ at horizon entry is of the order of unity. For $σ\ll 1$, we find a semi-analytic formula $β_{0}\simeq 0.05556 σ^{5}$, which is comparable with the Khlopov-Polnarev formula. We find that the production probability in the matter-dominated era is much larger than that in the radiation-dominated era for $σ\lesssim 0.05$, while they are comparable with each other for $σ\gtrsim 0.05$. We also discuss how $σ$ can be written in terms of primordial curvature perturbations.

astro-ph.CO↗

arXiv2016

Critical behavior of spherically symmetric domain wall collapse

Critical collapse of a spherically symmetric domain wall is investigated. The domain wall is made of a minimally coupled scalar field with a double well potential. We consider a sequence of the initial data which describe a momentarily static domain wall characterized by its initial radius. The time evolution is performed by a full general relativistic numerical code for spherically symmetric systems. In this paper, we use the maximal slice gauge condition, in which spacelike time slices may penetrate the black hole horizon differently from other widely used procedures. In this paper, we consider two specific shapes of the double well potential, and observe the Type II critical behavior in both cases. The mass scaling, sub-critical curvature scaling, and those fine structures are confirmed. The index of the scaling behavior agrees with the massless scalar case.

gr-qc↗

arXiv2016

Another Approach to Test Gravity around a Black Hole

Pulsars orbiting around the black hole at our galactic center provide us a unique testing site for gravity. In this work, we propose an approach to probe the gravity around the black hole introducing two phenomenological parameters which characterize deviation from the vacuum Einstein theory. The two phenomenological parameters are associated with the energy momentum tensor in the framework of the Einstein theory. Therefore, our approach can be regarded as the complement to the parametrized post-Newtonian framework in which phenomenological parameters are introduced for deviation of gravitational theories from general relativity. In our formulation, we take the possibility of existence of a relativistic and exotic matter component into account. Since the pulsars can be regarded as test particles, as the first step, we consider geodesic motion in the system composed of a central black hole and a perfect fluid whose distribution is static and spherically symmetric. It is found that the mass density of the fluid and a parameter of the equation of state can be determined with precision with $0.1\%$ if the density on the pulsar orbit is larger than $10^{-9}~{\rm g/cm^3}$.

gr-qc↗

arXiv2016

Inverse Construction of the $Λ$LTB Model from a Distance-redshift Relation

Spherically symmetric dust universe models with a positive cosmological constant $Λ$, known as $Λ$-Lemaître-Tolman-Bondi($Λ$LTB) models, are considered. We report a method to construct the $Λ$LTB model from a given distance-redshift relation observed at the symmetry center. The spherical inhomogeneity is assumed to be composed of growing modes. We derive a set of ordinary differential equations for three functions of the redshift, which specify the spherical inhomogeneity. Once a distance-redshift relation is given, with careful treatment of possible singular points, we can uniquely determine the model by solving the differential equations for each value of $Λ$. As a demonstration, we fix the distance-redshift relation as that of the flat $Λ$CDM model with $(Ω^{\rm dis}_{\rm m0}, Ω^{\rm dis}_{\rm Λ0})=(0.3,0.7)$, where $Ω^{\rm dis}_{\rm m0}$ and $Ω^{\rm dis}_{\rm Λ0}$ are the normalized matter density and the cosmological constant, respectively. Then, we construct the $Λ$LTB model for several values of $Ω_{\rm Λ0}:=Λ/(3H_0^2)$, where $H_0$ is the present Hubble parameter observed at the symmetry center. We obtain void structure around the symmetry center for $Ω_{Λ0}<Ω^{\rm dis}_{Λ0}$. We show the relation between the ratio $Ω_{\Lambda0}/Ω^{\rm dis}_{Λ0}$ and the amplitude of the inhomogeneity.

astro-ph.CO↗

arXiv2016

Spherical polytropic balls cannot mimic black holes

The so-called black hole shadow is a dark region which is expected to appear in a fine image of optical observation of black holes. It is essentially an absorption cross section of black hole, and the boundary of shadow is determined by unstable circular orbits of photons (UCOP). If there exists a compact object possessing UCOP but no black hole horizon, it can provide us with the same shadow image with black holes, and a detection of shadow image cannot be a direct evidence of black hole existence. Then, this paper examine whether or not such compact objects can exist under some suitable conditions. We investigate thoroughly the static spherical polytropic ball of perfect fluid with single polytrope index, and then investigate a representative example of the piecewise polytropic ball. Our result is that the spherical polytropic ball which we have investigated cannot possess UCOP, if the sound speed at center is subluminal (slower-than-light). This means that, if the polytrope treated in this paper is a good model of stellar matter in compact objects, the detection of shadow image is regarded as a good evidence of black hole existence. As a by-product, we have found the upper bound of the mass-to-radius radio (M/R) of polytropic ball with single index, M/R < 0.281, under the subluminal-sound-speed condition.

gr-qc↗

arXiv2016

Relationship between the CMB, SZ Cluster Counts, and Local Hubble Parameter Measurements in a Simple Void Model

The discrepancy between the amplitudes of matter fluctuations inferred from Sunyaev-Zel'dovich (SZ) cluster number counts, the primary temperature, and the polarization anisotropies of the cosmic microwave background (CMB) measured by the Planck satellite can be reconciled if the local universe is embedded in an under-dense region as shown by Lee, 2014. Here using a simple void model assuming the open Friedmann-Robertson-Walker geometry and a Markov Chain Monte Carlo technique, we investigate how deep the local under-dense region needs to be to resolve this discrepancy. Such local void, if exists, predicts the local Hubble parameter value that is different from the global Hubble constant. We derive the posterior distribution of the local Hubble parameter from a joint fitting of the Planck CMB data and SZ cluster number counts assuming the simple void model. We show that the predicted local Hubble parameter value of $H_{\rm loc}=70.1\pm0.34~{\rm km\,s^{-1}Mpc^{-1}}$ is in better agreement with direct local Hubble parameter measurements, indicating that the local void model may provide a consistent solution to the cluster number counts and Hubble parameter discrepancies.

astro-ph.CO↗

arXiv2015

Maximum mass of a barotropic spherical star

The ratio of total mass $M$ to surface radius $R$ of spherical perfect fluid ball has an upper bound, $M/R < B$. Buchdahl obtained $B = 4/9$ under the assumptions; non-increasing mass density in outward direction, and barotropic equation of states. Barraco and Hamity decreased the Buchdahl's bound to a lower value $B = 3/8$ $(< 4/9)$ by adding the dominant energy condition to Buchdahl's assumptions. In this paper, we further decrease the Barraco-Hamity's bound to $B \simeq 0.3636403$ $(< 3/8)$ by adding the subluminal (slower-than-light) condition of sound speed. In our analysis, we solve numerically Tolman-Oppenheimer-Volkoff equations, and the mass-to-radius ratio is maximized by variation of mass, radius and pressure inside the fluid ball as functions of mass density.

gr-qc↗

arXiv2015

Spherical Domain Wall Collapse in a Dust Universe

To clarify observational consequence of bubble nucleations in inflationary era, we analyse dynamics of a spherical domain wall in an expanding universe. We consider a spherical shell of the domain wall with tension $σ$ collapsing in a spherically-symmetric dust universe, which is initially separated into the open Friedmann-Lemaître-Robertson-Walker universe inside the shell and the Einstein-de Sitter universe outside. The domain wall shell collapses due to the tension, and sweeps the dust fluid. The universe after the collapse becomes inhomogeneous and is described by the Lemaître-Tolman-Bondi model. We construct solutions describing this inhomogeneous universe by solving dynamical equations obtained from Israel's junction conditions applied to this system. We find that a black hole forms after the domain wall collapse for any initial condition, and that the black hole mass at the moment of its formation is universally given by $M_{\rm BH}\simeq 17 σ/H_{\rm hc}$, where $H_{\rm hc}$ is the Hubble parameter at the time when the shell radius becomes equal to the Hubble radius. We also find that the dust fluid is distributed as $ρ\propto R^{3/2}$ near the central region after the collapse, where $R$ is the area radius. These features would provide observable signatures of a spherical domain wall generated in the early universe.

gr-qc↗

arXiv2015

Expanding universe with nonlinear gravitational waves

We test the validity of Isaacson's formula which states that high frequency and low amplitude gravitational waves behave as a radiation fluid on average. For this purpose, we numerically construct a solution of the vacuum Einstein equations which contains nonlinear standing gravitational waves. The solution is constructed in a cubic box with periodic boundary conditions. The time evolution is solved in a gauge in which the trace of the extrinsic curvature $K$ of the time slice becomes spatially uniform. Then, the Hubble expansion rate $H$ is defined by $H=-K/3$ and compared with the effective scale factor $L$ defined by the proper volume, area and length of the cubic box. We find that, even when the wave length of the gravitational waves is comparable to the Hubble scale, the deviation from Isaacson's formula $H\propto L^{-2}$ is at most 3\% without taking a temporal average and is below 0.1\% with a temporal average.

gr-qc↗

arXiv2015

Cosmological long-wavelength solutions and primordial black hole formation

We construct cosmological long-wavelength solutions without symmetry in general gauge conditions compatible with the long-wavelength scheme. We then specify the relationship among the solutions in different time slicings. Applying this general framework to spherical symmetry, we derive the correspondence relation between long-wavelength solutions in the constant mean curvature slicing with conformally flat spatial coordinates and asymptotic quasihomogeneous solutions in the comoving gauge and compare the numerical results of PBH formation in these two different approaches. To discuss the PBH formation, it is convenient and conventional to use $\tildeδ_{c}$, the value which the averaged density perturbation at threshold in the comoving slicing would take at horizon entry in the lowest-order long-wavelength expansion. We numerically find that within compensated models, the sharper the transition from the overdense region to the FRW universe is, the larger the $\tildeδ_{c}$ becomes. We suggest that, for the equation of state $p=(Γ-1)ρ$, we can apply the analytic formulas for the minimum $\tildeδ_{c, {\rm min}}\simeq [3Γ/(3Γ+2)]\sin^{2}\left[π\sqrt{Γ-1}/(3Γ-2)\right]$ and the maximum $\tildeδ_{c, {\rm max}}\simeq 3Γ/(3Γ+2)$. As for the threshold peak value of the curvature variable $ψ_{0,c}$, we find that the sharper the transition is, the smaller the $ψ_{0,c}$ becomes. We analytically explain this feature. Using simplified models, we also analytically deduce an environmental effect that $ψ_{0,c}$ can be significantly larger (smaller) if the underlying density perturbation of much longer wavelength is positive (negative).

gr-qc↗

arXiv2015

Systematic error due to isotropic inhomogeneities

Usually the effects of isotropic inhomogeneities are not seriously taken into account in the determination of the cosmological parameters because of Copernican principle whose statement is that we do not live in the privileged domain in the universe. But Copernican principle has not been observationally confirmed yet in sufficient accuracy, and there is the possibility that there are non-negligible large-scale isotropic inhomogeneities in our universe. In this paper, we study the effects of the isotropic inhomogeneities on the determination of the cosmological parameters and show the probability that non-Copernican isotropic inhomogeneities mislead us into believing, for example, the phantom energy of the equation of state, $p=wρ$ with $w<-1$, even in case that $w=-1$ is the true value.

astro-ph.CO↗

arXiv2014

Comparison of two approximation schemes for solving perturbations in a LTB cosmological model

Recently, the present authors studied perturbations in the Lemaitre-Tolman-Bondi cosmological model by applying the second-order perturbation theory in the dust Friedmann-Lemaitre-Robertson-Walker universe model. Before this work, the same subject was studied in some papers by analyzing linear perturbations in the Lemaitre-Tolman-Bondi cosmological model under the assumption proposed by Clarkson, Clifton and February, in which two of perturbation variables are negligible. However, it is a non-trivial issue in what situation the Clarkson-Clifton-February assumption is valid. In this paper, we investigate differences between these two approaches. It is shown that, in general, these two approaches are not compatible with each other. That is, in our perturbative procedure, the Clarkson-Clifton-February assumption is not valid at the order of our interest.

astro-ph.CO↗