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Anzhong Wang

Publications and source records attributed to Anzhong Wang.

At least 235 records · Page 13Linked to original sources
arXiv2008

Cosmological constant and late transient acceleration of the universe in the Horava-Witten Heterotic M-Theory on S^{1}/Z_{2}

Orbifold branes are studied in the framework of the 11-dimensional Horava-Witten heterotic M-Theory. It is found that the effective cosmological constant can be easily lowered to its current observational value by the mechanism of large extra dimensions. The domination of this constant over the evolution of the universe is only temporarily. Due to the interaction of the bulk and the branes, the universe will be in its decelerating expansion phase again in the future, whereby all problems connected with a far future de Sitter universe are resolved.

hep-ph↗
arXiv2007

Current constraints on interacting holographic dark energy

Although there is mounting observational evidence that the cosmic expansion is undergoing a late-time acceleration, the physical mechanism behind such a phenomenon is yet unknown. In this paper, we investigate a holographic dark energy (HDE) model with interaction between the components of the dark sector in the light of current cosmological observations. We use both the new \emph{gold} sample of 182 type Ia supernovae (SNe Ia) and the 192 SNe Ia ESSENCE data, the baryon acoustic oscillation measurement from the Sloan Digital Sky Survey and the shift parameter from the three-year Wilkinson Microwave Anisotropy Probe data. In agreement with previous results, we show that these observations suggest a very weak coupling or even a noninteracting HDE. The phantom crossing behavior in the context of these scenarios is also briefly discussed.

astro-ph↗
arXiv2007

Friedmann Equations and Thermodynamics of Apparent Horizons

With the help of a masslike function which has dimension of energy and equals to the Misner-Sharp mass at the apparent horizon, we show that the first law of thermodynamics of the apparent horizon $dE=T_AdS_A$ can be derived from the Friedmann equation in various theories of gravity, including the Einstein, Lovelock, nonlinear, and scalar-tensor theories. This result strongly suggests that the relationship between the first law of thermodynamics of the apparent horizon and the Friedmann equation is not just a simple coincidence, but rather a more profound physical connection.

hep-th↗
arXiv2007

Energy conditions and current acceleration of the universe

The energy conditions provide a very promising model-independent study of the current acceleration of the universe. However, in order to connect these conditions with observations, one often needs first to integrate them, and then find the corresponding constraints on some observational variables, such as the distance modulus. Those integral forms can be misleading, and great caution is needed when one interprets them physically. A typical example is that the transition point of the deceleration parameter $q(z)$ is at about $z \simeq 0.76$ in the $Λ$CDM model. However, with the same model when we consider the dimensionless Hubble parameter $E(z)$, which involves the integration of $q(z)$, we find that $E(z)$ does not cross the line of $q(z) = 0$ before $z = 2$. Therefore, to get the correct result, we cannot use the latter to determine the transition point. With these in mind, we carefully study the constraints from the energy conditions, and find that, among other things, the current observational data indeed strongly indicate that our universe has ocne experienced an accelerating expansion phase between the epoch of galaxy formation and the present.

astro-ph↗
arXiv2007

Direct evidence of acceleration from distance modulus redshift graph

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.

astro-ph↗
arXiv2007

Thermodynamical properties of dark energy

We have investigated the thermodynamical properties of dark energy. Assuming that the dark energy temperature $T\sim a^{-n}$ and considering that the volume of the Universe enveloped by the apparent horizon relates to the temperature, we have derived the dark energy entropy. For dark energy with constant equation of state $w>-1$ and the generalized Chaplygin gas, the derived entropy can be positive and satisfy the entropy bound. The total entropy, including those of dark energy, the thermal radiation and the apparent horizon, satisfies the generalized second law of thermodynamics. However, for the phantom with constant equation of state, the positivity of entropy, the entropy bound, and the generalized second law cannot be satisfied simultaneously.

gr-qc↗
arXiv2007

Colliding Branes and Formation of Spacetime Singularities

We construct a class of analytic solutions with two free parameters to the five-dimensional Einstein field equations, which represents the collision of two timelike 3-branes. We study the local and global properties of the spacetime, and find that spacelike singularities generically develop after the collision, due to the mutual focus of the two branes. Non-singular spacetime can be constructed only in the case where both of the two branes violate the energy conditions.

hep-th↗
arXiv2007

Reconstruction of the deceleration parameter and the equation of state of dark energy

The new 182 gold supernova Ia data, the baryon acoustic oscillation measurement and the shift parameter determined from the Sloan Digital Sky Survey and the three-year Wilkinson Microwave Anisotropy Probe data are combined to reconstruct the dark energy equation of state parameter $w(z)$ and the deceleration parameter $q(z)$. We find that the strongest evidence of acceleration happens around the redshift $z\sim 0.2$ and the stringent constraints on $w(z)$ lie in the redshift range $z\sim 0.2-0.5$. At the sweet spot, $-1.2<w(z)<-0.6$ for the dark energy parametrization $w(z)=w_0+w_a z/(1+z)^2$ at the $3σ$ confidence level. The transition redshift $z_t$ when the Universe underwent the transition from deceleration to acceleration is derived to be $z_t=0.36^{+0.23}_{-0.08}$. The combined data is also applied to find out the geometry of the Universe, and we find that at the $3σ$ confidence level, $|Ω_k|\alt 0.05$ for the simple one parameter dark energy model, and $-0.064<Ω_k<0.028$ for the $Λ$CDM model.

astro-ph↗
arXiv2007

Thermodynamical properties of the Universe with dark energy

We have investigated the thermodynamical properties of the Universe with dark energy. Adopting the usual assumption in deriving the constant co-moving entropy density that the physical volume and the temperature are independent, we observed some strange thermodynamical behaviors. However, these strange behaviors disappeared if we consider the realistic situation that the physical volume and the temperature of the Universe are related. Based on the well known correspondence between the Friedmann equation and the first law of thermodynamics of the apparent horizon, we argued that the apparent horizon is the physical horizon in dealing with thermodynamics problems. We have concentrated on the volume of the Universe within the apparent horizon and considered that the Universe is in thermal equilibrium with the Hawking temperature on the apparent horizon. For dark energy with $w\ge -1$, the holographic principle and the generalized second law are always respected.

gr-qc↗
arXiv2006

Existence of black holes in Friedmann-Robertson-Walker universe dominated by dark energy

We study the existence of black holes in a homogeneous and isotropic expanding Friedmann-Robertson-Walker (FRW) universe dominated by dark energy. We show that black holes can exist in such a universe by considering some specific McVittie solutions. Although these solutions violate all three energy conditions, the FRW background does satisfy the weak energy condition.

astro-ph↗
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

Acceleration from M theory and Fine-tuning

The compactification of M theory with time dependent hyperbolic internal space gives an effective scalar field with exponential potential which provides a transient acceleration in Einstein frame in four dimensions. Ordinary matter and radiation are present in addition to the scalar field coming from compactification. We find that we have to fine-tune the initial conditions of the scalar field so that our Universe experiences acceleration now. During the evolution of our Universe, the volume of the internal space increases about 12 times. The time variation of the internal space results in a large time variation of the fine structure constant which violates the observational constraint on the variation of the fine structure constant. The large variation of the fine structure constant is a generic feature of transient acceleration models.

gr-qc↗
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

Observational constraints on the acceleration of the Universe

We propose a new parametrization of the deceleration parameter to study its time-variation behavior. The advantage of parameterizing the deceleration parameter is that we do not need to assume any underlying theory of gravity. By fitting the model to the 157 gold sample supernova Ia data, we find strong evidence that the Universe is currently accelerating and it accelerated in the past. By fitting the model to the 115 nearby and Supernova Legacy Survey supernova Ia data, the evidence that the Universe is currently accelerating is weak, although there is still a strong evidence that the Universe once accelerated in the past. The results obtained from the 157 gold sample supernova Ia data and those from the 115 supernova Ia data are not directly comparable because the two different data sets measure the luminosity distance up to different redshifts. We then use the Friedmann equation and a dark energy parametrization to discuss the same problem. When we fit the model to the supernova Ia data alone, we find weak evidence that the Universe is accelerating and the current matter density is higher than that measured from other experiments. After we add the Sloan Digital Sky Survey data to constrain the dark energy model, we find that the behavior of the deceleration parameter is almost the same as that obtained from parameterizing the deceleration parameter.

astro-ph↗
arXiv2006

Black Hole Formation from Collapsing Dust Fluid in a Background of Dark Energy

The gravitational collapse of a spherically symmetric star, made of a dust fluid, $ρ_{DM}$, in a background of dark energy, $p = wρ,\; (w < -1/3)$, is studied. It is found that when only dark energy is present, black holes are {\em never} formed. When both of them are present, black holes can be formed, due to the condensation of the dust fluid. Initially the dust fluid may not play an important role, but, as time increases, it will dominate the collapse and finally leads to formation of black holes. This result remains true even when the interaction between the dust fluid and dark energy does not vanish. When $w < -1$ (phantoms), some models can also be interpreted as representing the death of a white hole that ejects both dust and phantoms. The ejected matter re-collapses to form a black hole.

astro-ph↗
arXiv2005

Asymptotics of Solutions of a Perfect Fluid Coupled with a Cosmological Constant in Four-Dimensional Spacetime with Toroidal Symmetry

Asymptotics of solutions of a perfect fluid when coupled with a cosmological constant in four-dimensional spacetime with toroidal symmetry are studied. In particular, it is found that the problem of self-similar solutions of the first kind for a fluid with the equation of state, $p = k ρ$, can be reduced to solving a master equation of the form, $$ 2 F(q, k)\frac{q''(ξ)}{q'(ξ)} - G(q,k) q'(ξ) = \frac{4}ξ. $$ For $k = 0$ and $k = -1/3$ the general solutions are obtained and their main local and global properties are studied in detail.

gr-qc↗
arXiv2005

On Gauge Choice of Spherically Symmetric 3-Branes

Gauge choice for a spherically symmetric 3-brane embedded in a D-dimensional bulk with arbitrary matter fields on and off the brane is studied. It is shown that Israel's junction conditions across the brane restrict severely the dependence of the matter fields on the spacetime coordinates. As examples, a scalar field or a Yang-Mills potential can be only either time-dependent or radial-coordinate dependent for the chosen gauge, while for a perfect fluid it must be co-moving.

hep-th↗