International Journal of Modern Physics D

We study the geometric and physical foundations of Finsler gravity theories with metric compatible connections defined on tangent bundles, or (pseudo) Riemannian manifolds, endowed with nonholonomic frame structure. Several generalizations and alternatives to Einstein gravity are considered, including modifications with broken local Lorentz invariance. It is also shown how such theories (and general relativity) can be equivalently re-formulated in Finsler like variables. We focus on prospects in modern cosmology and Finsler acceleration of Universe. Einstein–Finsler gravity theories are elaborated following almost the same principles as in the general relativity theory but extended to Finsler metrics and connections. Finally, some examples of generic off-diagonal metrics and generalized connections, defining anisotropic cosmological Einstein–Finsler spaces are analyzed; certain criteria for the Finsler accelerating evolution are formulated.

This work presents a complete cyclic cosmological scenario based on nonlinear magnetic field. It is constructed from a model composed of five fluids, namely baryonic matter, dark matter, radiation, neutrinos and a cosmological magnetic field. The first four fluids are treated in the standard way and the fifth fluid, the magnetic field, is described by a nonlinear electrodynamics. The free parameters are fitted by observational data (SNIa, CMB, extragalactic magnetic fields, etc.) and by simple theoretical considerations. As a result arises a cyclic cosmological model that preserves the main successes of the standard Big Bang model and solves some other problems such as the initial singularity, the present acceleration and the Big Rip.

The issue of causality in f(T) gravity is investigated by examining the possibility of existence of the closed timelike curves in the Gödel-type metric. By assuming a perfect fluid as the matter source, we find that the fluid must have an equation of state parameter greater than minus one in order to allow the Gödel solutions to exist, and furthermore the critical radius r_c, beyond which the causality is broken down, is finite and it depends on both matter and gravity. Remarkably, for certain f(T) models, the perfect fluid that allows the Gödel-type solutions can even be normal matter, such as pressureless matter or radiation. However, if the matter source is a special scalar field rather than a perfect fluid, then r_c → ∞ and the causality violation is thus avoided.

The mass evolution of Schwarzschild black holes by the absorption of scalar fields is investigated in the scenario of the generalized Chaplygin gas (GCG). The GCG works as a unification picture of dark matter plus dark energy that naturally accelerates the expansion of the Universe. Through elements of the quasi-stationary approach, we consider the mass evolution of Schwarzschild black holes accreted by nonminimally coupled cosmological scalar fields reproducing the dynamics of the GCG. As a scalar field nonminimally coupled to the metrics, such an exotic content has been interconnected with accreting black holes. The black hole increasing masses by the absorption of the gas reflects some consistence of the accretion mechanism with the hypothesis of the primordial origin of supermassive black holes. Our results effectively show that the nonminimal coupling with the GCG dark sector accelerates the increasing of black hole masses. Meanwhile some exotic features can also be depicted for specific ranges of the nonminimal coupling in which the GCG dynamics is substantially modified.

We consider a k = 0 Friedman–Robertson–Walker (FRW) model within loop quantum cosmology (LQC) and explore the issue of its semiclassical limit. The model is exactly solvable and allows us to construct analytical (Gaussian) coherent-state solutions for each point on the space of classical states. We propose physical criteria that select from these coherent states, those that display semiclassical behavior, and study their properties in the deep Planck regime. Furthermore, we consider generalized squeezed states and compare them to the Gaussian states. The issue of semiclassicality preservation across the bounce is studied and shown to be generic for all the states considered. Finally, we comment on some implications these results have, depending on the topology of the spatial slice. In particular, we consider the issue of the recovery, within our class of states, of a scaling symmetry present in the classical description of the system when the spatial topology is noncompact.

The Bardeen model describes a regular spacetime, i.e. a singularity-free black hole spacetime. In this paper, by analyzing the behavior of the effective potential for the particles and photons, we investigate the timelike and null geodesic structures in the Bardeen spacetime. At the same time, all kinds of orbits, which are allowed according to the energy level corresponding to the effective potentials, are numerically simulated in detail. We find many-world bound orbits, two-world escape orbits and escape orbits in this spacetime. We also find that bound orbits precession directions are opposite and their precession velocities are different, the inner bound orbits shift along counter-clockwise with high velocity while the exterior bound orbits shift along clockwise with low velocity.