International Journal of Modern Physics D

We demonstrate an alternative (with respect to the ones existing in the literature) and more-typical-for-physicists derivation of an exact solution to the Einstein–Maxwell equations for the motion of a charged spherical membrane with tangential tension. We stress that there exists physically acceptable range of parameters for which the static and stable state of the membrane producing the Reissner–Nordstrom (RN) repulsive gravity effect exists. The concrete realization of such a state for the Nambu–Goto membrane is described. The point is that the membrane is able to cut out the central naked singularity region and at the same time join in an appropriate way the RN repulsive region.

As a result we have a model of an everywhere-regular material source exhibiting a repulsive gravitational force in the vicinity of its surface: this construction gives a more sensible physical status to the RN solution in the naked singularity case.

We examine the nonequatorial orbits of charged particles under the combined influence of Schwarzschild curvature and external purely poloidal magnetic fields — the uniform and dipole ones — corresponding to analytical solutions to the Maxwell equations in the given space–time. Particle confinement in bound cross-equatorial orbits can occur, due to a magnetic bottle effect, but also natural escape trajectories can be obtained, without the need for additional devices — like electric or toroidal magnetic fields — previously invoked to this end. The conditions leading to either of these outcomes, entrapment or escape, are examined, and a comparison with the flat space–time case is made, evaluating the role played by gravity in the overall charged particle dynamics.

We investigate the phase space structure of the quintom paradigm in the framework of a spatially flat, open or closed isotropic and homogeneous universe. We examine the dynamical evolution under the assumption of late time dark energy domination, without specifying the explicit quintom potential form. The obtained cosmological behavior is qualitatively different than that acquired from the single phantom model.

Causal solutions of the Gödel type, for which the line element is ds² = dt² - 2be^mxdtdv - ce^2mxdv² - dx² - dz² with c = 0, are known to exist for gravitational theories containing a cosmological constant Λ and quadratic higher-derivative terms defined by the Lagrangian L = -(1/2)κ^-2(R + 2Λ) + A₁R² + A₂R_ijR^ij. Here, we show that acausal solutions, for which c < 0, containing closed time-like lines, can be constructed only if A₂ = 0. Extension of this analysis to the heterotic superstring theory, including a generic massless scalar field ϕ plus quadratic and quartic gravitational terms and , again yields a causal solution with c = 0, and also Λ = 0, as required for anomaly freedom, while solutions with c < 0 are ruled out. More general rotational space–times appear to be intractable analytically, and therefore it remains a matter of conjecture that the heterotic superstring admits only classical Lorentzian solutions which respect causality. For the energy density ρ(ϕ) of the scalar field is positive semi-definite only when g⁰⁰ ≥ 0, which is equivalent to the causality condition g₁₁ ≤ 0 or c ≥ 0 in a Lorentzian space–time for which det g_ij < 0; while ρ(ϕ) is unbounded from below in the presence of closed time-like lines, when g₁₁ > 0, implying instability of ϕ, which will react back on the metric until it becomes causal.

We show that the introduction of a hyperbolic phase of the Brans–Dicke (BD) field results in a flat vacuum cosmological solution of the Hubble parameter H and a fractional rate of change of the BD scalar field F, which asymptotically approach constant values. At later stages, the hyperbolic phase of the BD field behaves like dark matter.

By considering the product of the usual four-dimensional space–time with two dimensional space of constant curvature, an interesting black hole solution has recently been found for Einstein–Gauss–Bonnet gravity. It turns out that this as well as all others could easily be made to radiate Vaidya null dust. However, there exists no Kerr analog in this setting. To get the physical feel of the four-dimensional black hole space–times, we study asymptotic behavior of stresses at the two ends, r → 0 and r → ∞.

We present a numerical solution on a five-dimensional spherically symmetric space–time, in Einstein–Yang–Mills — Gauss–Bonnet theory, using a two-point boundary value routine. It turns out that the Gauss–Bonnet contribution has a profound influence on the behavior of the particle-like solution: it increases the number of nodes of the YM field. When a negative cosmological constant is incorporated in the model, it turns out that there is no horizon and no singular behavior of the model. For a positive cosmological constant the model has singular behavior.

We review the attractor properties of the simplest chaotic model of inflation, in which a minimally coupled scalar field is endowed with a quadratic scalar potential. The equations of motion in a flat Friedmann–Robertson–Walker universe are written as an autonomous system of equations, and the solutions of physical interest appear as critical points. This new formalism is then applied to the study of inflation dynamics, in which we can go beyond the known slow-roll approximation.

In quantum gravity, one seeks to combine quantum mechanics and general relativity. In attempting to do so, one comes across the "problem of time" impasse: the notion of time is conceptually different in each of these theories. In this paper, I consider the timeless records approach to resolving this. Records are localized, information-containing subconfigurations of a single instant. Records theory is the study of these and of how science (or history) is to be abstracted from correlations between them. I critically evaluate motivations for this approach that have previously appeared in the literature. I provide a ground-level structure for records theory and discuss what kind of further tools are needed, illustrated with some toy models: ordinary mechanics, relational particle dynamics, detector models and inhomogeneous perturbations about homogeneous cosmology.

We perform an analysis where Einstein's field equation is derived by means of very simple thermodynamical arguments. Our derivation is based on a consideration of the properties of a very small, spacelike two-plane in a uniformly accelerating motion.