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I. Brevik

Publications and source records attributed to I. Brevik.

103 records · Page 6Linked to original sources
arXiv2000

Casimir Energy and Thermodynamic Properties of the Relativistic Piecewise Uniform String

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calculated. The great adaptibility of this string model with respect to various regularization methods is pointed out. We survey several regularization methods: the cutoff method, the complex contour integration method, and the zeta-function method. The most powerful method in the present case is the contour integration method. The Casimir energy turns out to be negative, and more so the larger is the number of pieces in the string. The thermodynamic free energy F is calculated for a two-piece string in the limit when the tension ratio x=T_{I}/T_{II} approaches zero.

hep-th↗
arXiv1999

Magnetohydrodynamics in the Inflationary Universe

Magnetohydrodynamic (MHD) waves are analysed in the early Universe, in the inflationary era, assuming the Universe to be filled with a nonviscous fluid of the Zel'dovich type ($p=ρ$) in a metric of the de Sitter form. A spatially uniform, time dependent, magnetic field ${\bf B_0}$ is assumed to be present. The Einstein equations are first solved to give the time dependence of the scale factor, assuming that the matter density, but not the magnetic field, contribute as source terms. The various modes are thereafter analysed; they turn out to be essentially of the same kind as those encountered in conventional nongravitational MHD, although the longitudinal magnetosonic wave is not interpretable as a physical energy-transporting wave as the group velocity becomes superluminal. We determine the phase speed of the various modes; they turn out to be scale factor independent. The Alfvén velocity of the transverse magnetohydrodynamic wave becomes extremely small in the inflationary era, showing that the wave is in practice 'frozen in'.

gr-qc↗
arXiv1999

Energy Production in the Formation of a Finite Thickness Cosmic String

The classical electromagnetic modes outside a long, straight, superconducting cosmic string are calculated, assuming the string to be surrounded by a superconducting cylindric surface of radius R. Thereafter, by use of a Bogoliubov-type argument, the electromagnetic energy W produced per unit length in the lowest two modes is calculated when the string is formed "suddenly". The essential new element in the present analysis as compared with prior work of Parker [Phys. Rev. Lett. {\bf 59}, 1369 (1987)] and Brevik and Toverud [Phys. Rev. D {\bf 51}, 691 (1995)], is that the radius {\it a} of the string is assumed finite, thus necessitating Neumann functions to be included in the fundamental modes. We find that the theory is changed significantly: W is now strongly concentrated in the lowest mode $(m,s)=(0,1)$, whereas the proportionality $W \propto (Gμ/t)^2$ that is characteristic for zero-width strings is found in the next mode (1,1). Here G is the gravitational constant, $μ$ the string mass per unit length, and t the GUT time.

gr-qc↗
arXiv1999

The Casimir Problem of Spherical Dielectrics: A Solution in Terms of Quantum Statistical Mechanics

The Casimir energy for a compact dielectric sphere is considered in a novel way, using the quantum statistical method introduced by H\oye - Stell and others. Dilute media are assumed. It turns out that this method is a very powerful one: we are actually able to derive an expression for the Casimir energy that contains also the negative part resulting from the attractive van der Waals forces between the molecules. It is precisely this part of the Casimir energy that has turned out to be so difficult to extract from the formalism when using the conventional field theoretical methods for a continuous medium. Assuming a frequency cutoff, our results are in agreement with those recently obtained by Barton [J. Phys. A: Math. Gen. 32(1999)525].

quant-ph↗
arXiv1999

Quantum Cosmology from \cal{N} = 4 Super Yang - Mills Theory

We consider quantum \cal{N} = 4 super Yang-Mills theory interacting in a covariant way with \cal{N} = 4 conformal supergravity. The induced large N effective action for such a theory is calculated on a dilaton-gravitational background using the conformal anomaly found via AdS/CFT correspondence. Considering such an effective action as a quantum correction to the classical gravity action we study quantum cosmology. In particular, the effect from dilaton to the scale factor (which without dilaton corresponds to the inflationary universe) is investigated. It is shown that, dependent on the initial conditions for the dilaton, the dilaton may slow down, or accelerate, the inflation process. At late times, the dilaton is decaying exponentially.

hep-th↗
arXiv1999

Mass and Decay Spectra of the Piecewise Uniform String

Mass and decay spectra are calculated for quantum massive excitations of a piecewise uniform bosonic string. The physical meaning of the critical temperatures characterizing the radiation in the decay of a massive microstate in string theory is discussed.

hep-th↗
arXiv1998

Casimir Theory of the Relativistic Piecewise Uniform String

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calculated. The string is relativistic in the sense that the velocity of transverse waves is always equal to c. The great adaptibility of this string model with respect to various regularization methods is pointed out. We survey several regularization methods: the cutoff method, the complex contour integration method, and the zeta-function method. The most powerful method in the present case is the contour integration method. The Casimir energy turns out to be negative, and the more so the larger is the number of pieces in the string. The thermodynamic free energy F is calculated for a two-piece string in the limit when the tension ratio x = T_I/T_II approaches zero. For large values of the length ratio s = L_II/L_I, the Hagedorn temperature becomes proportional to the square root of s.

hep-th↗
arXiv1998

Vanishing of Gravitational Particle Production in the Formation of Cosmic Strings

We consider the gravitationally induced particle production from the quantum vacuum which is defined by a free, massless and minimally coupled scalar field during the formation of a gauge cosmic string. Previous discussions of this topic estimate the power output per unit length along the string to be of the order of $10^{68}$ ergs/sec/cm in the s-channel. We find that this production may be completely suppressed. A similar result is also expected to hold for the number of produced photons.

hep-th↗
arXiv1998

Thermodynamic Properties of the Piecewise Uniform String

The thermodynamic free energy F is calculated for a gas whose particles are the quantum excitations of a piecewise uniform bosonic string. The string consists of two parts of length L_I and L_II, endowed with different tensions and mass densities, adjusted in such a way that the velocity of sound always equals the velocity of light. The explicit calculation is done under the restrictive condition that the tension ratio x = T_I/T_II approaches zero. Also, the length ratio s = L_II/L_I is assumed to be an integer. The expression for F is given on an integral form, in which s is present as a parameter. For large values of s, the Hagedorn temperature becomes proportional to the square root of s.

hep-th↗
arXiv1997

On the Casimir energy for a 2N-piece relativistic string

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calculated. The string consists of 2N pieces of equal length, of alternating type I and type II material, and is taken to be relativistic in the sense that the velocity of sound always equals the velocity of light. By means of a new recursion formula we manage to calculate the Casimir energy for arbitrary integers N. Agreement with results obtained in earlier works on the string is found in all special cases. As basic regularization method we use the contour integration method. As a check, agreement is found with results obtained from the ζfunction method (the Hurwitz function) in the case of low N (N = 1-4). The Casimir energy is generally negative, and the more so the larger is the value of N. We illustrate the results graphically in some cases. The generalization to finite temperature theory is also given.

hep-th↗
arXiv1996

The effective potential of gauged NJL model in magnetic field

The formalism, which permits to study the phase structure of gauged NJL-model for arbitrary external fields, is developed. The effective potential in the gauged NJL model in the weak magnetic field is found. It is shown that in fixed gauge coupling case the weak magnetic field doesn't influence chiral symmetry breaking condition. The analogy with the situation near black hole is briefly mentioned.

hep-th↗