Modern Physics Letters A

We study a flat three-brane in the presence of a linear k field with nonzero cosmological constant Λ₄. In this model the crossing of the phantom divide (PD) occurs when the k-essence energy density becomes negative. We show that in the high energy regime the effective equation of state has a resemblance of a modified Chaplygin gas while in the low energy regime it becomes linear. We find a scale factor that begins from a singularity and evolves to a de Sitter stable stage while other solutions have a super-accelerated regime and end with a big rip. We use the energy conditions to show when the effective equation of state of the brane-universe crosses the PD.

The importance for cosmology of the recently introduced ELKOs requires our deepest understanding of them and of all of their fundamental properties. Among these fundamental properties, a special one is causality: in the present paper, we show that causality is always preserved for ELKOs.

We show that a general principle of physical independence or physical invariance of mathematical background manifold leads to a replacement of the common derivative operators by the covariant co-derivative ones. This replacement naturally induces a background matrix, by means of which we obtain an effective Lagrangian for the minimal standard model with supplement terms characterizing Lorentz invariance violation or anisotropy of spacetime. We construct a simple model of the background matrix and find that the strength of Lorentz violation of proton in the photopion production is of the order 10^-23.

In this work we present a scalar field theory invariant under space-time anisotropic transformations with a dynamic exponent z. It is shown that this theory possesses symmetries similar to Hořava gravity and that in the limit z = 0 the equations of motion of the non-relativistic MOND theory are obtained. This result allow us to conjecture the existence of a Hořava type gravity that in the limit z = 0 is consistent with MOND.

We show the natural relation between the Wigner Hamiltonian and the conformal Hamiltonian. A model in (super)conformal quantum mechanics with (super)conformal symmetry in the Wigner–Heisenberg algebra picture [x, p_x] = i(1+cP) (P being the parity operator) is presented. In this context, the energy spectrum, the Casimir operator, raising and lowering operators are defined.

In this paper the stationary Klein–Gordon equation is considered for the Coulomb potential in noncommutative space. The energy shift due to noncommutativity is obtained via the perturbation theory. Furthermore, we show that the degeneracy of the initial spectral line is broken in transition from commutative space to noncommutative space.

We exploit the 't Hooft–Polyakov monopole to construct closed algebra of the quantum field operators and the BRST charge Q_BRST. In the first-class configuration of the Dirac quantization, by including the Q_BRST-exact gauge-fixing term and the Faddeev–Popov ghost term, we find the BRST invariant Hamiltonian to investigate the de Rham type cohomology group structure for the monopole system. The Bogomol'nyi bound is also discussed in terms of the first-class topological charge defined on the extended internal two-sphere.