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.     

Quintom cosmologies admitting either tracking or phantom attractors

In this Letter we investigate the evolution of a class of cosmologies fueled by quintom dark energy and dark matter. Quintom dark energy is a hybrid of quintessence and phantom which involves the participation of two real scalar fields playing the roles of those two types of dark energy. In that framework we examine, from a dynamical systems perspective, the possibility that those fields are coupled among them by considering an exponential potential with an interesting functional dependence similar but not identical to others studied before. The model we consider represents a counterexample to the typical behavior of quintom models with exponential potentials because it admits either tracking attractors (w=0$w=0$), or phantom attractors (w<−1$w<-1$).     

On phantom thermodynamics

The thermodynamic properties of dark energy fluids described by an equation of state parameter ω=p/ρ$\omega =p/\rho$ are rediscussed in the context of FRW type geometries. Contrarily to previous claims, it is argued here that the phantom regime ω<−1$\omega <-1$ is not physically possible since that both the temperature and the entropy of every physical fluids must be always positive definite. This means that one cannot appeal to negative temperature in order to save the phantom dark energy hypothesis as has been recently done in the literature. Such a result remains true as long as the chemical potential is zero. However, if the phantom fluid is endowed with a non-null chemical potential, the phantom field hypothesis becomes thermodynamically consistent, that is, there are macroscopic equilibrium states with T>0$T>0$ and S>0$S>0$ in the course of the Universe expansion.     

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.     

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${\omega }_D<-1$.     

Phantom and non-phantom dark energy: The cosmological relevance of non-locally corrected gravity

In this Letter we have investigated the cosmological dynamics of non-locally corrected gravity involving a function of the inverse d'Alembertian of the Ricci scalar, f(^□−1R)$f({\square }^{\mathrm{-1}}R)$. Casting the dynamical equations into local form, we derive the fixed points of the dynamics and demonstrate the existence and stability of a one parameter family of dark energy solutions for a simple choice, f(^□−1R)∼exp(α^□−1R)$f({\square }^{\mathrm{-1}}R)\sim \exp (\alpha {\square }^{\mathrm{-1}}R)$. The effective EoS parameter is given by, weff=(α−1)/(3α−1)$w_{\mathrm{eff}}=(\alpha -1)/(3\alpha -1)$ and the stability of the solutions is guaranteed provided that 1/3<α<2/3$1/3<\alpha <2/3$. For 1/3<α<1/2$1/3<\alpha <1/2$ and 1/2<α<2/3$1/2<\alpha <2/3$, the underlying system exhibits phantom and non-phantom behavior respectively; the de Sitter solution corresponds to α=1/2$\alpha =1/2$. For a wide range of initial conditions, the system mimics dust like behavior before reaching the stable fixed point. The late time phantom phase is achieved without involving negative kinetic energy fields. A brief discussion on the entropy of de Sitter space in non-local model is included.     

On phantom thermodynamics with negative temperature

We discuss the thermodynamic properties of the Friedmann–Robertson–Walker universe with dark energy fluids labelled by ω=p/ρ<−1/3$\omega =p/\rho <-1/3$. Using the integrability condition, we show that the phantom phase of ω<−1$\omega <-1$ can still be thermodynamically allowed even when the temperature takes on negative values because in that case, there exists at least a condition of keeping physical values for p and ρ.     

Generalized Chaplygin gas model: Constraints from Hubble parameter versus redshift data

We examine observational constraints on the generalized Chaplygin gas (GCG) model for dark energy from the 9 Hubble parameter data points, the 115 SNLS Sne Ia data and the size of baryonic acoustic oscillation peak at redshift, z=0.35$z=0.35$. At a 95.4% confidence level, a combination of three data sets gives 0.67?As?0.83$0.67?A_s?0.83$ and −0.21?α?0.42$-0.21?\alpha ?0.42$, which is within the allowed parameters ranges of the GCG as a candidate of the unified dark matter and dark energy. It is found that the standard Chaplygin gas model (α=1$\alpha =1$) is ruled out by these data at the 99.7% confidence level.     

Black hole and holographic dark energy

We discuss the connection between black hole and holographic dark energy. We examine the issue of the equation of state (EOS) for holographic energy density as a candidate for the dark energy carefully. This is closely related to the EOS for black hole, because the holographic dark energy comes from the black hole energy density. In order to derive the EOS of a black hole, we may use its dual (quantum) systems. Finally, a regular black hole without the singularity is introduced to describe an accelerating universe inside the cosmological horizon. Inspired by this, we show that the holographic energy density with the cosmological horizon as the IR cutoff leads to the dark energy-dominated universe with ωΛ=−1${\omega }_{\Lambda }=-1$.     

Brane tachyon dynamics

The dynamics of a tachyon attached to a Dvali, Gabadadze and Porrati (DGP) brane is investigated. Exponential potential and inverse power law potential are explored, respectively. The quasi-attractor behavior, for which the universe will eventually go into a phase similar to the slow-roll inflation, is discovered in both cases of exponential potential and inverse power law potential. The equation of state (EOS) of the virtual dark energy for a single scalar can cross the phantom divide in the branch θ=−1$\theta =-1$ for both potentials, while the EOS of the virtual dark energy for a single scalar cannot cross this divide in the branch θ=1$\theta =1$.     

Rotating Einstein–Maxwell–dilaton black holes in D dimensions

We construct exact charged rotating black holes in Einstein–Maxwell–dilaton theory in D   spacetime dimensions, D?5$D?5$, by embedding the D  -dimensional Myers–Perry solutions in D+1$D+1$ dimensions, and performing a boost with a subsequent Kaluza–Klein reduction. Like the Myers–Perry solutions, these black holes generically possess N=[(D−1)/2]$N=[(D-1)/2]$ independent angular momenta. We present the global and horizon properties of these black holes, and discuss their domains of existence.     

Wormholes with a barotropic equation of state admitting a one-parameter group of conformal motions

The theoretical construction of a traversable wormhole proposed by Morris and Thorne maintains complete control over the geometry by assigning both the shape and redshift functions, thereby leaving open the determination of the stress–energy tensor. This paper examines the effect of introducing the linear barotropic equation of state p_r=ωρ$p_r=\omega \rho$ on the theoretical construction. If either the energy density or the closely related shape function is known, then the Einstein field equations do not ordinarily yield a finite redshift function. If, however, the wormhole admits a one-parameter group of conformal motions, then both the redshift and shape functions exist provided that −3<ω<−1$-3<\omega <-1$. In a cosmological setting, the equation of state p=ωρ$p=\omega \rho$, ω<−1$\omega <-1$, is associated with phantom dark energy, which is known to support traversable wormholes. The restriction −3<ω<−1$-3<\omega <-1$ that arises in the present wormhole setting can be attributed to the assumption of conformal symmetry.     

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$w<-1$.     

Interacting holographic dark energy in Brans–Dicke theory

We study cosmological application of interacting holographic energy density in the framework of Brans–Dicke cosmology. We obtain the equation of state and the deceleration parameter of the holographic dark energy in a non-flat universe. As system's IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as L=ar(t)$L=ar(t)$. We find that the combination of Brans–Dicke field and holographic dark energy can accommodate wD=−1$w_D=-1$ crossing for the equation of state of noninteracting   holographic dark energy. When an interaction between dark energy and dark matter is taken into account, the transition of wD$w_D$ to phantom regime can be more easily accounted for than when resort to the Einstein field equations is made.     

Modified holographic dark energy in DGP brane world

In this Letter, the cosmological dynamics of a modified holographic dark energy which is derived from the UV/IR duality by considering the black hole mass in higher dimensions as UV cutoff, is investigated in Dvali–Gabadadze–Porrati (DGP) brane world model. We choose Hubble horizon and future event horizon as IR cutoff respectively. And the two branches of the DGP model are both taken into account. When Hubble horizon is considered as IR cutoff, the modified holographic dark energy (HDE) behaves like an effect dark energy that modification of gravity in pure DGP brane world model acts and it can drive the expansion of the universe speed up at late time in ?=−1$?=-1$ branch which in pure DGP model cannot undergo an accelerating phase. When future event horizon acts as IR cutoff, the equation of state parameter of the modified HDE can cross the phantom divide.     

A parametric model for dark energy

Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in cosmology but also in high energy physics theories. Following this approach, we explore in this Letter some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift z   over the entire cosmological evolution, i.e., z∈[−1,∞)$z\in [-1,\infty )$. This parameterization allows us to divide the parametric plane (w₀,w₁)$(w_0,w_1)$ in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. By assuming a flat universe, a statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution H(z)$H(z)$ is performed to check the observational viability of the EoS parameterization here proposed.     

Route to Lambda in conformally coupled phantom cosmology

In this Letter we investigate acceleration in the flat cosmological model with a conformally coupled phantom field and we show that acceleration is its generic feature. We reduce the dynamics of the model to a 3-dimensional dynamical system and analyze it on a invariant 2-dimensional submanifold. Then the concordance FRW model with the cosmological constant Λ   is a global attractor situated on a 2-dimensional invariant space. We also study the behaviour near this attractor, which can be approximated by the dynamics of the linearized part of the system. We demonstrate that trajectories of the conformally coupled phantom scalar field with a simple quadratic potential crosses the cosmological constant barrier infinitely many times in the phase space. The universal behaviour of the scalar field and its potential is also calculated. We conclude that the phantom scalar field conformally coupled to gravity gives a natural dynamical mechanism of concentration of the equation of state coefficient around the magical value weff=−1$w_{\mathrm{eff}}=-1$. We demonstrate route to Lambda through the infinite times crossing the weff=−1$w_{\mathrm{eff}}=-1$ phantom divide.     

Brane world cosmology in the sourced-Taub background

A new brane world in the sourced-Taub background is proposed. The gravity field equations in the internal source region and external vacuum region are investigated, respectively. We find that the equation of state for the effective dark energy of a dust brane in the source region can cross the phantom divide w=−1$w=-1$. Furthermore, there is a drop on H(z)$H(z)$ diagram, which presents a possible mechanism for the recent direct data of H(z)$H(z)$.