118431152
Dynamics of generalized tachyon field
We investigate the dynamics of generalized tachyon field in FRW spacetime. We obtain the autonomous dynamical system for the general case. Because the general autonomous dynamical system cannot be solved analytically, we discuss two cases in detail: β=1 and β=2. We find the critical points and study their stability. At these critical points, we also consider the stability of the generalized tachyon field, which is as important as the stability of critical points. The possible final states of the universe are discussed.
119105820
Geometrical diagnostic for purely kinetic k-essence dark energy
Abstract Geometrical diagnostic, involving the statefinder { r , s } and Om ( x ) , is widely used to discriminate different dark energy models. We apply the statefinder { r , s } and Om ( x ) to purely kinetic k-essence dark energy model with Dirac–Born–Infeld-like Lagrangian which can be considered as scalar field realizations of Chaplygin gas. We plot the evolution trajectories of this model in the statefinder parameter-planes and Om ( x ) parameter-plane. We find that the statefinder { r , s } and Om ( x ) fail to distinguish purely kinetic k-essence model from ΛCDM model at 68.3% confidence level for z ≪ 1 .
119037938
New types of f(T) gravity
Recently f(T) theories based on modifications of teleparallel gravity, where torsion is the geometric object describing gravity instead of curvature, have been proposed to explain the present cosmic accelerating expansion. The field equations are always second order, remarkably simpler than f(R) theories. In analogy to the f(R) theory, we consider here three types of f(T) gravity, and find that all of them can give rise to cosmic acceleration with interesting features, respectively.
120520435
Bound on the Equation of State of Dark Energy from the Generalized Second Law of Thermodynamics
Assuming that the equation of state of dark energy is a constant, we obtain the allowed interval of the equation of state of dark energy: wD≥−1, bounded from the generalized second law of thermodynamics, in a universe enveloped by the apparent horizon and containing a Schwarzschild black hole.
121495245
Theoretical Constraint on Purely Kinetic k -Essence
Purely kinetic k-essence models in which the Lagrangian contains only a kinetic factor and does not depend explicitly on the field itself are considered, and a theoretical constraint is obtained: Fx = F0a−3. Under this theoretical constraint, we discuss a kind of purely k-essence with form F(X) = -(1 + 2Xn)1/2n, which can be considered as the generalized tachyon field, and find that this kind of k-essence is not likely a candidate of dark energy to describe the present accelerated expansion of the Universe. This is contrary to a previous suggestion that k-essence with such a form may be used to describe phantom cosmologies.
120945447
Observational Constraints on Purely Kinetic k -Essence Dark Energy Models
We consider constraints on purely kinetic k-essence models from the latest observational data (including 182 gold SNIa data, the shift parameter, and the acoustic scale). We find the best-fit values of the parameters are Ωm = 0.37 ± 0.01 and k0 = 0.064 ± 0.013 at 68.3% confidence level. The phase transition from deceleration to acceleration is found to occur at redshift zq = 0 ~ 0.48−0.52 in 68.3% confidence level limits. By applying model-comparison statistics (F-test, AICc, and BIC), we find that the purely kinetic k-essence scenario is favored over the ?CDM model by the combined data. These results are also confirmed by combined data 307 SNIa + R + la.
119262542
Coexistence and competition of ferromagnetism and p-wave superconductivity in holographic model
By combining a holographic $p$-wave superconductor model and a holographic ferromagnetism model, we study the coexistence and competition of ferromagnetism and $p$-wave superconductivity. It is found that the results depend on the self-interaction of magnetic moment of the complex vector field and which phase appears first. In the case where the ferromagnetic phase appears first, if the interaction is attractive, the system shows the ferromagnetism and superconductivity can coexist at low temperatures. If the interaction is repulsive, the system will only be in a pure ferromagnetic state. In the case where the superconducting phase appears first, the attractive interaction will lead to a magnetic $p$-wave superconducting phase at low temperatures. If the interaction is repulsive, the system will be in a pure $p$-wave superconducting phase or a ferromagnetic phase when the temperature is lowered.
119108999
Holographic model for the paramagnetism/antiferromagnetism phase transition
In this paper we build a holographic model of paramagnetism/antiferromagnetism phase transition, which is realized by introducing two real antisymmetric tensor fields coupling to the background gauge field strength and interacting with each other in a dyonic black brane background. In the case without external magnetic field and in low temperatures, the magnetic moments condense spontaneously in antiparallel manner with the same magnitude and the time reversal symmetry is also broken spontaneously (if boundary spatial dimension is more than 2, spatial rotational symmetry is broken spontaneously as well), which leads to an antiferromagnetic phase. In the case with weak external magnetic field, the magnetic susceptibility density has a peak at the critical temperature and satisfies the Curie-Weiss law in the paramagnetic phase of antiferromagnetism. In the strong external magnetic field case, there is a critical magnetic field $B_c$ in antiferromagnetic phase: when magnetic field reaches $B_c$, the system will return into the paramagnetic phase by a second order phase transition.
119285015
P-V criticality in the extended phase space of Gauss-Bonnet black holes in AdS space
A bstractWe study the P − V criticality and phase transition in the extended phase space of charged Gauss-Bonnet black holes in anti-de Sitter space, where the cosmological constant appears as a dynamical pressure of the system and its conjugate quantity is the thermodynamic volume of the black holes. The black holes can have a Ricci flat (k = 0), spherical (k = 1), or hyperbolic (k = −1) horizon. We find that for the Ricci flat and hyperbolic Gauss-Bonnet black holes, no P − V criticality and phase transition appear, while for the black holes with a spherical horizon, even when the charge of the black hole is absent, the P − V criticality and the small black hole/large black hole phase transition will appear, but it happens only in d = 5 dimensions; when the charge does not vanish, the P − V criticality and the small black hole/large phase transition always appear in d = 5 dimensions; in the case of d ≥ 6, to have the P − V criticality and the small black hole/large black hole phase transition, there exists an upper bound for the parameter $ b=\widetilde{\alpha }{{\left| Q \right|}^{{{-2 \left/ {{\left( {d-3} \right)}} \right.}}}} $, where $ \widetilde{\alpha } $ is the Gauss-Bonnet coefficient and Q is the charge of the black hole. We calculate the critical exponents at the critical point and find that for all cases, they are the same as those in the van der Waals liquid-gas system.