arXiv subjects

Yuan-Zhong Zhang

Publications and source records attributed to Yuan-Zhong Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2006

An Algorithm for Generating Rotating Brans-Dicke Wormhole Solutions

Using the ansatz of Matos and Núñez, the present article proposes an algorithm for generating several classes, not all independent, of asymptotically flat rotating wormhole solutions in the Brans-Dicke Theory. The algorithm allows us to associate a real number}$n$ \textit{with} \textit{each static and generated rotating solution. We shall also demonstrate how to match a rotating wormhole to a flat spacetime at the matter boundaries. The vacuum string extensions of the solutions are straightforward. The physical interpretations of the solutions are deferred.

gr-qc↗

arXiv2006

On curvature coupling and quintessence fine-tuning

We discuss the phenomenological model in which the potential energy of the quintessence field depends linearly on the energy density of the spatial curvature. We find that the pressure of the scalar field takes a different form when the potential of the scalar field also depends on the scale factor and the energy momentum tensor of the scalar field can be expressed as the form of a perfect fluid. A general coupling was proposed to explain the current accelerating expansion of the Universe and solve the fine-tuning problem.

hep-th↗

arXiv2006

Probing the curvature and dark energy

Two new one-parameter tracking behavior dark energy representations $ω=ω_0/(1+z)$ and $ω=ω_0 e^{z/(1+z)}/(1+z)$ are used to probe the geometry of the Universe and the property of dark energy. The combined type Ia supernova (SN Ia), Sloan Digital Sky Survey (SDSS), and Wilkinson Microwave Anisotropy Probe (WMAP) data indicate that the Universe is almost spatially flat and that dark energy contributes about 72% of the matter content of the present universe. The observational data also tell us that $ω(0)\sim -1$. It is argued that the current observational data can hardly distinguish different dark energy models to the zeroth order. The transition redshift when the expansion of the Universe changed from deceleration phase to acceleration phase is around $z_{\rm T}\sim 0.6$ by using our one-parameter dark energy models.

astro-ph↗

arXiv2006

Exact scaling solutions and fixed points for general scalar field

We show that the most general dark energy model that possesses a scaling solution $ρ_ϕ\propto a^n$ is the k-essence model, which includes both of the quintessence and tachyon models. The exact scaling solutions are then derived. The potential that gives the tracking solution in which dark energy exactly tracks the background matter field is the inverse squared potential. The quintessence field with exponential potential can be obtained from the k-essence field with the inverse squared potential. We also find the fixed points and study their main properties, whereby the scalar field dominant fixed point is identified.

gr-qc↗

arXiv2006

Constraints on the DGP Model from Recent Supernova Observations and Baryon Acoustic Oscillations

Although there is mounting observational evidence that the expansion of our universe is undergoing a late-time acceleration, the mechanism for this acceleration is yet unknown. In the so-called Dvali-Gabadadze-Porrati (DGP) model this phenomena is attributed to gravitational leakage into extra dimensions. In this work, we mainly focus our attention to the constraints on the model from the gold sample of type Ia supernovae (SNeIa), the first year data from the Supernova Legacy Survey (SNLS) and the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS). At 99.73% confidence level, the combination of the three databases provides Omega_m=0.270^{+0.018}_{-0.017} and Omega_{r_c}=0.216^{+0.012}_{-0.013} (hence a spatially closed universe with Omega_k=-0.350^{+0.080}_{-0.083}), which seems to be in contradiction with the most recent WMAP results indicating a flat universe. Based on this result, we also estimated the transition redshift (at which the universe switches from deceleration to acceleration) to be 0.70 < z_{q=0} < 1.01, at 2 sigma confidence level.

astro-ph↗

arXiv2006

Parametrization of K-essence and Its Kinetic Term

We construct the non-canonical kinetic term of a k-essence field directly from the effective equation of state function $w_k(z)$, which describes the properties of the dark energy. Adopting the usual parametrizations of equation of state we numerically reproduce the shape of the non-canonical kinetic term and discuss some features of the constructed form of k-essence.

astro-ph↗

arXiv2005

Hologrphy and holographic dark energy model

The holographic principle is used to discuss the holographic dark energy model. We find that the Bekenstein-Hawking entropy bound is far from saturation under certain conditions. A more general constraint on the parameter of the holographic dark energy model is also derived.

hep-th↗

arXiv2005

Observational Constraints on Variable Chaplygin Gas

We investigate observational constraints on the variable Chaplygin gas model from the gold sample of type Ia supernova data and the recent measurements of the X-ray gas mass fractions in galaxy clusters. Combining these databases, we obtain a tight constraint on the two model parameters. Our results indicate that the original Chaplygin gas model is ruled out by the data at 99.7% confidence level.

astro-ph↗

arXiv2005

The Holographic dark energy reexamined

We have reexamined the holographic dark energy model by considering the spatial curvature. We have refined the model parameter and observed that the holographic dark energy model does not behave as phantom model. Comparing the holographic dark energy model to the supernova observation alone, we found that the closed universe is favored. Combining with the Wilkinson Microwave Anisotropy Probe (WMAP) data, we obtained the reasonable value of the spatial curvature of our universe.

hep-th↗

arXiv2005

Parametrization of Quintessence and Its Potential

We develop a theoretical method of constructing the quintessence potential directly from the effective equation of state function $w(z)$, which describes the properties of the dark energy. We apply our method to four parametrizations of equation of state parameter and discuss the general features of the resulting potentials. In particular, it is shown that the constructed quintessence potentials are all in the form of a runaway type.

astro-ph↗

arXiv2005

Attractor Behavior of Phantom Cosmology

We investigate the cosmological attractor of the minimally coupled, self-interacting phantom field with a positive energy density but negative pressure. It is proved that the phantom cosmology is rigid in the sense that there exists a unique attractor solution. We plot the trajectories in the phase space numerically for the phantom field with three typical potentials. Phase portraits indicate that an initial kinetic term decays rapidly and the trajectories reach the unique attractor curve. We find that the curve corresponds to the slow-climb solution.

astro-ph↗

arXiv2005

Phantom Inflation and Primordial Perturbation Spectrum

In this paper we study the inflation model driven by the phantom field. We propose a possible exit from phantom inflation to our observational cosmology by introducing an additional normal scalar field, similar to hybrid inflation model. Then we discuss the primordial perturbation spectra from various phantom inflation models and give an interesting compare with those of normal scalar field inflation models.

astro-ph↗

arXiv2004

Comments on "New Brans-Dicke Wormholes"

It is shown that the recently claimed two new Brans-Dicke wormhole solutions [F. He and S-W. Kim, Phys. Rev. D{\bf 65}, 084022 (2002)] are not really new solutions. They are just the well known Brans-Dicke solutions of Class I and II in a different conformal gauge.

gr-qc↗

arXiv2004

Cosmological Evolution of Interacting Phantom Energy with Dark Matter

We investigate the cosmological evolution of an interacting phantom energy model in which the phantom field has interaction with the dark matter. We discuss the existence and stability of scaling solutions for two types of specific interactions. One is motivated by the conformal transformation in string theory and the other is motivated by analogy with dissipation. In the former case, there exist no scaling solutions. In the latter case, there exist stable scaling solutions, which may give a phenomenological solution of the coincidence problem. Furthermore, the universe either accelerates forever or ends with a singularity, which is determined by not only the model parameters but also the initial velocity of the phantom field.

astro-ph↗

arXiv2004

Volume Integral Theorem for Exotic Matter

We answer an important question in general relativity about the volume integral theorem for exotic matter by suggesting an exact integral quantifier for matter violating Averaged Null Energy Condition (ANEC). It is checked against some well known static, spherically symmetric traversable wormhole solutions of general relativity with a sign reversed kinetic term minimally coupled scalar field. The improved quantifier is consistent with the principle that traversable wormholes can be supported by arbitrarily small quantities of exotic matter.

gr-qc↗

arXiv2004

Inflation in Oscillating Universe

We make use of possible high energy correction to the Friedmann equation to implement the bounce and study the behavior of massive scalar field before and after bounce semianalytically and numerically. We find that the slow-roll inflation can be preceded by the kinetic dominated contraction. During this process, the field initially in the bottom of its potential can be driven by the anti-frictional force resulted from the contraction and roll up its potential hill, and when it rolls down after the bounce, it can driven a period inflation. The required e-folds number during the inflation limits the energy scale of bounce. Further unlike that expected, the field during the contraction can not be driven to arbitrary large value, even though the bounce occurs at Planck scale.

gr-qc↗

arXiv2004

Acoustic Wormholes

Acoustic analogs of static, spherically symmetric massive traversable Lorentzian wormholes are constructed as a {\em formal} extension of acoustic black holes. The method is straightforward but the idea is interesting in itself. The analysis leads to a new acoustic invariant for the massless counterpart of the Einstein-Rosen model of an elementary particle. It is shown that there is a marked, in a sense even counterintuitive, physical difference between the acoustic analogs of black holes and wormholes. The analogy allows us to also portray the nature of curvature singularity in the acoustic language. It is demonstrated that the light ray trajectories in an optical medium are the same as the sound trajectories in its acoustic analog. The implications of these analogies in the laboratory set up and in the different context of phantom energy accretion have been speculated.

gr-qc↗

arXiv2004

Interacting Phantom Energy

We investigate the role of a suitable interaction between a matter fluid and a phantom field for the coincidence problem. There exists a stationary scaling solution which is a stable attractor at late times. Furthermore, the cosmic doomsday is avoided in one region of the parameter space.

astro-ph↗