Magnetic strings in f(R) gravity

We construct a new class of spinning magnetic string solutions in f(R) gravity with constant scalar curvature. These solutions which produce a longitudinal magnetic field have no curvature singularity and no horizon, but have a conic geometry with a deficit angle. We also generalize this class of solutions to the case of spinning magnetic solutions with one rotation parameter. We find that the spinning string has a net electric charge which is proportional to the rotation parameter. With choosing a suitable counterterm, we remove the divergences of the action. The conserved quantities of the solutions are also calculated by using the counterterm method.     

Magnetic Dilaton Rotating Strings in the Presence of Exponential Nonlinear Electrodynamics

In this paper, we construct a new class of four-dimensional spinning magnetic dilaton string solutions which produces a longitudinal nonlinear electromagnetic field. The Lagrangian of the matter field has the exponential form. We study the physical properties of the solution in ample details. Geometrical, causal and geodisical structures of the solutions are investigated, separately. We confirm that the spacetime is both null and geodesically complete. We find that these solutions have no curvature singularity and no horizon, but have a conic geometry. We investigate the effects of variation of charge and the intensity of the dilaton field, on the deficit angle. Due to the presence of the dilaton field, the asymptotic behavior of the solutions are neither flat nor (anti-) de Sitter [(A)dS]. Furthermore, we extend our study to the higher dimensions and obtain the (n+1)-dimensional magnetic rotating dilaton strings with k≤[n/2] rotation parameters and calculate conserved quantities of the solutions. Although these solutions are not asymptotically (A)dS, we use counterterm method to calculate conserved quantities. We also calculate electric charge and show that the net electric charge of the spinning string is proportional to the rotating parameter and the electric field only exists when the rotation parameter does not vanish.     

Magnetic Branes in Brans-Dicke-Maxwell Theory

We present a new class of magnetic brane solutions in (n+1)-dimensional Brans-Dicke-Maxwell theory in the presence of a quadratic potential for the scalar field. These solutions are neither asymptotically flat nor (anti)-de Sitter. Our strategy for constructing these solutions is applying a conformal transformation to the corresponding solutions in dilaton gravity. This class of solutions represents a spacetime with a longitudinal magnetic field generated by a static brane. They have no curvature singularity and no horizons but have a conic geometry with a deficit angle δ. We generalize this class of solutions to the case of spinning magnetic brane with all rotation parameters. We also use the counterterm method and calculate the conserved quantities of the solutions.     

Magnetic strings in Einstein–Born–Infeld-dilaton gravity

A class of spinning magnetic string in 4-dimensional Einstein-dilaton gravity with Liouville type potential which produces a longitudinal nonlinear electromagnetic field is presented. These solutions have no curvature singularity and no horizon, but have a conic geometry. In these spacetimes, when the rotation parameter does not vanish, there exists an electric field, and therefore the spinning string has a net electric charge which is proportional to the rotation parameter. Although the asymptotic behavior of these solutions are neither flat nor (A)dS, we calculate the conserved quantities of these solutions by using the counterterm method. We also generalize these four-dimensional solutions to the case of (n+1)$(n+1)$-dimensional rotating solutions with k?[n/2]$k?[n/2]$ rotation parameters, and calculate the conserved quantities and electric charge of them.     

Curvature-corrected dilatonic black holes and black hole—string transition

We investigate extremal charged black hole solutions in the four-dimensional string frame Gauss-Bonnet gravity with the Maxwell field and the dilaton. Without curvature corrections, the extremal electrically charged dilatonic black holes have singular horizon and zero Bekenstein entropy. When the Gauss-Bonnet term is switched on, the horizon radius expands to a finite value provided curvature corrections are strong enough. Below a certain threshold value of the Gauss-Bonnet coupling the extremal black hole solutions cease to exist. Since decreasing Gauss-Bonnet coupling corresponds to decreasing string coupling g _s , the situation can tentatively be interpreted as classical indication on the black hole—string transition. Previously the extremal dilaton black holes were studied in the Einstein-frame version of the Gauss-Bonnet gravity. Here we work in the string frame version of the theory with the S-duality symmetric dilaton function as required by the heterotic string theory. The article is published in the original.     

Charged rotating dilaton black branes in AdS universe

We present the metric for the (n + 1)-dimensional charged rotating dilaton black branes with cylindrical or toroidal horizons in the background of anti-de Sitter spacetime. We find the suitable counterterm which removes the divergences of the action in the presence of the dilaton potential in all higher dimensions. We plot the Penrose diagrams of the spacetime and reveal that the spacetime geometry crucially modifies in the presence of the dilaton field. The conserved and thermodynamic quantities of the black branes are also computed.     

On the motion of a quantum particle in the spinning cosmic string space–time

We analyze the energy spectrum and the wave function of a particle subjected to magnetic field in the spinning cosmic string space–time and investigate the influence of the spinning reference frame and topological defect on the system. To do this we solve Schrödinger equation in the spinning cosmic string background. In our work, instead of using an approximation in the calculations, we use the quasi-exact ansatz approach which gives the exact solutions for some primary levels.     

Wormhole solutions in Gauss–Bonnet–Born–Infeld gravity

A new class of solutions which yields an (n + 1)-dimensional spacetime with a longitudinal nonlinear magnetic field is introduced. These spacetimes have no curvature singularity and no horizon, and the magnetic field is non singular in the whole spacetime. They may be interpreted as traversable wormholes which could be supported by matter not violating the weak energy conditions. We generalize this class of solutions to the case of rotating solutions and show that the rotating wormhole solutions have a net electric charge which is proportional to the magnitude of the rotation parameter, while the static wormhole has no net electric charge. Finally, we use the counterterm method and compute the conserved quantities of these spacetimes.     

String creation in cosmologies with a varying dilaton

FRW solutions of the string theory low-energy effective actions are described, yielding a dilaton which first decreases and then increases. We study string creation in these backgrounds and find an exponential divergence due to an initial space-like singularity. We conjecture that this singularity may be removed by the effects of back-reaction, leading to a solution which at early times is de Sitter space.     

Asymptotic singular behaviour of inhomogeneous cosmologies in Einstein-Maxwell-dilaton-axion theory

We present the study of exact inhomogeneous cosmological solutions to a four-dimensional low energy limit of string theory containing non-minimal interacting electromagnetic, dilaton and axion fields. We focus at Einstein-Rosen solutions of Einstein-Maxwell-dilaton-axion equations. Several cases are analyzed and some of them have asymptotically velocity-term dominated (AVTD) singularities.     

D-brane solutions in a light-like linear dilaton background

The light-like linear dilaton background presents a simple time dependent solution of type II supergravity equations of motion that preserves 1/2 supersymmetry in ten dimensions. We construct supergravity D-brane solutions in a linear dilaton background starting from the known intersecting brane solutions in string theory. By applying a Penrose limit on the intersecting (NS1–NS5–NS5′)-brane solution, we find out a D5-brane in a linear dilaton background. We solve the Killing spinor equations for the brane solutions explicitly, and show that they preserve 1/4 supersymmetry. We also find a M5-brane solution in eleven-dimensional supergravity.     

Kalb–Ramond Dipole Solution in Low-Energy Bosonic String Theory

We construct a new solution subspace for the bosonic string theory toroidally compactified to 3 dimensions. This subspace corresponds to the complex harmonic scalar field coupled to the effective 3–dimensional gravity. We calculate a class of the asymptotically flat and free of the Dirac string peculiarity solutions which describes a Kalb–Ramond dipole source with the generally nontrivial dilaton characteristics.     

Dilaton dependence of the two-loop effective action

We construct a family of equivalent two-loop string theory effective actions, incorporating dilaton background field contributions, which subsumes all those presently available in the literature. We identify in particular two one-parameter subsets of this family, one of which consists of actions with no dilaton dependence (beyond an overall factor) and another which corresponds in a natural way to renormalisation scheme dependence in the underlying non-linear σ-model. Corresponding to the family of actions we construct a family of renormalisation group dilaton β-functions and show that they are related by renormalisation group transformations.     

Stabilization of extra dimensions and the dimensionality of the observed space

We present a simple model for the late time stabilization of extra dimensions. The basic idea is that brane solutions wrapped around extra dimensions, which is allowed by string theory, will resist expansion due to their winding mode. The momentum modes in principle work in the opposite way. It is this interplay that leads to dynamical stabilization. We use the idea of democratic wrapping, where in a given decimation of extra dimensions, all possible winding cases are considered. To further simplify the study we assumed a symmetric decimation in which the total number of extra dimensions is taken to be Np where N can be called the order of the decimation. We also assumed that extra dimensions all have the topology of tori. We show that with these rather conservative assumptions, there exist solutions to the field equations in which the extra dimensions are stabilized and that the conditions do not depend on p. This fact means that there exists at least one solution to the asymmetric decimation case. If we denote the number of observed space dimensions (excluding time) by m, the condition for stabilization is m≥3 for pure Einstein gravity and m≤3 for dilaton gravity massaged by a string theory parameter, namely the dilaton coupling to branes. PACS 98.80.-k; 11.25.Uv     

Nonrotating cosmic strings interacting with gravitational waves in Einstein-Maxwell-dilaton gravity

We present methods for the construction of exact diagonal cylindrically symmetric solutions in a four dimensional low energy limit of string theory, the Einstein-Maxwell-dilaton gravity. The methods allow us to generate exact string backgrounds from known solutions to the equations of Einstein or Einstein gravity coupled to a massless scalar field. We also give and analyze explicit examples of such solutions. It is shown that they are free of curvature singularities,(quasi)regular on the axis of symmetry, asymptotically flat and describe nonrotating cosmic strings interacting with gravitational, dilaton and electromagnetic waves.     

Solution of the dilaton problem in open bosonic string theories

All string theories naively contain a massless dilaton which couples with the strength of gravity in direct violation of experiment. We present a simple mechanism for giving the dilaton a mass In unoriented open bosonic string theories.This essay received an honourable mention from the Gravity Research Foundation, 1990. — Ed.     

Test particle trajectories near cosmic strings

We present a detailed analysis of the motion of test particle in the gravitational field of cosmic strings in different situations using the Hamilton-Jacobi (H-J) formalism. We have discussed the trajectories near static cosmic string, cosmic string in Brans-Dicke theory and cosmic string in dilaton gravity.     

Nonequilibrium dynamics of strings in time-dependent plane wave backgrounds

We formulate and study the nonequilibrium dynamics of strings near the singularity of the time-dependent plane wave background in the framework of the Nonequilibrium Thermo Field Dynamics (NETFD). In particular, we construct the Hilbert space of the thermal string oscillators at nonequilibrium and generalize the NETFD to describe the coordinates of the center of mass of the thermal string. The equations of motion of the thermal fields and the Hamiltonian are derived. Due to the time-dependence of the oscillator frequencies, a counterterm is present in the Hamiltonian. This counterterm determines the correlation functions in a perturbative fashion. We compute the two point correlation function of the thermal string at zero order in the power expansion.     

Five-dimensional black hole string backgrounds and brane universe acceleration

We examine background solutions of black hole type arising from the string effective action in five dimensions. We derive the unique metric — dilaton vacuum which is a Schwarzschild type black hole. It is found that it can be extended to incorporate electric charge without changing the topology of the three space. Kalb–Ramond charge may also be introduced if the three space is closed. The basic features of the cosmology induced on a three brane evolving in this background are also discussed.