arXiv subjects

Chul-Moon Yoo

Publications and source records attributed to Chul-Moon Yoo.

At least 109 records · Page 6Linked to original sources

arXiv2010

Redshift Drift in LTB Void Universes

We study the redshift drift, i.e., the time derivative of the cosmological redshift in the Lemaître-Tolman-Bondi (LTB) solution in which the observer is assumed to be located at the symmetry center. This solution has often been studied as an anti-Copernican universe model to explain the acceleration of cosmic volume expansion without introducing the concept of dark energy. One of decisive differences between LTB universe models and Copernican universe models with dark energy is believed to be the redshift drift. The redshift drift is negative in all known LTB universe models, whereas it is positive in the redshift domain $z \lesssim 2$ in Copernican models with dark energy. However, there have been no detailed studies on this subject. In the present paper, we prove that the redshift drift of an off-center source is always negative in the case of LTB void models. We also show that the redshift drift can be positive with an extremely large hump-type inhomogeneity. Our results suggest that we can determine whether we live near the center of a large void without dark energy by observing the redshift drift.

astro-ph.CO↗

arXiv2010

CMB observations in LTB universes: Part II: the kSZ effect in an LTB universe

We study the kinematic Sunyaev-Zel'dovich (kSZ) effect in a Lemître-Tolman-Bondi (LTB) universe model whose distance-redshift relation agrees with that of the concordance $Λ$CDM model at redshifts $z\lesssim2$. This LTB universe model has a void with size comparable to the Hubble horizon scale. We first determine the decoupling epoch in this LTB universe model by an approximate analytical condition under a few simplified assumptions on the physical quantities at that epoch. Then we calculate the cosmic microwave background (CMB) anisotropy observed in the rest frame of clusters of galaxies which are assumed to be at rest in the spatial comoving coordinates of the LTB universe model. We find that the obtained temperature anisotropies are dominated by dipole, although there may exist higher multi-poles in general. We may interpret this dipole anisotropy as the drift velocity of a cluster of galaxies relative to the CMB rest frame. Hence it gives rise to the kSZ effect. We calculate this effect and compare it with observational data. We find that if we assume the conventional adiabatic perturbation scenario at the time of decoupling, the drift velocity of clusters of galaxies becomes unacceptably large. Conversely, this observational constraint may be relaxed by introducing a non-adiabatic (i.e., primordially isocurvature) component of inhomogeneities at the time of decoupling. However, our result indicates that the necessary isocurvature perturbation amplitude is very large.

astro-ph.CO↗

arXiv2010

Zeta(n) via hyperbolic functions

We present here an approach to a computation of $ζ(2)$ by changing variables in the double integral using hyperbolic trig functions. We also apply this approach to present $ζ(n)$, when $n>2$, as a definite improper integral of single variable.

math.CA↗

arXiv2010

Analytical model for CMB temperature angular power spectrum from cosmic (super-)strings

We present a new analytical method to calculate the small angle CMB temperature angular power spectrum due to cosmic (super-)string segments. In particular, using our method, we clarify the dependence on the intercommuting probability $P$. We find that the power spectrum is dominated by Poisson-distributed string segments. The power spectrum for a general value of $P$ has a plateau on large angular scales and shows a power-law decrease on small angular scales. The resulting spectrum in the case of conventional cosmic strings is in very good agreement with the numerical result obtained by Fraisse et al.. Then we estimate the upper bound on the dimensionless tension of the string for various values of $P$ by assuming that the fraction of the CMB power spectrum due to cosmic (super-)strings is less than ten percents at various angular scales up to $\ell=2000$. We find that the amplitude of the spectrum increases as the intercommuting probability. As a consequence, strings with smaller intercommuting probabilities are found to be more tightly constrained.

astro-ph.CO↗

arXiv2010

CMB observations in LTB universes: Part I: Matching peak positions in the CMB spectrum

Acoustic peaks in the spectrum of the cosmic microwave background in spherically symmetric inhomogeneous cosmological models are studied. At the photon-baryon decoupling epoch, the universe may be assumed to be dominated by non-relativistic matter, and thus we may treat radiation as a test field in the universe filled with dust which is described by the Lemaître-Tolman-Bondi (LTB) solution. First, we give an LTB model whose distance-redshift relation agrees with that of the concordance $Λ$CDM model in the whole redshift domain and which is well approximated by the Einstein-de Sitter universe at and before decoupling. We determine the decoupling epoch in this LTB universe by Gamow's criterion and then calculate the positions of acoustic peaks. Thus obtained results are not consistent with the WMAP data. However, we find that one can fit the peak positions by appropriately modifying the LTB model, namely, by allowing the deviation of the distance-redshift relation from that of the concordance $Λ$CDM model at $z>2$ where no observational data are available at present. Thus there is still a possibility of explaining the apparent accelerated expansion of the universe by inhomogeneity without resorting to dark energy if we abandon the Copernican principle. Even if we do not take this extreme attitude, it also suggests that local, isotropic inhomogeneities around us may seriously affect the determination of the density contents of the universe unless the possible existence of such inhomogeneities is properly taken into account.

astro-ph.CO↗

arXiv2010

Skewness in CMB temperature fluctuations from curved cosmic (super-)strings

We compute the one-point probability distribution function of small-angle cosmic microwave background temperature fluctuations due to curved cosmic (super-)strings with a simple model of string network by performing Monte Carlo simulations. Taking into account of the correlation between the curvature and the velocity of string segments, there appear non-Gaussian features, specifically non-Gaussian tails and a skewness, in the one-point pdf. The obtained sample skewness for the conventional field-theoretic cosmic strings is $g_1\approx -0.14$, which is consistent with the result reported by Fraisse et al.. We also discuss the dependence of the pdf on the intercommuting probability. We find that the standard deviation of the Gaussian part increases and non-Gaussian features are suppressed as the intercommuting probability decreases. For sufficiently small intercommuting probability, the skewness is given by $\lesssim$ (a\ few) $\times 10^{-2}$.

astro-ph.CO↗

arXiv2009

Non-Gaussianity in Cosmic Microwave Background Temperature Fluctuations from Cosmic (Super-)Strings

We compute analytically the small-scale temperature fluctuations of the cosmic microwave background from cosmic (super-)strings and study the dependence on the string intercommuting probability $P$. We develop an analytical model which describes the evolution of a string network and calculate the numbers of string segments and kinks in a horizon volume. Then we derive the probability distribution function (pdf) which takes account of finite angular resolution of observation. The resultant pdf consists of a Gaussian part due to frequent scatterings by long string segments and a non-Gaussian tail due to close encounters with kinks. The dispersion of the Gaussian part is reasonably consistent with that obtained by numerical simulations by Fraisse et al.. On the other hand, the non-Gaussian tail contains two phenomenological parameters which are determined by comparison with the numerical results for P=1. Extrapolating the pdf to the cases with $P<1$, we predict that the non-Gaussian feature is suppressed for small $P$.

astro-ph↗

arXiv2009

Solving the Inverse Problem with Inhomogeneous Universes

We construct the Lemaître-Tolman-Bondi (LTB) dust universe whose distance-redshift relation is equivalent to that in the concordance $Λ$ cold dark matter ($Λ$CDM) cosmological model. In our model, the density distribution and velocity field are not homogeneous, whereas the big-bang time is uniform, which implies that the universe is homogeneous at its beginning. We also study the effects of local clumpiness in the density distribution as well as the effects of large-scale inhomogeneities on the distance-redshift relation, and show that these effects may reduce the amplitude of large-scale inhomogeneities necessary for having a distance-redshift relation that is the same as that of the concordance $Λ$CDM universe. We also study the temporal variation of the cosmological redshift and show that, by the observation of this quantity, we can distinguish our LTB universe model from the concordance $Λ$CDM model, even if their redshift-distance relations are equivalent to each other.

astro-ph↗

arXiv2009

Yet Another Realization of Kerr/CFT Correspondence

The correspondence between the Kerr black hole and a boundary CFT has been conjectured recently. The conjecture has been proposed first only for the half of the CFT, namely for left movers. For right movers, the correspondence has been also found out through the suitable asymptotic boundary condition. However, the boundary conditions for these two studies are exclusive to each other. The boundary condition for left movers does not allow the symmetry of right movers, and vice versa. We propose new boundary condition which allows both of left and right movers.

hep-th↗

arXiv2009

Dynamical Compactification and Inflation in Einstein-Yang-Mills Theory with Higher Derivative Coupling

We study cosmology of the Einstein-Yang-Mills theory in ten dimensions with a quartic term in the Yang-Mills field strength. We obtain analytically a class of cosmological solutions in which the extra dimensions are static and the scale factor of the four-dimensional Friedmann-Lemaitre-Robertson-Walker metric is an exponential function of time. This means that the model can explain inflation. Then we look for solutions that describe dynamical compactification of the extra dimensions. The effective cosmological constant $λ_1$ in the four-dimensional universe is determined from the gravitational coupling, ten-dimensional cosmological constant, gauge coupling and higher derivative coupling. By numerical integration, the solution with $λ_1=0$ is found to behave as a matter-dominated universe which asymptotically approaches flat space-time, while the solution with a non-vanishing $λ_1$ approaches de Sitter space-time in the asymptotic future.

hep-th↗

arXiv2009

Another Realization of Kerr/CFT Correspondence

We study another realization of the Kerr/CFT correspondence. By imposing new asymptotic conditions for the near horizon geometry of Kerr black hole, an asymptotic symmetry which contains all of the exact isometries can be obtained. In particular, the Virasoro algebra can be realized as an enhancement of SL(2,R) symmetry of the AdS geometry. By using this asymptotic symmetry, we discuss finite temperature effects and show the correspondence concretely.

hep-th↗

arXiv2009

Topology Changing Process of Coalescing Black Holes on Eguchi-Hanson Space

We numerically study the event horizons of two kinds of five-dimensional coalescing black hole solutions with different asymptotic structures: the five-dimensional Kastor-Traschen solution (5DKT) and the coalescing black hole solution on Eguchi-Hanson space (CBEH). Topologies of the spatial infinity are ${\rm S}^3$ and $L(2;1)={\rm S}^3/{\mathbb Z}_2$, respectively. We show that the crease sets of event horizons are topologically ${\rm R}^1$ in 5DKT and ${\rm R}^1\times {\rm S}^1$ in CBEH, respectively. If we choose the time slices which respect space-time symmetry, the first contact points of the coalescing process is a point in the 5DKT case but a ${\rm S}^1$ in the CBEH case. We also find that in CBEH, time slices can be chosen so that a black ring with ${\rm S}^1\times {\rm S}^2$ topology can be also formed during a certain intermediate period unlike the 5DKT.

gr-qc↗

arXiv2009

Conformal Symmetry for Rotating D-branes

We apply the Kerr/CFT correspondence to the rotating black p-brane solutions. These solutions give the simplest examples from string theory point of view. Their near horizon geometries have structures of AdS, even though black p-brane solutions do not have AdS-like structures in the non-rotating case. The microscopic entropy which can be calculated via the Cardy formula exactly agrees with Bekenstein-Hawking entropy.

hep-th↗

arXiv2009

Sound Modes in Holographic Hydrodynamics for Charged AdS Black Hole

In the previous paper we studied the transport coefficients of Quark-Gluon Plasma in finite temperature and finite density in vector and tensor modes. In this paper, we extend it to the scalar modes. We work out the decoupling problem and hydrodynamic analysis for the sound mode in charged AdS black hole and calculate the sound velocity, the charge susceptibility and the electrical conductivity. We find that Einstein relation among the conductivity, the diffusion constant and the susceptibility holds exactly.

hep-th↗

arXiv2008

Magnification Probability Distribution Functions of Standard Candles in a Clumpy Universe

Lensing effects on light rays from point light sources, such like Type Ia supernovae, are simulated in a clumpy universe model. In our universe model, it is assumed that all matter in the universe takes the form of randomly distributed objects each of which has finite size and is transparent for light rays. Monte-Carlo simulations are performed for several lens models, and we compute probability distribution functions of magnification. In the case of the lens models that have a smooth density profile or the same degree of density concentration as the spherical NFW (Navarro-Frenk-White) lens model at the center, the so-called gamma distributions fit well the magnification probability distribution functions if the size of lenses is sufficiently larger than the Einstein radius. In contrast, the gamma distributions do not fit the magnification probability distribution functions in the case of the SIS (Singular Isothermal Sphere) lens model. We find, by using the power law cusp model, that the magnification probability distribution function is fitted well using the gamma distribution only when the slope of the central density profile is not very steep. These results suggest that we may obtain information about the slope of the central density profiles of dark matter halo from the lensing effect of Type Ia supernovae.

astro-ph↗

arXiv2008

Horizons of Coalescing Black Holes on Eguchi-Hanson Space

Using the numerical method, we study dynamics of coalescing black holes on the Eguchi-Hanson base space. Effects of a difference in spacetime topology on the black hole dynamics is discussed. We analyze appearance and disappearance process of marginal surfaces. In our calculation, the area of a coverall black hole horizon at the creation time in the coalescing black holes solutions on Eguchi-Hanson space is larger than that in the five-dimensional Kastor-Traschen solutions. This fact suggests that the black hole production on the Eguchi-Hanson space is easier than that on the flat space.

gr-qc↗

arXiv2007

Can inhomogeneties accelerate the cosmic volume expansion?

If expanding and contracting regions coexist in the universe, the speed of the cosmic volume expansion can be accelerated. We construct simple inhomogeneous dust-filled universe models in which the speed of the cosmic volume expansion is accelerated for finite periods. These models are constructed by removing spherical domains from the Einstein-de Sitter universe and filling each domain with a Lemaitre-Tolman-Bondi dust sphere possessing the same gravitational mass as the removed region. This represents an exact solution of the Einstein equations. We find that acceleration of the cosmic volume expansion is realized in some cases when the size of the contracting region is comparable to the horizon radius of the Einstein-de Sitter universe though this model is very different from the universe observed today. This result implies that non-linear general relativistic effects of inhomogeneities are very important to realize the acceleration of the cosmic volume expansion.

gr-qc↗

arXiv2007

Lensing Effects on Gravitational Waves in a Clumpy Universe -Effects of Inhomogeneity on the Distance-Redshift Relation-

The distance-redshift relation determined by means of gravitational waves in the clumpy universe is simulated numerically by taking into account the effects of gravitational lensing. It is assumed that all of the matter in the universe takes the form of randomly distributed point masses, each of which has the identical mass $M_L$. Calculations are carried out in two extreme cases: $λ\gg GM_L/c^2$ and $λ\ll GM_L/c^2$, where $λ$ denotes the wavelength of gravitational waves. In the first case, the distance-redshift relation for the fully homogeneous and isotropic universe is reproduced with a small distance dispersion, whereas in the second case, the distance dispersion is larger. This result suggests that we might obtain information about the typical mass of lens objects through the distance-redshift relation gleaned through observation of gravitational waves of various wavelengths. In this paper, we show how to set limitations on the mass $M_L$ through the observation of gravitational waves in the clumpy universe model described above.

astro-ph↗