第五讲暗能量和德西特时空

最近的天文观测表明，宇宙是在加速膨胀，而不是原来认为的减速膨胀．为解释加速膨胀，必须在宇宙的物质能量中引入暗能量这一成分，文章讨论了暗能量的可能侯选者，特别强调了宇宙常数问题、德西特时空问题以及和德西特时空相关的一些基本问题．     

Letter: Inflationary Dilatonic de Sitter Universe from Super Yang-Mills Theory Perturbed by Scalars and Spinors

In this paper a quantum N = 4 super Yang-Mills theory perturbed by dilaton-coupled scalars and spinor fields, is considered. The induced effective action for such a theory is calculated on a dilaton-gravitational background using the conformal anomaly found via the AdS/CFT correspondence. Considering such an effective action (using the large N method) as a quantum correction to the classical gravity action with cosmological constant, we study the effect from the dilaton on the scale factor (this corresponds to an inflationary universe without dilaton). It is shown that, depending on the initial conditions for the dilaton, the dilaton may slow down, or accelerate, the inflation process. At late times, the dilaton is decaying exponentially. Different possible cases corresponding to a dilatonic dS Universe are analyzed with respect to the equations of motion.     

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 observationaldata sets by statistical analysis and is shown consistent with (not deviated away much from) the concordant Λ 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 ΛCDM modeland 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.     

Singularities and Entropy in Bulk Viscosity Dark Energy Model

In this paper bulk viscosity is introduced to describe the effects of cosmic non-perfect fluid on the cosmos evolution and to build the unified dark energy (DE) with (dark) matter models. Also we derive a general relation between the bulk viscosity form and Hubble parameter that can provide a procedure for the viscosity DE model building. Especially, a redshift dependent viscosity parameterζ∝λ₀ +λ₁(1+z)ⁿ proposed in the previous work [X.H. Meng and X. Dou, Commun. Theor. Phys. 52 (2009) 377] is investigated extensively in this present work. Further more we use the recently released supernova dataset
(the Constitution dataset) to constrain the model parameters. In order to differentiate the proposed concrete dark energy models from the well known $\Lambda$CDM model, statefinder diagnostic method is applied to this bulk viscosity model, as a complementary to the Om parameter diagnostic and the deceleration parameter analysis performed by us before. The DE model evolution behavior and tendency are shown in the plane of the statefinder diagnostic parameter pair {r,s} as axes where the fixed point represents theΛCDM model. The possible singularity property in this bulk viscosity
cosmology is also discussed to which we can conclude that in the different parameter regions chosen properly, this concrete viscosity DE model can have various late evolution behaviors and the late time singularity could be avoided. We also calculate the cosmic entropy in the bulk viscosity dark energy frame, and find that the total entropy in the viscosity DE model increases monotonously with respect to the scale factor evolution, thus this monotonous
increasing property can indicate an arrow of time in the universe evolution, though the quantum version of the arrow of time is still very puzzling.     

Dark Energy Model with Non-Minimal Coupling and CosmologicalConstant Boundary

In this paper, we study a kind of dark energy models in the framework of the non-minimal coupling. With this kind of models, dark energy could cross the cosmological constant boundary, and at early time, dark energy could have ``tracking" behavior.     

Exponential Potentials and Attractor Solution of Dilatonic Cosmology

We present the scalar-tensor gravitational theory with an exponential potential in which Pauli metric is regarded as the physical space-time metric. We show that it is essentially equivalent to coupled quintessence (CQ) model. However for baryotropic fluid being radiation there are in fact no coupling between dilatonic scalar field and radiation. We present the critical points for baryotropic fluid and investigate the properties of critical points when the baryotropic matter is specified to ordinary matter. It is possible for all the critical points to be attractors as long as the parameters λ and β satisfy certain conditions. To demonstrate the attractor behaviors of these critical points, We numerically plot the phase plane for each critical point. Finally with the bound on β from the observation and the fact that our universe is undergoing an accelerating expansion, we conclude that present accelerating expansion is not the eventual stage of universe. Moreover, we numerically describe the evolution of the density parameters Ω and the decelerating factor q, and computer the present values of some cosmological parameters, which are consistent with current observational data.     

理解暗能量和暗物质之间的相互作用

从热力学角度研究了暗能量和暗物质之间的相互作用. 假设相互作用是平衡态上的涨落并考虑此涨落导致的熵的修正, 导出了相互作用的物理表述, 把我们模型和观测结果作了比较.     

Entropy-Corrected Holographic Dark Energy

The holographic dark energy (HDE) is now an interesting candidate of dark energy, which has been studied extensively in the literature. In the derivation of HDE, the black hole entropy plays an important role. In fact, the entropy-area relation can be modified due to loop quantum gravity or other reasons. With the modified entropy-area relation, we propose the so-called ``entropy-corrected holographic dark energy' (ECHDE) in the present work. We consider many aspects of ECHDE and find some interesting results. In addition, we briefly consider the so-called ``entropy-corrected agegraphic dark energy' (ECADE).     

Entropy-Corrected Holographic Dark Energy

The holographic dark energy （HDE） is now an studied extensively in the literature. In the derivation of HDE, interesting candidate of dark energy, which has been the black hole entropy plays an important role. In fact, the entropy-area relation can be modified due to loop quantum gravity or other reasons. With the modified entropyarea relation, we propose the so-called ＂entropy-corrected holographic dark energy＂ （ECHDE） in the present work. We consider many aspects of ECHDE and tlnd some interesting results. In addition, we briefly consider the so-called ＂entropy-eorreeted agegraphic dark energy＂ （ECADE）.     

A Pressure Parametric Dark Energy Model

In this paper, we propose a new pressure parametric model of the total cosmos energy components in a spatially flat Friedmann-Robertson-Walker (FRW) universe and then reconstruct the model into quintessence and phantom scenarios, respectively. By constraining with the datasets of the type Ia supernova (SNe Ia), the baryon acoustic oscillation (BAO) and the observational Hubble parameter data(OHD), we find that Ω_m0=0.270_-0.034^+0.039 at the 1σ level and our universe slightly biases towards quintessence behavior. Then we use two diagnostics including Om(a) diagnostic and statefinder to discriminate our model from the cosmology constant cold dark matter (ΛCDM) model. From Om(a) diagnostic, we find that our model has a relatively large deviation from the ΛCDM model at high redshifts and gradually approaches the ΛCDM model at low redshifts and in the future evolution, but they can be easily differentiated from each other at the 1σ level all along. By the statefinder, we find that both of quintessence case and phantom case can be well distinguished from the ΛCDM model and will gradually deviate from each other. Finally, we discuss the fate of universe evolution (named the rip analysis) for the phantom case of our model and find that the universe will run into a little rip stage.     

Holographic Dark Energy Model is Consistent with Pantheon SN Ia Data

We constrain three cosmological models,i.e.,ACDM model, holographic dark energy(HDE) model and R_h = ct model by using the recent Pantheon compilation of type la supernovae(SN la), the direction measurements of Hubble parameter H(z), and the baryon acoustic oscillations(BAO). The spatial curvature is considered in the ACDM model and the HDE model. We show that the HDE model in a spatially flat and HDE dominate universe has the same behavior as Rh = ct model if the characteristic parameter of the HDE model C_0 approaches to infinity. Numerical results show that the ACDM model is the best favoured one among the three models. The HDE model is consistent with observational data, the best fitting value of C_0 is around 0.8, which implies that the Rh = ct model should be modified to be compatible with the present cosmological observational data. Combing all the datasets, we give strict constraint on the Hubble constant,where h_0=0.694 ± 0.020 for the ACDM model and h_0= 0.689 ±0.019 for the HDE model.Our results imply that the tension of Hubble constant between Planck collaborations and Riess et al. has been partially relaxed. The constraint on the spatial curvature is also given,where Ω_(k0) =-0.066 ± 0.165 for the ACDM model andΩ_(k0)=0.029 ± 0.067 for the HDE model.     

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.     

Constraining a New Type of Dark Energy Model from Present Observations

We propose a new type of dark energy (DE) model, in which the equation of state of DE ω de is a simple function of the fractional energy density de instead of the redshift z. We assume three DE models of this type, and fit them with present observations to get constraints of DE, which are also compared with the CPL model. It is shown that a suitable ω de (Ωde ) model can give smaller χ2 or smaller errors of ω de than that of the CPL model. This new type of DE model can help to study the essential properties and nature of DE.     

带有背景矢量场的标量场暗能量模型

讨论了带有背景矢量场的动力学标量场作为暗能量在闭宇宙的情形下的演化. 选取适当的标量场势函数后, 在得到的暗能量有效状态方程中, 参数w在红移z≈0.2处可以越过－1, 而且在z≈1.7处宇宙从减速膨胀的状态转变为加速膨胀状态, 这与最近的宇宙学观测相符.     

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 can be used to calculate the mass of star. In addition, we find that star＇s mass has a minimum when matter state parameter γ→0.