N. Ozdemir

Motion of charged particles around a rotating black hole in a magnetic field

We study the effects of an external magnetic field, which is assumed to be uniform at infinity, on the marginally stable circular motion of charged particles in the equatorial plane of a rotating black hole. We show that the magnetic field has its greatest effect in enlarging the region of stability towards the event horizon of the black hole. Using the Hamilton-Jacobi formalism we obtain the basic equations governing the marginal stability of the circular orbits and their associated energies and angular momenta. As instructive examples, we review the case of the marginal stability of the circular orbits in the Kerr metric, as well as around a Schwarzschild black hole in a magnetic field. For large enough values of the magnetic field around a maximally rotating black hole we find the limiting analytical solutions to the equations governing the radii of marginal stability. We also show that the presence of a strong magnetic field provides the possibility of relativistic motions in both direct and retrograde innermost stable circular orbits around a Kerr black hole.

Vacuum fluctuations of a massless spin- field around multiple cosmic strings

We study the interaction of a massless quantized spinor field with the gravitational field of N parallel static cosmic strings by using a perturbative approach. We show that the presence of more than one cosmic string gives rise to an additional contribution to the energy density of vacuum fluctuations, thereby leading to a vacuum force of attraction between two parallel cosmic strings.

Spinning cosmic strings: A General class of solutions

In this work, we give a general class of solutions of the spinning cosmic string in Einsteins theory of gravity. After treating same problem in Einstein Cartan (EC) theory of gravity, the exact solution satisfying both exterior and interior space-times representing a spin fluid moving along the symmetry axis is presented in the EC theory. The existence of closed timelike curves in this spacetime are also examined.In this work, we give a general class of solutions of the spinning cosmic string in Einsteins theory of gravity. After treating same problem in Einstein–Cartan (EC) theory of gravity, the exact solution satisfying both exterior and interior space–times representing a spin fluid moving along the symmetry axis is presented in the EC theory. The existence of closed timelike curves in this space–time are also examined.

Gravitomagnetic effects and cosmic strings

In this paper, some gravitomagnetic effects are explored in the presence of cosmic strings. In particular, the gravitomagnetic clock effects and gyroscope precession are considered in the equatorial plane of a spacetime that is the nonlinear superposition of a Kerr black hole with a cosmic string along its axis of rotation. Moreover, an exact solution of the Einstein field equations corresponding to the Godel universe with a cosmic string is introduced. The influence of the cosmic string on the gravitomagnetic effects in this spacetime is briefly studied.

Particle creation if a cosmic string snaps

We calculate the Bogolubov coefficients for aC(0) metric which describes the snapping of a cosmic string. In this background, we show that there are noregular solutions with particle interpretation, but we find ageneralized solution with integrable discontinuity, which exhibits particle creation. We also find a regular solution if we allow wave packets.

QUANTUM FLUCTUATIONS FOR A SCALAR FIELD IN THE SPACE–TIME OF GRAVITATIONAL IMPLUSIVE WAVE

Quantum fluctuations for a massless scalar field in the background metric of spherical impulsive gravitational waves through Minkowski and de Sitter spaces are investigated. It is shown that there exist finite fluctuations for de Sitter space.Quantum fluctuations for a massless scalar field in the background metric of spherical implusive gravitational waves propagating through Minkowski and de Sitter spaces are investigated. It is shown that there exist finite fluctuations for de Sitter space.

Can an Unruh Detector Feel a Cosmic String

Unruhs detector calculation is used to study the effect of the defect angle

CANCELLATION OF ULTRAVIOLET DIVERGENCES FOR LOCALLY FLAT METRICS

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Reduction of space–time by a constraint for C(0) metrics

in a space-time with a cosmic string for both the excitation and deexcitation cases. It is found that a rotating detector results in a non-zero effect for both finite (small) and infinite (large) time.Unruhs detector calculation is used to study the effect of the defect angle β in a space-time with a cosmic string for both the excitation and deexcitation cases. It is found that a rotating detector results in a non-zero effect for both finite (small) and infinite (large) time.

Semiclassical corrections to the metric of multiple cosmic strings

We extend the work done for cosmic strings on the perturbative calculation of vacuum polarization of a massless field in the space-time of multiple cosmic strings and show that for a more general class of locally flat metrics the one loop calculation do not introduce any new divergences to the VEV of the energy of a scalar particle or a spinor particle. We explicitly perform the calculation for the configuration where we have one cosmic string in the presence of a dipole made out of cosmic strings both for the scalar and the spinor cases.We extend the work done for cosmic strings on the perturbative calculation of vacuum polarization of a massless field in the space–time of multiple cosmic strings and show that for a more general class of locally flat metrics, the one-loop calculation does not introduce any new divergences to the vev of the energy of a scalar particle or a spinor particle.