119180210
Restrictions on purely kinetic k-essence
We restrict purely kinetic k-essence. Assuming the equation of state is a power law of the kinetic energy: w = w0X α, to obtain accelerated phases, we must have α > 0 as one of necessary conditions, constrained from the conditions for stability and causality, and the k-essence must behave like phantom. We also study the evolutions of the equation of state and the speed of sound with numerical simulation. PACS: 95.36.+x, 98.80.-k, 98.80.Es
118714602
SPATIAL RICCI SCALAR DARK ENERGY MODEL
Inspired by the holographic principle, we suggest that the density of dark energy is proportional to the spatial Ricci scalar curvature (SRDE). Such a model is phenomenologically viable. The best fit values of its parameters at 68% confidence level are found to be Ωm 0 = 0.259±0.016 and α = 0.261±0.0122, constrained from the Union+CFA3 sample of 397 SNIa and the BAO measurement. We find that the equation of state of SRDE crosses -1 at z ≃ -0.14. The present value of the deceleration parameter q(z) for SRDE is found to be qz = 0~-0.85. The phase transition from deceleration to acceleration of the Universe for SRDE occurs at the redshift zq = 0~0.4. After studying the perturbation of each component of the Universe, we show that the matter power spectra and cosmic microwave background temperature anisotropy are slightly affected by SRDE, compared with ΛCDM.
119246835
Conformal transformation in f(T) theories
It is well known that f(R) theories are dynamically equivalent to a particular class of scalar-tensor theories. In analogy to the f(R) extension of the Einstein-Hilbert action of general relativity, f(T) theories are generalizations of the action of teleparallel gravity. The field equations are always second order, remarkably simpler than f(R) theories. It is interesting to investigate whether f(T) theories have the similar conformal features possessed in f(R) theories. It is shown, however, that f(T) theories are not dynamically equivalent to teleparallel action plus a scalar field via conformal transformation, since there appears an additional scalar-torsion coupling term. We discuss briefly what constraint of this coupling term may be put on f(T) theories from observations of the solar system.
118724058
The evolution of the power law k-essence cosmology
We investigate the evolution of the power law k-essence field in FRWL spacetime. The autonomous dynamical system and critical points are obtained. The corresponding cosmological parameters, such as Ωϕ and wϕ, are calculated at these critical points. We find it is possible to achieve an equation of state crossing through −1 for k-essence field. The results we obtained indicate that the power law k-essence dark energy model can be compatible with observations.
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.
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 .
118361149
Paramagnetism-ferromagnetism phase transition in a dyonic black hole
Coupling an antisymmetric tensor field to the electromagnetic field in a dyonic Reissner-Nordstrom-anti-de Sitter black hole background, we build a holographic model for the paramagnetism/ferromagnetism phase transition. In the case of zero magnetic field, the time reversal symmetry is broken spontaneously and spontaneous magnetization happens at low temperatures. The critical exponents are in agreement with the ones from mean field theory. In the case of nonzero magnetic field, the model realizes the hysteresis loop of a single magnetic domain and the magnetic susceptibility satisfies the Curie-Weiss law.
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.
12639048
Towards complete phase diagrams of a holographic p-wave superconductor model
A bstractWe study in detail the phase structure of a holographic p-wave superconductor model in a five dimensional Einstein-Maxwell-complex vector field theory with a negative cosmological constant. To construct complete phase diagrams of the model, we consider both the soliton and black hole backgrounds. In both two cases, there exist second order, first order and zeroth order phase transitions, and the so-called “retrograde condensation” also happens. In particular, in the soliton case with the mass of the vector field being beyond a certain critical value, we find a series of phase transitions happen such as “insulator/superconductor/insulator/superconductor”, as the chemical potential continuously increases. We construct complete phase diagrams in terms of temperature and chemical potential and find some new phase boundaries.