A Two-Dimensional Brans—Dicke Star Model with Exotic Matter and Dark Energy

A two-dimensional Brans-Dicke star model with exotic matter and dark energy is studied in this paper,the field equation and balance equation are derived at finite temperature,the analytic solutions of these equations canbe used to calculate the mass of star.In addition,we find that star's mass has a minimum when matter state parameterγ→0.

Crossing w=1 in Gauss-Bonnet Brane World with Induced Gravity

Recent type Ia supernovas data seemingly favor a dark energy model whose equation of state w（z） crosses -1 very recently, which is a much more amazing problem than the acceleration of the universe. In this paper we show that it is possible to realize such a crossing without introducing any phantom component in a Gauss-Bonnet brane worm with induced gravity, where a four-dimensional curvature scalar on the brahe and a five-dimensional Gauss-Bonnet term in the bu/k are present. In this realization, the Gauss-Bonnet term and the mass parameter in the bulk play a crucial role.

Unification of Dark Matter and Dark Energy in a Modified Entropic Force Model

In Verlinde＇s entropic force scenario of gravity, Newton＇s laws and Einstein equations can be obtained from the first principles and general assumptions. However, the equipartition law of energy is invalid at very low temperatures. We show clearly that the threshold of the equipartition law of energy is related with horizon of the universe. Thus, a one-dimensional Debye （ODD） model in the direction of radius of the modified entropic force （MEF） may be suitable in description of the accelerated expanding universe. We present a Friedmann cosmic dynamical model in the ODD-MEF framework. We examine carefully constraints on the ODD-MEF model from the Union2 compilation of the Supernova Cosmology Project （SCP） collaboration, the data from the observation of the large-scale structure （LSS） and the cosmic microwave background （CMB）, i.e. SNe Ia＋LSS＋CMB. The combined numerical analysis gives the best-fit value of the model parameters （ -10^-9 and Ωm0= 0.224, with 2 = Xmin= 591.156. The corresponding age of the universe agrees with the result of D. Spergel et al. [J.M. Bardeen, B. Carter, and S.W. Hawking, Commun. Math. Phys. 31 （1973） 161] at 95% confidence level. The numerical result also yields an accelerated expanding universe without invoking any kind of dark energy. Taking ζ（= 2πωD/ H0 ） as a running parameter associated with the structure scale r, we obtain a possible unified scenario of the asymptotic flatness of the radial velocity dispersion of spiral galaxies, the accelerated expanding universe and the Pioneer 10/11 anomaly in the entropic force framework of Verlinde.

Friedmann Cosmology with Bulk Viscosity:A Concrete Model for Dark Energy

The universe content is considered as a non-perfect fluid with bulk viscosity and can be described by a general equation of state （endowed some deviation from the conventionally assumed cosmic perfect fluid model）. An explicitly bulk viscosity dark energy model is proposed to confront consistently with the current observational data sets by statistical analysis and is shown consistent with （not deviated away much from） the concordant A Cold Dark Matter （CDM） model by comparing the decelerating parameter. Also we compare our relatively simple viscosity dark energy model with a more complicated one by contrast with the concordant ACDM model and find our model improves for the viscosity dark energy model building. Finally we discuss the perspectives of dark energy probes for the coming years with observations.

Holographic Gas as Dark Energy

We investigate the statistical nature of holographic gas, which may represent the quasi-particle excitations of a strongly correlated gravitational system. We find that the holographic entropy can be obtained by modifying degeneracy. We calculate thermodynamical quantities and investigate stability of the holographic gas. When applying to cosmology, we find that the holographic gas behaves as holographic dark energy, and the parameter c in holographic dark energy can be calculated from our model. Our model of holographic gas generally predicts c 〈 1, implying that the fate of our universe is phantom-like.

Gauge Model with massive Gravitons

Gauge theory of gravity is formulated based on principle of local gauge invariance. Because the model hasstrict local gravitational gauge symmetry, and gauge theory of gravity is a perturbatively renormalizable quantum model.However, in the original model, all gauge gravitons are massless. We want to ask whether there exist massive gravitonsin Nature. In this paper, we will propose a gauge model with massive gravitons. The mass term of gravitational gaugefield is introduced into the theory without violating the strict local gravitational gauge symmetry. Massive gravitons canbe considered to be possible origin of dark energy and dark matter in the Universe.

Reconstructing Quintom from WMAP 5-year Observations:Generalized Ghost Condensate

In the 5-year WMAP data analysis, a new parametrization form for dark energy equation-of-state was used, and it has been shown that the equation-of-state, w（z）, crosses the cosmological-constant boundary w = -1. Based on this observation, in this paper, we investigate the reconstruction of quintom dark energy model. As a single-real-sealarfield model of dark energy, the generalized ghost condensate model provides us with a successful mechanism for realizing the quintom-like behavior. Therefore, we reconstruct this scalar-field quintom dark energy model from the WMAP 5-year observational results. As a comparison, we also discuss the quintom reconstruction based on other specific dark energy ansatzs, such as the CPL parametrization and the holographic dark energy scenarios.

Revisiting the interacting model of new agegraphic dark energy

In this paper, a new version of the interacting model of new agegraphic dark energy （INADE） is proposed and analyzed in detail. The interaction between dark energy and dark matter is reconsidered. The interaction term Q = bh0ραdcρ1-α dm is adopted, which abandons the Hubble expansion rate H and involves both Pae and Pare. Moreover, the new initial condition for the agegraphie dark energy is used, which solves the problem of accommodating baryon matter and radiation in the model. The solution of the model can be given using an iterative algorithm. A concrete example for the calculation of the model is given. Furthermore, the model is constrained by using the combined Planck data （Planck＋BAO＋SNIa＋H0） and the combined WMAP-9 data （WMAP＋BAO＋SNIa＋H0）. Three typical cases are considered： （A） Q = bHoPde, （B） Q = bHo√ρdeρdm,and （C） Q = bHopdm, which correspond to α = 1, 1/2, and 0, respectively. The departures of the models from the ACDM model are measured by the ABIC and AAIC values. It is shown that the INADE model is better than the NADE model in the fit, and the 1NADE（A） model is the best in fitting data among the three cases.

Reconstruction of Scalar Field Dark Energy Models in Kaluza-Klein Universe

This paper is devoted to study the modified holographic dark energy model by taking its different aspects in the t/at Kaiuza-Klein universe. We construct the equation of state parameter which evolutes the universe from quintessence region towards the vacuum. It is found that the modified holographic model exhibits instability against small perturbations in the early epoch of the universe but becomes stable in the later times. We also develop its correspondence with some scalar field dark energy models. It is interesting to mention here that ail the results are consistent with the present observations.

Dark energy and fate of the Universe

We explore the ultimate fate of the Universe by using a divergence-free parametrization for dark energy w（z）=w0＋wa, [In（2 ＋ z） / （1 ＋ z） - In 2] . Unlike the Chevallier-Polarski-Linder parametrization, this parametrization has well behaved, bounded behavior for both high redshifts and negative redshifts, and thus can genuinely cover many theoretical dark energy models. Alter constraining the parameter space of this parametrization by using the current cosmological observations, we find that, at the 95.4% confidence level, our Universe can still exist at least 16.7 Gyr before it ends in a big rip. Moreover, for the phantom energy dominated Universe, we find that a gravitationally bound system will be destroyed at a time t = P√2｜ 1 ＋ 3w（ 1）] / [6π] 1 ＋ w（-1）｜], where P is the period of a circular orbit around this system, before the big rip.

Constraining the lattice fluid dark energy from SNe Ia,BAO and OHD

Sanchez and Lacombe have developed a lattice fluid theory based on a well-defined statistical mechanical model.Taking the lattice fluid as a candidate of dark energy,we investigate the cosmic evolution of this fluid.Using the combined observational data of Type Ia Supernova(SNe Ia) Union2.1,Baryon Acoustic Oscillations(BAO) data from 6dFGS,SDSS and WiggleZ survey,and Observational Hubble Data(OHD),we find the best fit value of the parameters:A = 0.3 ± 0.2(1σ) ± 0.4(2σ) ± 0.8(3σ) and Ω m = 0.31 ± 0.02(1σ) ± 0.05(2σ) ± 0.07(3σ).The cosmological implications of the model are presented.

Discriminate spatial Ricci scalar dark energy from ΛCDM

We apply two geometrical diagnostics, the statefinder {s, r} and Om（x）, to discriminate the Spatial Ricci scalar dark energy model from the ACDM model. We plot the evolution trajectories of those models in the statefinder plane and Om（x） plane. We show that the Spatial Ricci scalar dark energy model can be distinguished from the ACDM model at 68.3% confidence level for z ≤ 1.

Agegraphic Dark Energy with the Sign-Changeable Interaction in Non-Flat Universe

In this paper,we investigate the agegraphic dark energy(ADE) model by including the sign-changeable interaction between ADE and dark matter in non-flat universe.The interaction Q can change its sign from Q < 0 to Q > 0 as the universe expands.This indicates that at first dark matter decays to ADE,and then ADE decays to dark matter.We study the dynamical behavior of the model by using the phase-plane analysis.It is shown numerically that the coupling constant β plays an important role in the evolution of the universe.The equation of state(Eo S) of ADE with the sign-changeable interaction is more likely to cross the phantom divide w_d =-1 from top to bottom with the increasing of the |β|.Whereas in ADE model with usual interaction,wd can cross the phantom divide from bottom to top.We also find that our model is consistent with the observational data.

Interacting model of new agegraphic dark energy:observational constraints and age problem

Many dark energy models fail to pass the cosmic age test because of the old quasar APM 08279+5255 at redshift z=3.91,the ΛCDMmodel and holographic dark energy models being no exception.Inthis paper,we focus onthe topic of the age problem inthe new agegraphic dark energy(NADE)model.We determine the age of the universe in the NADE model by fitting the observational data,including type Ia supernovae(SNIa),baryon acoustic oscillations(BAO)and the cosmic microwave background(CMB).We find that the NADE model also faces the challenge of the age problem caused by the old quasar APM 08279+5255.In order to overcome such a difficulty,we consider the possible interaction between dark energy and dark matter.We show that this quasar can be successfully accommodated in the interacting new agegraphic dark energy(INADE)model at the 2σlevel under the current observational constraints.

Correspondence between Yang–Mills Condensate Dark Energy with Various Kinds of Scalar Field Models of Dark Energy

In this work, we study a new kind of dark energy （DE）, which is named as ＂Yang-Mills condensate＂ （YMC）. We study the stability and wde - w＇de analysis of YMC DE model. Then we correspond it with quintessence, k- essence, tachyon, phantom, dilaton, DBI-essence and hessenee scalar field models of DE in FRW spacetime to reconstruct potentials as well as the dynamics for these scalar fields for describing the acceleration of the universe. We also analyze the models in graphically to interpret the nature of the scalar fields and corresponding potentials.

Reconstruction of f(R) Gravity with Ordinary and Entropy-Corrected(m,n)-Type Holographic Dark Energy Model

In this assignment we will present a reconstruction scheme between f(R) gravity with ordinary and entropy corrected(m, n)-type holographic dark energy. The correspondence is established and expressions for the reconstructed f(R) models are determined. To study the evolution of the reconstructed models plots are generated. The stability of the calculated models are also investigated using the squared speed of sound in the background of the reconstructed gravities.

Redshift drift constraints on holographic dark energy

The Sandage-Loeb （SL） test is a promising method for probing dark energy because it measures the redshift drift in the spectra of Lyman-o： forest of distant quasars, covering the ＂redshift desert＂ of 2 ≤ z ≤ 5, which is not covered by existing cosmological observations. Therefore, it could provide an important supplement to current cosmological observations. In this paper, we explore the impact of SL test on the precision of cosmological constraints for two typical holographic dark energy models, i.e., the original holographic dark energy （HDE） model and the Ricci holographic dark energy （RDE） model. To avoid data inconsistency, we use the best-fit models based on current combined observational data as the fiducial models to simulate 30 mock SL test data. The results show that SL test can effectively break the existing strong degeneracy between the present-day matter density Ωm0 and the Hubble constant 1-1o in other cosmological observations. For the considered two typical dark energy models, not only can a 30-year observation of SL test improve the constraint precision of Ωm0 and h dramatically, but can also enhance the constraint precision of the model parameters c and α significantly.

Analysis of Generalized Ghost Dark Energy in LQC and Galileon Gravity

A so-called ghost dark energy was recently proposed to explain the present acceleration of the universe.The energy density of ghost dark energy,which originates from Veneziano ghost of Quantum Chromodynamics(QCD),in a time dependent background,can be written in the form,ρD=αH + βH^2 where H is the Hubble parameter.We investigate the generalized ghost dark energy(GGDE) model in the setup of loop quantum Cosmology(LQC) and Galileon Cosmology.We study the cosmological implications of the models.We also obtain the equation of state and the deceleration parameters and differential equations governing the evolution of this dark energy model for LQC and Galileon Cosmology.

Effects of Anisotropy on Scalar Field Ghost Dark Energy and the Non-Equilibrium Thermodynamics in Fractal Cosmology

Since the fractal cosmology has been created in early universe, therefore their models were mostly isotropic. The majority of previous studies had been based on FRW universe, while in the early universe, the best model for describing fractal cosmology is actually the anisotropic universe. Therefore in this work, by assuming the anisotropic universe, the cosmological implications of ghost and generalized ghost dark energy models with dark matter in fractal cosmology has been discussed. Moreover, the different kinds of dark energy models such as quintessence and tachyon field, with the generalized ghost dark energy in fractal universe has been investigated. In addition, we have reconstructed the Hubble parameter, H, the energy density, p, the deceleration parameter, q, the equations of state parameter, wD, for both ghost and generalized ghost dark energy models. This correspondence allows us to reconstruct the potential and the dynamics of a fractal canonical scalar field according to the evolution of generalized ghost dark energy density. Eventually, thermodynamics of the cosmological apparent horizon in fractal cosmology was investigated and the validity of the Generalized second law of thermodynamics （GSLT） have been examined in an anisotropic universe. The results show the influence of the anisotropy on the OSLT of thermodynamics in a fractal cosmology.