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Search Results: 1 - 10 of 63291 matches for " Zong-Hong Zhu "
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Gravitational lensing statistical properties in general FRW cosmologies with dark energy component(s): analytic results
Zong-Hong Zhu
Physics , 2001, DOI: 10.1142/S0218271800000591
Abstract: Various astronomical observations have been consistently making a strong case for the existence of a component of dark energy with negative pressure in the universe. It is now necessary to take the dark energy component(s) into account in gravitational lensing statistics and other cosmological tests. By using the comoving distance we derive analytic but simple expressions for the optical depth of multiple image, the expected value of image separation and the probability distribution of image separation caused by an assemble of singular isothermal spheres in general FRW cosmological models with dark energy component(s). We also present the kinematical and dynamical properties of these kinds of cosmological models and calculate the age of the universe and the distance measures, which are often used in classical cosmological tests. In some cases we are able to give formulae that are simpler than those found elsewhere in the literature, which could make the cosmological tests for dark energy component(s) more convenient.
Gravitational Lensing Statistics as a Probe of Dark Energy
Zong-Hong Zhu
Physics , 2000, DOI: 10.1142/S0217732300001304
Abstract: By using the comoving distance, we derive an analytic expression for the optical depth of gravitational lensing, which depends on the redshift to the source and the cosmological model characterized by the cosmic mass density parameter $\Omega_m$, the dark energy density parameter $\Omega_x$ and its equation of state $\omega_x = p_x/\rho_x$. It is shown that, the larger the dark energy density is and the more negative its pressure is, the higher the gravitational lensing probability is. This fact can provide an independent constraint for dark energy.
Magnification cross-sections of gravitational lensing by galaxies in general FLRW cosmologies
Zong-Hong Zhu
Physics , 2000,
Abstract: For a wide variety of cosmological models characterized by the cosmic mass density $\Omega_M$ and the normalized cosmological constant $\Omega_{\Lambda}$, we derive an analytic expression for the estimate of magnification cross-sections by an ensemble of isothermal spheres as models of galactic mass distributions. This provides a simple approach to demonstrate how the lensing probability by galaxies depends on the cosmological parameters. An immediate consequence is that, while a non-zero cosmological constant indeed leads to a significant increase of the lensing probability as it has been shown in the literature, only a small fraction of sky to $z\sim 3$ can be moderately ($\mu \sim 1.3$) lensed by galaxies even in a $\Lambda$-dominated flat universe. Therefore, whether or not there is a nonzero cosmological constant, it is unlikely that the overall quasar counts have been seriously contaminated by the presence of galactic lenses.
Limits from Weak Gravity Conjecture on Chaplygin-Gas-Type Models

WU Xing,ZHU Zong-Hong,

中国物理快报 , 2008,
Abstract: The weak gravity conjecture is proposed as a criterion to distinguish the landscape from the swampland in string theory. As an application in cosmology of this conjecture, we use it to impose theoretical constraint on parametersof the Chaplygin-gas-type models. Our analysis indicates that the Chaplygin-gas-type models realized in quintessence field are in the swampland.
Natural Phantom Dark Energy, Wiggling Hubble Parameter $H(z)$ and Direct $H(z)$ Data
Zhang, Hongsheng;Zhu, Zong-Hong
High Energy Physics - Phenomenology , 2007, DOI: 10.1088/1475-7516/2008/03/007
Abstract: Recent direct $H(z)$ data indicate that the parameter $H(z)$ may wiggle with respect to $z$. On the other hand the luminosity distance data of supernovae flatten the wiggles of $H(z)$ because of integration effect. It is expected that the fitting results can be very different in a model permitting a wiggling $H(z)$ because the data of supernovae is highly degenerated to such a model. As an example the natural phantom dark energy is investigated in this paper. The dynamical property of this model is studied. The model is fitted by the direct $H(z)$ data set and the SNLS data set, respectively. And the results are quite different, as expected. The quantum stability of this model is also shortly discussed. We find it is a viable model if we treat it as an effective theory truncated by an upperbound.
Time drift of subtended angles as a new cosmological probe
Hongbao Zhang,Zong-Hong Zhu
Physics , 2009,
Abstract: We here propose the time drift of subtended angles as a new possible cosmological probe. In particular, with the coming era of microarcsecond astrometry, our proposal can be used to measure the Hubble expansion rate of our universe in a direct way.
Crossing $w=-1$ by a single scalar on a Dvali-Gabadadze-Porrati brane
Hongsheng Zhang,Zong-Hong Zhu
Physics , 2006, DOI: 10.1103/PhysRevD.75.023510
Abstract: Recent type Ia supernovae data seem to favor a dark energy model whose equation of state $w(z)$ crosses -1, which is a much more amazing problem than the acceleration of the universe. Either the case that $w(z)$ evolves from above -1 to below -1 or the case that $w(z)$ runs from below -1 to above -1, is consistent with present data. In this paper we show that it is possible to realize the crossing behaviours of both of the two cases by only a single scalar field in frame of Dvali-Gabadadze-Porrati braneworld. At the same time we prove that there does not exist scaling solution in a universe with dust.
The distance duality relation and the temperature profile of Galaxy Clusters
Shuo Cao,Zong-Hong Zhu
Physics , 2011,
Abstract: The validity of distance duality relation, $\eta=D_L(z)(1+z)^{-2}/D_A(z)=1$, an exact result required by the Etherington reciprocity theorem, where $D_A(z)$ and $D_L(z)$ are the angular and luminosity distances, plays an essential part in cosmological observations and model constraints. In this paper, we investigate some consequences of such a relation by assuming $\eta$ a constant or a function of the redshift. In order to constrain the parameters concerning $\eta$, we consider two groups of cluster gas mass fraction data including 52 X-ray luminous galaxy clusters observed by Chandra in the redshift range $0.3\sim 1.273$ and temperature range $T_{\rm gas}> 4$ keV, under the assumptions of two different temperature profiles [1]. We find that the constant temperature profile is in relatively good agreement with no violation of the distance duality relation for both parameterizations of $\eta$, while the one with temperature gradient (the Vikhlinin et al. temperature profile) seems to be incompatible even at 99% CL.
Curvaton reheating mechanism in inflation on warped Dvali-Gabadadze-Porrati brane
Hongsheng Zhang,Zong-Hong Zhu
Physics , 2006, DOI: 10.1016/j.physletb.2006.07.055
Abstract: An impressed feature of inflation on warped Dvali-Gabadadze-Porrati (DGP) brane is that the inflationary phase exits spontaneously for a scalar inflaton field with exponential potential, which presents a graceful exit mechanism for the inflation. But its reheating mechanism leaves open. We investigate the curvaton reheating in inflation on warped DGP brane model. The reheating may occur in effctively 5 dimensional or 4 dimensional stage. We study the permitted parameter space of the curvaton field in detail. We demonstrate how the inflation model of the warped DGP brane is improved by the curvaton mechanism.
WKB Wave Functions with the Induced Gravity Theory
Zong-Hong Zhu,Li Cao
Physics , 2000, DOI: 10.1142/S0217732399002248
Abstract: The Wheeler-DeWitt equation for the induced gravity theory is constructed in the minisuperspace approximation, and then solved using the WKB method under three types of boundary condition proposed respectively by Hartle & Hawking (``no boundary''), Linde and Vilenkin (``tunneling from nothing''). It is found that no matter how the gravitational and cosmological ``constants'' vary in the classical models, they will acquire constant values when the universe comes from quantum creation, and that, in particular, the resulting tunneling wave function under the Linde or Vilenkin boundary condition reaches its maximum value if the cosmological constant vanishes.
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