Constraints on the interacting holographic dark energy model

We examined the interacting holographic dark energy model in a universe with spatial curvature. Using the near-flatness condition and requiring that the universe is experiencing an accelerated expansion, we have constrained the parameter space of the model and found that the model can accommodate a transition of the dark energy from ωD>−1${\omega }_D>-1$ to ωD<−1${\omega }_D<-1$.     

Signature of the interaction between dark energy and dark matter in galaxy clusters

We investigate the influence of an interaction between dark energy and dark matter upon the dynamics of galaxy clusters. We obtain the general Layser–Irvine equation in the presence of interactions, and find how, in that case, the virial theorem stands corrected. Using optical, X-ray and weak lensing data from 33 relaxed galaxy clusters, we put constraints on the strength of the coupling between the dark sectors. Available data suggests that this coupling is small but positive, indicating that dark energy might be decaying into dark matter. Systematic effects between the several mass estimates, however, should be better known, before definitive conclusions on the magnitude and significance of this coupling could be established.     

Stability of the curvature perturbation in dark sectors' mutual interacting models

We consider perturbations in a cosmological model with a small coupling between dark energy and dark matter. We prove that the stability of the curvature perturbation depends on the type of coupling between dark sectors. When the dark energy is of quintessence type, if the coupling is proportional to the dark matter energy density, it will drive the instability in the curvature perturbations; however if the coupling is proportional to the energy density of dark energy, there is room for the stability in the curvature perturbations. When the dark energy is of phantom type, the perturbations are always stable, no matter whether the coupling is proportional to the one or the other energy density.     

Cosmological model with energy transfer

The observations of SNIa suggest that we live in the acceleration epoch when the densities of the cosmological constant term and matter are almost equal. This leads to the cosmic coincidence conundrum. As the explanation for this problem we propose the FRW model with dark matter and dark energy which interact each other exchanging energy. We show that the cubic correction to the Hubble law, measured by distant supernovae type Ia, probes this interaction. We demonstrate that influences between nonrelativistic matter and vacuum sectors are controlled by third and higher derivatives of the scale factor. As an example we consider flat decaying Λ(t)$\Lambda (t)$ FRW cosmologies. We point out the possibility of measure of the energy transfer by the cubic and higher corrections to Hubble's law. The statistical analysis of SNIa data is used as an evidence of energy transfer. We find that there were the transfer from the dark energy sector to the dark matter one without any assumption about physics governing this process. We confront this hypothesis about the transfer with SNIa observations and find that the transfer the phantom and matter sector is admissible for Ω_m,0=0.27${\Omega }_{\mathrm{m},0}=0.27$. We also demonstrate that it is possible to differentiate between the energy transfer model and the variable coefficient equation of state model.     

A note on crossing the phantom divide in hybrid dark energy model

Recently a lot of attention has been given to building dark energy models in which the equation-of-state parameter w   can cross the phantom divide w=−1$w=-1$. However, to our knowledge, these models with crossing the phantom divide only provide the possibility that w can cross −1. They do not answer another question: why crossing phantom divide occurs recently? Since in many existing models whose equation-of-state parameter can cross the phantom divide, w undulates around −1 randomly, why are we living in an epoch  w<−1$w<-1$? This can be regarded as the second cosmological coincidence problem. In this Letter, we propose a possible approach to alleviate this problem within a hybrid dark energy model.     

Holographic Ricci dark energy in Randall–Sundrum braneworld: Avoidance of big rip and steady state future

In the holographic Ricci dark energy (RDE) model, the parameter α plays an important role in determining the evolutionary behavior of the dark energy. When α<1/2, the RDE will exhibit a quintom feature, i.e., the equation of state of dark energy will evolve across the cosmological constant boundary w=−1. Observations show that the parameter α is indeed smaller than 1/2, so the late-time evolution of RDE will be really like a phantom energy. Therefore, it seems that the big rip is inevitable in this model. On the other hand, the big rip is actually inconsistent with the theoretical framework of the holographic model of dark energy. To avoid the big rip, we appeal to the extra dimension physics. In this Letter, we investigate the cosmological evolution of the RDE in the braneworld cosmology. It is of interest to find that for the far future evolution of RDE in a Randall–Sundrum braneworld, there is an attractor solution where the steady state (de Sitter) finale occurs, in stead of the big rip.     

Bianchi Type-IX String Cosmological Models for Perfect Fluid Distribution in General Relativity

The present study deals with Bianchi type-IX string cosmological models for perfect fluid distribution. We consider two cases： （i） ρ ＋ λ = 0, （ii） ρ - λ = 0, where ρ and λ are the rest energy density and the tension density of a string cloud, respectively. The physical and geometrical properties of the models are discussed.     

Bulk viscous L.R.S. Bianchi type V tilted stiff fluid cosmological model in general relativity

Locally rotationally symmetric (L.R.S.) Bianchi type V bulk viscous tilted stiff fluid cosmological model is investigated. To get the deterministic model of the universe, we have also assumed a condition A=Bⁿ between metric potentials A, B where n is the constant. The behaviour of the model in presence and absence of bulk viscosity and singularities in the model are also discussed. In general, the models represent accelerating, shearing, tilted and non-rotating universe. The models have point type singularity in presence and absence of bulk viscosity both.     

Generalized holographic dark energy model

In this paper, the model of the holographic Chaplygin gas has been extended to two general cases: first the case of a modified variable Chaplygin gas and second the case of the viscous generalized Chaplygin gas. The dynamics of the model is expressed by the use of scalar fields and scalar potentials.     

A new model of agegraphic dark energy

In this note, we propose a new model of agegraphic dark energy based on the Károlyházy relation, where the time scale is chosen to be the conformal time η   of the Friedmann–Robertson–Walker (FRW) universe. We find that in the radiation-dominated epoch, the equation-of-state parameter of the new agegraphic dark energy wq=−1/3$w_q=-1/3$ whereas Ωq=n²a²${\Omega }_q=n^2{a}^2$; in the matter-dominated epoch, wq=−2/3$w_q=-2/3$ whereas Ωq=n²a²/4${\Omega }_q=n^2{a}^2/4$; eventually, the new agegraphic dark energy dominates; in the late time wq→−1$w_q\to -1$ when a→∞$a\to \infty$, and the new agegraphic dark energy mimics a cosmological constant. In every stage, all things are consistent. The confusion in the original agegraphic dark energy model proposed in [R.G. Cai, Phys. Lett. B 657 (2007) 228, arXiv: 0707.4049 [hep-th]] disappears in this new model. Furthermore, Ωq?1${\Omega }_q?1$ is naturally satisfied in both radiation-dominated and matter-dominated epochs where a?1$a?1$. In addition, we further extend the new agegraphic dark energy model by including the interaction between the new agegraphic dark energy and background matter. In this case, we find that wq$w_q$ can cross the phantom divide.     

Bianchi Type-Ⅲ String Cosmological Models with Time Dependent Bulk Viscosity

Bianchi type-Ⅲ string cosmological models with bulk viscous fluid for massive string are investigated. To obtain the determinate model of the universe, we assume that the coeffcient of bulk viscosity ξ is inversely proportional to the expansion θ in the model and expansion θ in the model is proportional to the shear g. This leads to B =lC^n, where l and n are constants. Behaviour of the model in the presence and absence of bulk viscosity is discussed. The physical implications of the models are also discussed in detail.     

Bianchi Type-V Bulk Viscous Barotropic Fluid Cosmological Model with Variable G and Λ

We investigate the Bianchi type-V bulk viscous barotropic fluid cosmological model with variable gravitational constant G and the cosmological constant A, assuming the condition on metric A/A=B/B=C/C=m/tn potential aswhere A, B, and C are functions of time t, while m and n are constants. To obtain the deterministic mo del, we also assume the relations P = p - 3η H, p = 7P, η = ηop^s, where p is the isotropic pressure, η the bulk viscosity,0≤r≤1 H the Hubble constant, ηo and s are constants. Various physical aspects of the model are discussed.The case of n = 1 is also discussed to compare the results with the actual universe.     

Avoiding the big-rip jeopardy in a quintom dark energy model with higher derivatives

In the framework of a single scalar field quintom model with higher derivative, we construct in this Letter a dark energy model of which the equation of state (EOS) w   crosses over the cosmological constant boundary. Interestingly during the evolution of the universe w<−1$w<-1$ happens just for a period of time with a distinguished feature that w   starts with a value above −1, transits into w<−1$w<-1$, then comes back to w>−1$w>-1$. This avoids the big-rip jeopardy induced by w<−1$w<-1$.     

Constraints on Deceleration Parameter of a 5D Bounce Cosmological Model from Recent Cosmic Observations

We study the constraint on deceleration parameter q from the recent SNela Gold dataset and observational Hubble data by using a model-independent deceleration parameter q（z） = 1/2 - a/（1 ＋ z）^b under the flve-dimensional bounce cosmological model. For the cases of SNeIa Gold dataset, Hubble data, and their combination, the present results show that the constraints on transition redshift ZT are 0.35-0.07^＋0.14,0.68-0.58^＋1.47,and 0.55-0.09^＋0.18 with 1σ errors,respectively.     

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.     

The variation of the electromagnetic coupling and quintessence

The properties of quintessence are examined through the study of the variation of the electromagnetic coupling. We consider two simple quintessence models with a modified exponential potential and study the parameter space constraints derived from the existing observational bounds on the variation of the fine structure constant and the most recent Wilkinson Microwave Anisotropy Probe observations.