Yuan-Zhong Zhang

Publications (7)0 Total impact

• Source

  Available from: ArXiv

  Article: Direct evidence of acceleration from distance modulus redshift graph

  Yungui Gong, Anzhong Wang, Qiang Wu, Yuan-Zhong Zhang
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  ABSTRACT: The energy conditions give upper bounds on the luminosity distance. We apply these upper bounds to the 192 essence supernova Ia data to show that the Universe had experienced accelerated expansion. This conclusion is drawn directly from the distance modulus-reshift graph. In addition to be a very simple method, this method is also totally independent of any cosmological model. From the degeneracy of the distance modulus at low redshift, we argue that the choice of $w_0$ for probing the property of dark energy is misleading. One explicit example is used to support this argument.
  04/2007;
• Source

  Available from: arxiv.org

  Article: Holography and holographic dark energy model

  Yungui Gong, Yuan-Zhong Zhang
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  ABSTRACT: 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.
  11/2005;
• Source

  Available from: arxiv.org

  Article: On curvature coupling and quintessence fine-tuning

  Yungui Gong, Anzhong Wang, Yuan-Zhong Zhang
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  ABSTRACT: 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. Comment: 5 pages, 1 figure, v2: correct the comment on astro-ph/0509177, v3: significant changes are made to better present the paper;v4: use epl style, add new contents, conclusion remains, accepted for publication by Europhys. Lett
  10/2005;
• Source

  Available from: arxiv.org

  Article: Dark energy and the future fate of the Universe

  Yungui Gong, Yuan-Zhong Zhang
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  ABSTRACT: We consider the possibility of observing the onset of the late time inflation of our patch of the Universe. The Hubble size criterion and the event horizon criterion are applied to several dark energy models to discuss the problem of future inflation of the Universe. We find that the acceleration has not lasted long enough to confirm the onset of inflation by present observations for the dark energy model with constant equation of state, the holographic dark energy model and the generalized Chaplygin gas (GCG) model. For the flat $\Lambda$CDM model with $\Omega_{m0}=0.3$, we find that if we use the Hubble size criterion, we need to wait until the $a_v$ which is the scale factor at the time when the onset of inflation is observed reaches 3.59 times of the scale factor $a_T$ when the Universe started acceleration, and we need to wait until $a_v=2.3 a_T$ to see the onset of inflation if we use the event horizon criterion. For the flat holographic dark energy model with $d=1$, we find that $a_v=3.46 a_T$ with the Hubble horizon and $a_v=2.34 a_T$ with the event horizon, respectively. For the flat GCG model with the best supernova fitting parameter $\alpha=1.2$, we find that $a_v=5.50 a_T$ with the Hubble horizon and $a_v=2.08 a_T$ with the event horizon, respectively. Comment: 2 figures, 11 pages, Mod. Phys. Lett. A in press
  08/2005;
• Source

  Available from: ArXiv

  Article: Hologrphy and holographic dark energy model

  Yungui Gong, Yuan-Zhong Zhang
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  ABSTRACT: 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. Comment: no figures, use revtex, v2: use iop style, some typos corrected and references updated, will appear in CQG
  05/2005;
• Source

  Available from: arxiv.org

  Article: Probing the curvature and dark energy

  Yungui Gong, Yuan-Zhong Zhang
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  ABSTRACT: Two new one-parameter tracking behavior dark energy representations $\omega=\omega_0/(1+z)$ and $\omega=\omega_0 e^{z/(1+z)}/(1+z)$ 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 $\omega(0)\sim -1$. 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. Comment: v2: two more figures are added, use revtex, main conclusion unchanged, Accepted for publication in Phys. Rev. D, v3: some typos corrected v4: correct caculation errors in model 4
  02/2005;
• Source

  Available from: arxiv.org

  Article: Exact scaling solutions and fixed points for general scalar field

  Yungui Gong, Anzhong Wang, Yuan-Zhong Zhang
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  ABSTRACT: We show that the most general dark energy model that possesses a scaling solution ρϕ∝an 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.
  Physics Letters B.