Narrow Results

We study null bulk geodesic motion in the brane world cosmology in the RS2 scenario and in the static universe in the bulk of the charged topological AdS black hole. We obtain equations describing the null bulk geodesic motion as observed in one lower dimension. We find that the null geodesic motion in the bulk of the brane world cosmology in the RS2 scenario is observed to be under the additional influence of extra non-gravitational force by the observer on the three-brane, if the brane universe does not possess the Z₂ symmetry. As for the null geodesic motion in the static universe in the bulk of the charged AdS black hole, the extra force is realized even when the brane universe has the Z₂ symmetry.

We consider sectionally-elliptical conical space–times and compute the energy–momentum tensor of the source generating such configurations. It turns out that the problem can be reduced to the calculation of the Gaussian curvature of the bidimensional elliptical cone defined by the foliation t = const., z = const. We then employ the method of smoothing the conical singularity by taking a sequence of regular bidimensional manifolds in order to compute the curvature. Finally, we conclude that the gravitational effects produced by sectionally-elliptical and sectionally-circular conical space–times are completely equivalent.

We study a varying electric charge brane world cosmology in the RS2 model obtained from a varying-speed-of-light brane world cosmology by redefining the system of units. We elaborate conditions under which the flatness problem and the cosmological constant problem can be resolved by such cosmological model.

Gravitational field of cosmic strings in theories with extra spatial dimensions differs significantly from that in the 4D Einstein's theory. This means that all gravity induced properties of cosmic strings need to be revised. Here we consider the effect of vacuum polarization outside a straight infinitely thin cosmic string embedded in RS2 brane world.

The role of cosmic defects in cosmology is entering its new phase—as a test for several fundamental physics, including unification theories and inflation. We discuss how to use the Cosmic Microwave Background (CMB) to detect cosmic strings, a type of cosmic defects, and how to use this result to constrain the underlying physics. In particular, we use the simulations for the Array for Microwave Background Anisotropy (AMiBA) to demonstrate the power of this approach. The required resolution and sensitivity in such a method are discussed, and so is the possible scientific impact.

In dilaton gravity theories, we consider a string-like topological defect formed during U(1) gauge symmetry-breaking phase transition in the early Universe, and far from the cosmic string we have vacuum solutions of the generalized Einstein equation. We discuss how they can be related to the flatness of galactic rotation curves.

In the present paper we present and analyze exact cylindrically symmetric diagonal and nondiagonal solutions in the so-called dilaton–axion gravity. It is shown that they are free of curvature singularities, (quasi)regular on the axis of symmetry, asymptotically flat and describe rotating or nonrotating cosmic strings interacting with gravitational and dilaton–axion waves.

In this paper, based on the gauge potential decomposition and the ϕ-mapping theories, we study the topological structures and properties of the cosmic strings that arise in the Abelian–Higgs gauge theory in the zero-thickness limit. After a detailed discussion, we conclude that the topological tensor current introduced in our model is a better and more basic starting point than the generally used Nambu–Goto effective action for studying cosmic strings.

The model of cosmic string formed from two gravitating and interacting scalar fields is considered. It is shown that the regular solutions exist at special choice of the model's parameters only.

We investigated a simple D-term inflation with taking account of higher order corrections in the Kähler potential. These terms may solve the cosmic string problem in D-term inflation model. The mass per unit length of cosmic strings formed after inflation can be suppressed enough. In addition, the change of the potential slope leads simultaneously a more tilted scalar spectral index n_s ≃ 0.96 – 0.97 than that in the model without these corrections.

In the light of ϕ-mapping topological current theory, the structure of cosmic strings are obtained from the Abelian Higgs model, which is an effective description to the brane world cosmic string system. In this topological description of the cosmic string, combining the result of decomposition of U(1) gauge potential, we analytically reach the familiar conclusions that in the brane world scenario the magnetic flux of the cosmic string is quantized and the RR charge of it is screened.

As two generalizations of Einstein's general relativity, the nontrivial spacetime torsion and the compact extra dimensions have been largely studied in the literature. In this paper, by combining a torsioned Kaluza–Klein scheme and the field-theory cosmic string theory, we discuss that a higher-dimensional torsion component can be expressed in terms of the usual four-dimensional field strength two-form, and this torsion form can then get trapped into the cores of the cosmic strings and further relate to the intrinsic spins of the strings.

We obtain the most general static cylindrically symmetric vacuum solutions of the Einstein field equations in (4 + N) dimensions. Under the assumption of separation of variables, we construct a family of Levi–Civita–Kasner vacuum solutions in (4 + N) dimensions. We discuss the dimensional reduction of the static solutions. Depending on the reduction procedure, they can be interpreted either as a scalar-vacuum generalization of Levi–Civita spacetimes, or as the effective 4D vacuum spacetime outside of an idealized string in braneworld theory.

In our previous research, we have constructed a second rank antisymmetric topological current to study various topological properties of cosmic strings in the early universe. In this paper, starting from the conservation equation of the current, we give a detailed discussion of the structure of the current itself, and finally obtain a new constrained equation for the motion of cosmic strings and a conserved, antisymmetric world sheet tensor which may have a deep relation with the structure of the spacetime.

We consider the quantum capture of nonrelativistic massive particle by the moving infinite curve (cosmic string in wire approximation). It is shown that the cusp appearing on a string at a certain point due to the string dynamics can make the wave function collapse at this point irrespective of the caption place.

In the standard model, stabilization of a classically unstable cosmic string may occur through the quantum fluctuations of a heavy fermion doublet. We review numerical results from a semiclassical expansion in a reduced version of the standard model. In this expansion, the leading quantum corrections emerge at one loop level for many internal degrees of freedom. The resulting vacuum polarization energy and the binding energies of occupied fermion energy levels are of the same order, and must therefore be treated on equal footing. Populating these bound states lowers the total energy compared to the same number of free fermions and assigns a charge to the string. Charged strings are already stabilized for a fermion mass only somewhat larger than the top quark mass. Though obtained in a reduced version, these results suggest that neither extraordinarily large fermion masses nor unrealistic couplings are required to bind a cosmic string in the standard model. Furthermore, we also review results for a quantum stabilization mechanism that prevents closed Nielsen–Olesen-type strings from collapsing.

Anisotropic cosmological models are constructed in f(R, T) gravity theory to investigate the dynamics of universe concerning the late time cosmic acceleration. Using a more general and simple approach, the effect of the coupling constant and anisotropy on the cosmic dynamics have been investigated. In this study, it is found that cosmic anisotropy substantially affects cosmic dynamics.

This paper is devoted to formulating exact solutions of axially symmetric spacetime through gravitational decoupling technique. For this purpose, we first evaluate an exact solution in the framework of cosmic strings by assuming some additional constraints on the metric coefficients and extend it to obtain two concrete anisotropic cosmological models. We investigate energy conditions as well as the speed of sound constraint to ensure the physical viability of the developed solutions. It is concluded that both anisotropic models meet all the energy bounds as well as stability criterion. The expanding behavior of the universe is also confirmed through different cosmological parameters.

The existence of time machines, understood as space–time constructions exhibiting physically realised closed timelike curves (CTC's), would raise fundamental problems with causality and challenge our current understanding of classical and quantum theories of gravity. In this paper, we investigate three proposals for time machines which share some common features: cosmic strings in relative motion, where the conical space–time appears to allow CTC's; colliding gravitational shock waves, which in Aichelburg–Sexl coordinates imply discontinuous geodesics; and the superluminal propagation of light in gravitational radiation metrics in a modified electrodynamics featuring violations of the strong equivalence principle. While we show that ultimately none of these constructions creates a working time machine, their study illustrates the subtle levels at which causal self-consistency imposes itself, and we consider what intuition can be drawn from these examples for future theories.

Some black hole-cosmic string models such as Reissner–Nordström, RN–de Sitter, Kerr–Newman and multi-black holes with cosmic string are given. Energy and angular momentum of a timelike particle in circular orbits in multi-black hole space–time are calculated. The geodesic equations for the timelike particles for the far region of the multi-black hole sources are calculated and small oscillations around the circular orbit obtained. It is seen that the particle's orbit precesses like the Lens–Thirring effect.