arXiv subjects

Chul-Moon Yoo

Publications and source records attributed to Chul-Moon Yoo.

At least 91 records · Page 5Linked to original sources

arXiv2014

Newtonian self-gravitating system in a relativistic huge void universe model

We consider a test of the Copernican Principle through observations of the large-scale structures, and for this purpose we study the self-gravitating system in a relativistic huge void universe model which does not invoke the Copernican Principle. If we focus on the the weakly self-gravitating and slowly evolving system whose spatial extent is much smaller than the scale of the cosmological horizon in the homogeneous and isotropic background universe model, the cosmological Newtonian approximation is available. Also in the huge void universe model, the same kind of approximation as the cosmological Newtonian approximation is available for the analysis of the perturbations contained in a region whose spatial size is much smaller than the scale of the huge void: the effects of the huge void are taken into account in a perturbative manner by using the Fermi-normal coordinates. By using this approximation, we derive the equations of motion for the weakly self-gravitating perturbations whose elements have relative velocities much smaller than the speed of light, and show the derived equations can be significantly different from those in the homogeneous and isotropic universe model, due to the anisotropic volume expansion in the huge void. We linearize the derived equations of motion and solve them. The solutions show that the behaviors of linear density perturbations are very different from those in the homogeneous and isotropic universe model.

gr-qc↗

arXiv2014

Impact of small scale inhomogeneities on observations of standard candles

We investigate the effect of small scale inhomogeneities on standard candle observations, such as type Ia supernovae (SNe) observations. Existence of the small scale inhomogeneities may cause a tension between SNe observations and other observations with larger diameter sources, such as the cosmic microwave background (CMB) observation. To clarify the impact of the small scale inhomogeneities, we use the Dyer-Roeder approach. We determined the smoothness parameter $α(z)$ as a function of the redshift $z$ so as to compensate the deviation of cosmological parameters for SNe from those for CMB. The range of the deviation which can be compensated by the smoothness parameter $α(z)$ satisfying $0\leqα(z)\leq1$ is reported. Our result suggests that the tension may give us the information of the small scale inhomogeneities through the smoothness parameter.

astro-ph.CO↗

arXiv2014

Black Hole Universe with $Λ$

Time evolution of a black hole lattice universe with a positive cosmological constant $Λ$ is simulated. The vacuum Einstein equations are numerically solved in a cubic box with a black hole in the center. Periodic boundary conditions on all pairs of opposite faces are imposed. Configurations of marginally trapped surfaces are analyzed. We describe the time evolution of not only black hole horizons, but also cosmological horizons. Defining the effective scale factor by using the area of a surface of the cubic box, we compare it with that in the spatially flat dust dominated FLRW universe with the same value of $Λ$. It is found that the behaviour of the effective scale factor is well approximated by that in the FLRW universe. Our result suggests that local inhomogeneities do not significantly affect the global expansion law of the universe irrespective of the value of $Λ$.

gr-qc↗

arXiv2013

Threshold of primordial black hole formation

Based on a physical argument, we derive a new analytic formula for the amplitude of density perturbation at the threshold of primordial black hole formation in the universe dominated by a perfect fluid with the equation of state $p=wρc^{2}$ for $w\ge 0$. The formula gives $δ^{\rm UH}_{H c}=\sin^{2}[π\sqrt{w}/(1+3w)]$ and $\tildeδ_{c}=[3(1+w)/(5+3w)]\sin^{2}[π\sqrt{w}/(1+3w)]$, where $δ^{\rm UH}_{H c}$ and $\tildeδ_{c}$ are the amplitude of the density perturbation at the horizon crossing time in the uniform Hubble slice and the amplitude measure used in numerical simulations, respectively, while the conventional one gives $δ^{\rm UH}_{H c}=w$ and $\tildeδ_{c}=3w(1+w)/(5+3w)$. Our formula shows a much better agreement with the result of recent numerical simulations both qualitatively and quantitatively than the conventional formula. For a radiation fluid, our formula gives $δ^{\rm UH}_{H c}=\sin^{2}(\sqrt{3}π/6)\simeq 0.6203$ and $\tildeδ_{c}=(2/3)\sin^{2}(\sqrt{3}π/6)\simeq 0.4135$. We also discuss the maximum amplitude and the cosmological implications of the present result.

astro-ph.CO↗

arXiv2013

Type-I cosmic string network

We study the network of Type-I cosmic strings using the field-theoretic numerical simulations in the Abelian-Higgs model. For Type-I strings, the gauge field plays an important role, and thus we find that the correlation length of the strings is strongly dependent upon the parameter β, the ratio between the self-coupling constant of the scalar field and the gauge coupling constant, namely, β=λ/2e^2. In particular, if we take the cosmic expansion into account, the network becomes densest in the comoving box for a specific value of βfor β<1.

astro-ph.CO↗

arXiv2013

Black Hole Universe: Time Evolution

Time evolution of a black hole lattice universe is simulated. The vacuum Einstein equations in a cubic box with a black hole at the origin are numerically solved with periodic boundary conditions on all pairs of opposite faces. Defining effective scale factors by using the area of a surface and the length of an edge of the cubic box, we compare them with that in the Einstein-deSitter universe. It is found that the behaviour of the effective scale factors is well approximated by that in the Einstein-deSitter universe. Our result suggests that local inhomogeneities do not significantly affect the global expansion law of the universe even if the inhomogeneity is extremely nonlinear.

gr-qc↗

arXiv2013

Two-point correlation function of density perturbations in a large void universe

We study the two-point correlation function of density perturbations in a spherically symmetric void universe model which does not employ the Copernican principle. First we solve perturbation equations in the inhomogeneous universe model and obtain density fluctuations by using a method of non-linear perturbation theory which was adopted in our previous paper. From the obtained solutions, we calculate the two-point correlation function and show that it has a local anisotropy at the off-center position differently from those in homogeneous and isotropic universes. This anisotropy is caused by the tidal force in the off-center region of the spherical void. Since no tidal force exists in homogeneous and isotropic universes, we may test the inhomogeneous universe by observing statistical distortion of the two-point galaxy correlation function.

astro-ph.CO↗

arXiv2013

Wave Effect in Gravitational Lensing by the Ellis Wormhole

We propose the use of modulated spectra of astronomical sources due to gravitational lensing to probe Ellis wormholes. The modulation factor due to gravitational lensing by the Ellis wormhole is calculated. Within the geometrical optics approximation, the normal point mass lens and the Ellis wormhole are indistinguishable unless we know the source's unlensed luminosity. This degeneracy is resolved with the significant wave effect in the low frequency domain if we take the deviation from the geometrical optics into account. We can roughly estimate the upper bound for the number density of Ellis wormholes as $n\lesssim 10^{-9}\{AU}^{-3}$ with throat radius $a\sim1\cm$ from the existing femto-lensing analysis for compact objects.

gr-qc↗

arXiv2012

Notes on Spinoptics in a Stationary Spacetime

In arXiv:1105.5629, equations of the modified geometrical optics for circularly polarized photon trajectories in a stationary spacetime are derived by using a (1+3)-decomposed form of Maxwell's equations. We derive the same results by using a four-dimensional covariant description. In our procedure, the null nature of the modified photon trajectory naturally appears and the energy flux is apparently null. We find that, in contrast to the standard geometrical optics, the inner product of the stationary Killing vector and the tangent null vector to the modified photon trajectory is no longer a conserved quantity along light paths. This quantity is furthermore different for left and right handed photon. A similar analysis is performed for gravitational waves and an additional factor of 2 appears in the modification due to the spin-2 nature of gravitational waves.

gr-qc↗

arXiv2012

Femto-lensing due to a Cosmic String

We consider the femto-lensing due to a cosmic string. If a cosmic string with the deficit angle $Δ\sim 100$ [femto-arcsec] $\sim10^{-18}$ [rad] exists around the line of sight to a gamma-ray burst, we may observe characteristic interference patterns caused by gravitational lensing in the energy spectrum of the gamma-ray burst. This "femto-lensing" event was first proposed as a tool to probe small mass primordial black holes. In this paper, we propose use of the femto-lensing to probe cosmic strings with extremely small tension. Observability conditions and the event rate are discussed. Differences between the cases of a point mass and a cosmic string are presented.

astro-ph.CO↗

arXiv2012

Black Hole Universe: Construction and Analysis of Initial Data

We numerically construct an one-parameter family of initial data of an expanding inhomogeneous universe model which is composed of regularly aligned black holes with an identical mass. They are initial data for vacuum solutions of the Einstein equations. We call this universe model the "black hole universe" and analyze the structure of these initial data. We study the relation between the mean expansion rate of the 3-space, which corresponds to the Hubble parameter, and the mass density of black holes. The result implies that the same relation as that of the Einstein-de Sitter universe is realized in the limit of the large separation between neighboring black holes. The applicability of the cosmological Newtonian $N$-body simulation to the dark matter composed of black holes is also discussed. The deviation of the spatial metric of the cosmological Newtonian $N$-body system from that of the black hole universe is found to be smaller than about 1% in a region distant from the particles, if the separation length between neighboring particles is 20 times larger than their gravitational radius. By contrast, the deviation of the square of the Hubble parameter of the cosmological Newtonian $N$-body system from that of the black hole universe is about 20% for the same separation length.

gr-qc↗

arXiv2012

Evolution of density perturbations in large void universe

We study the evolution of linear density perturbations in a large spherical void universe which accounts for the acceleration of the cosmic volume expansion without introducing dark energy. The density contrast of this void is not large within the light cone of an observer at the center of the void. Therefore, we describe the void structure as a perturbation with a dimensionless small parameter $κ$ in a homogeneous and isotropic universe within the region observable for the observer. We introduce additional anisotropic perturbations with a dimensionless small parameter $ε$, whose evolution is of interest. Then, we solve perturbation equations up to order $κε$ by applying second-order perturbation theory in the homogeneous and isotropic universe model. By this method, we can know the evolution of anisotropic perturbations affected by the void structure. We show that the growth rate of the anisotropic density perturbations in the large void universe is significantly different from that in the homogeneous and isotropic universe. This result suggests that the observation of the distribution of galaxies may give a strong constraint on the large void universe model.

astro-ph.CO↗

arXiv2012

Weak lensing of CMB by cosmic (super-)strings

We study the effect of weak lensing by cosmic (super-)strings on the anisotropies of cosmic microwave background (CMB). In developing a method to calculate the lensing convergence field due to strings, and thereby temperature and polarization angular power spectra of CMB, we clarify how the nature of strings, characterized by the intercommuting probability, can influence these CMB anisotropies. Assuming that the power spectrum is dominated by Poisson-distributed string segments, we find that the convergence spectrum peaks at low multipoles and is mostly contributed from strings located at relatively low redshifts. As the intercommuting probability decreases, the spectra of the convergence and hence the lensed temperature and polarizations are gained because the number density of string segments becomes larger. An observationally important feature of the string-induced CMB polarizations is that the string-lensed spectra decay more slowly on small scales compared with primordial scalar perturbations from standard inflation.

astro-ph.CO↗

arXiv2012

Probing the Inhomogeneous Universe with Gravitational Wave Cosmology

If we assume that we live in the center of a spherical inhomogeneous universe, we can explain the apparent accelerating expansion of the universe without introducing the unknown dark energy or modifying gravitational theory. Direct measurement of the cosmic acceleration can be a powerful tool in distinguishing $Λ$CDM and the inhomogeneous models. If $Λ$CDM is the correct model, we have shown that DECIGO/BBO has sufficient ability to detect the positive redshift drift of the source by observing gravitational waves from neutron star binaries for 5-10 years. This enables us to rule out any Lemaître-Tolman-Bondi (LTB) void model with monotonically increasing density profile. Furthermore, by detecting the positive redshift drift at $z\sim 0$, we can even rule out generic LTB models unless we allow unrealistically steep density gradient at $z\sim 0$. We also show that the measurement accuracy is slightly improved when we consider the joint search of DECIGO/BBO and the third generation Einstein Telescope. This test can be performed with GW observations alone without any reference to electromagnetic observations.

astro-ph.CO↗

arXiv2011

Direct Measurement of the Positive Acceleration of the Universe and Testing Inhomogeneous Models under Gravitational Wave Cosmology

One possibility for explaining the apparent accelerating expansion of the universe is that we live in the center of a spherically inhomogeneous universe. Although current observations cannot fully distinguish $Λ$CDM and these inhomogeneous models, direct measurement of the acceleration of the universe can be a powerful tool in probing them. We have shown that, if $Λ$CDM is the correct model, DECIGO/BBO would be able to detect the positive redshift drift (which is the time evolution of the source redshift $z$) in 3--5 year gravitational wave (GW) observations from neutron-star binaries, which enables us to rule out any Lemaître-Tolman-Bondi (LTB) void model with monotonically increasing density profile. We may even be able to rule out any LTB model unless we allow unrealistically steep density profile at $z\sim 0$. This test can be performed with GW observations alone, without any reference to electromagnetic observations, and is more powerful than the redshift drift measurement using Lyman $α$ forest.

astro-ph.CO↗

arXiv2011

Gregory-Laflamme instability of a slowly rotating black string

We study the Gregory-Laflamme instability of a 5-dimensional slowly rotating black string in which the 4-dimensional section is described by the Kerr black hole. We treat the rotation in a perturbative way introducing a small parameter for the rotation. It is found that rotation makes the Gregory-Laflamme instability stronger. Both the critical wavelength at the onset of instability and the growth time-scale are found to decrease as the rotation increases.

hep-th↗

arXiv2011

A Note on the Inverse Problem with LTB Universes

The inverse problem with Lemaître-Tolman-Bondi (LTB) universe models is discussed. The LTB solution for the Einstein equations describes the spherically symmetric dust-filled spacetime. The LTB solution has two physical functional degrees of freedom of the radial coordinate. The inverse problem is constructing an LTB model requiring that the LTB model be consistent with selected important observational data. In this paper, we assume that the observer is at the center and consider the distance-redshift relation $\da$ and the redshift-space mass density $μ$ as the selected important observational data. We give $\da$ and $μ$ as functions of the redshift $z$. Then, we explicitly show that, for general functional forms of $\da(z)$ and $μ(z)$, the regular solution does not necessarily exist in the whole redshift domain. We also show that the condition for the existence of the regular solution %in terms of $\da(z)$ and $μ(z)$ is satisfied by the distance-redshift relation and the redshift-space mass density in $Λ$CDM models. Deriving regular differential equations for the inverse problem with the distance-redshift relation and the redshift-space mass density in $Λ$CDM models, we numerically solve them for the case $(Ω_{\rm M0},Ω_{\Lambda0})=(0.3,0.7)$. A set of analytic fitting functions for the resultant LTB universe model is given. How to solve the inverse problem with the simultaneous big-bang and a given function $\da(z)$ for the distance-redshift relation is provided in the Appendix.

astro-ph.CO↗

arXiv2010

Perturbative Analysis of a Stationary Magnetosphere in an Extreme Black Hole Spacetime : On the Meissner-like Effect of an Extreme Black Hole

It is known that the Meissner-like effect is seen in a magnetosphere without an electric current in black hole spacetime: no non-monopole component of magnetic flux penetrates the event horizon if the black hole is extreme. In this paper, in order to see how an electric current affects the Meissner-like effect, we study a force-free electromagnetic system in a static and spherically symmetric extreme black hole spacetime. By assuming that the rotational angular velocity of the magnetic field is very small, we construct a perturbative solution for the Grad-Shafranov equation, which is the basic equation to determine a stationary, axisymmetric electromagnetic field with a force-free electric current. Our perturbation analysis reveals that, if an electric current exists, higher multipole components may be superposed upon the monopole component on the event horizon, even if the black hole is extreme.

gr-qc↗