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I. Brevik

Publications and source records attributed to I. Brevik.

At least 73 records · Page 4Linked to original sources
arXiv2009

Electromagnetic Casimir Effect in Wedge Geometry and the Energy-Momentum Tensor in Media

The wedge geometry closed by a circular-cylindrical arc is a nontrivial generalization of the cylinder, which may have various applications. If the radial boundaries are not perfect conductors, the angular eigenvalues are only implicitly determined. When the speed of light is the same on both sides of the wedge, the Casimir energy is finite, unlike the case of a perfect conductor, where there is a divergence associated with the corners where the radial planes meet the circular arc. We advance the study of this system by reporting results on the temperature dependence for the conducting situation. We also discuss the appropriate choice of the electromagnetic energy-momentum tensor.

hep-th↗
arXiv2008

A Brief Review of the Singularities in 4D and 5D Viscous Cosmologies Near the Future Singularity

Analytic properties of physical quantities in the cosmic fluid such as energy density ρ(t) and Hubble parameter H(t) are investigated near the future singularity (Big Rip). Both 4D and 5D cosmologies are considered (the Randall-Sundrum II model in the 5D case), and the fluid is assumed to possess a bulk viscosity ζ. We consider both Einstein gravity and modified gravity, where in the latter case the Lagrangian contains a term R^αwith αa constant. If ζis proportional to the power (2α-1) of the scalar expansion, the fluid can pass from the quintessence region into the phantom region as a consequence of the viscosity. A property worth noticing is that the 4D singularity on the brane becomes carried over to the bulk region.

gr-qc↗
arXiv2008

Casimir Energies: Temperature Dependence, Dispersion, and Anomalies

Assuming the conventional Casimir setting with two thick parallel perfectly conducting plates of large extent with a homogeneous and isotropic medium between them, we discuss the physical meaning of the electromagnetic field energy $W_{\rm disp}$ when the intervening medium is weakly dispersive but nondissipative. The presence of dispersion means that the energy density contains terms of the form $d[ωε(ω)] /dω$ and $d[ωμ(ω)] /dω$. We find that, as $W_{\rm disp}$ refers thermodynamically to a non-closed physical system, it is {\it not} to be identified with the internal thermodynamic energy $U$ following from the free energy $F$, or the electromagnetic energy $W$, when the last-mentioned quantities are calculated without such dispersive derivatives. To arrive at this conclusion, we adopt a model in which the system is a capacitor, linked to an external self-inductance $L$ such that stationary oscillations become possible. Therewith the model system becomes a non-closed one. As an introductory step, we review the meaning of the nondispersive energies, $F, U,$ and $W$. As a final topic, we consider an anomaly connected with local surface divergences encountered in Casimir energy calculations for higher spacetime dimensions, $D>4$, and discuss briefly its dispersive generalization. This kind of application is essentially a generalization of the treatment of Alnes {\it et al.} [J. Phys. A: Math. Theor. {\bf 40}, F315 (2007)] to the case of a medium-filled cavity between two hyperplanes.

quant-ph↗
arXiv2008

Viscous Dark Cosmology with Account of Quantum Effects

The analytic properties of the energy density rho(t) of the cosmic fluid, and the Hubble parameter H(t), are investigated near to the future singularity t=t_s assuming different forms for the equation of state. First, it is shown that the inclusion of quantum effects coming from the conformal anomaly modifies the singularity. Thereafter, we consider the effect coming from a bulk viscosity in the fluid. The viscosity tends to reduce the magnitude of t_s, but does not alter the singularity itself (the exponent). Main emphasis is laid on the simple case when the equation of state is p=w*rho, with w a constant.

gr-qc↗
arXiv2007

A FRW Dark Fluid with a Non-Linear Inhomogeneous Equation of State

A dark Friedman-Robertson-Walker fluid governed by a non-linear inhomogeneous equation of state is considered which can be viewed as a conveniently simple paradigm for a whole class of models which exhibit phase transitions from a non-phantom towards a phantom era (superacceleration transition). From another side, such dark fluid models may describe also quintessence-like cosmic acceleration. Thermodynamical considerations for the processes involved, which are of great importance in the characterization of the global evolution of the corresponding universe, are given too. Connecting the proposed equation of state with an anisotropic Kasner universe with viscosity, we are led to the plausible conjecture of a dark fluid origin of the anisotropies in the early universe.

gr-qc↗
arXiv2007

Entropy Bound for the TM Electromagnetic Field in the Half Einstein Universe

An explicit calculation is given of the entropy/energy ratio for the TM modes of the electromagnetic field in the half Einstein universe. This geometry provides a mathematically convenient and physically instructive example of how the electromagnetic and thermodynamic quantities behave as a function of the nondimensional parameter δ=1/2πaT, a being the scale factor and T the temperature. On physical grounds (related to the relaxation time), it is the case of small δ's that is pertinent to thermodynamics. We find that as long as δis small, the entropy/energy ratio behaves in the same way as for the TE modes. The entropy is thus bounded. The present kind of formalism makes it convenient to study also the influence from frequency dispersion. We discuss an example where a sharp cutoff dispersion relation can in principle truncate the electromagnetic oscillations in the Einstein cavity such that only the lowest mode survives.

hep-th↗
arXiv2007

Dark Energy Fluid with Time-Dependent, Inhomogeneous Equation of State

The four-dimensional Friedman flat universe, filled with an ideal fluid with a linear (oscillating) inhomogeneous equation of state (EoS) depending on time, is studied. The equations of motion are solved. It is shown that in some cases there appears a quasi-periodic universe, which repeats the cycles of phantom-type space acceleration. The appearance of future singularities resulting from various choices for the input parameters is discussed.

gr-qc↗
arXiv2006

What is the Temperature Dependence of the Casimir Effect?

There has been recent criticism of our approach to the Casimir force between real metallic surfaces at finite temperature, saying it is in conflict with the third law of thermodynamics and in contradiction with experiment. We show that these claims are unwarranted, and that our approach has strong theoretical support, while the experimental situation is still unclear.

quant-ph↗
arXiv2005

Temperature Dependence of the Casimir Effect

In view of the increasing accuracy of Casimir experiments, there is a need for performing accurate theoretical calculations. Using accurate experimental data for the permittivities we present, via the Lifshitz formula applied to the standard Casimir setup with two parallel plates, accurate theoretical results in case of the metals Au, Cu and Al. Both similar and dissimilar cases are considered. Concentrating in particular on the finite temperature effect, we show how the Casimir pressure varies with separation for three different temperatures, T={1, 300, 350}K. The metal surfaces are taken to be perfectly plane. The experimental data for the permittivities are generally yielding results that are in good agreement with those calculated from the Drude relation with finite relaxation frequency. We give the results in tabular form, in order to facilitate the assessment of the temperature correction which is on the 1% level. We emphasize two points: (i) The most promising route for a definite experimental verification of the finite temperature correction appears to be to concentrate on the case of large separations (optimum around 2 micrometres); and (ii) there is no conflict between the present kind of theory and the Nernst theorem in thermodynamics.

quant-ph↗
arXiv2005

Calculation of the Casimir Force between Similar and Dissimilar Metal Plates at Finite Temperature

The Casimir pressure is calculated between parallel metal plates, containing the materials Au, Cu, or Al. Our motivation for making this calculation is the need of comparing theoretical predictions, based on the Lifshitz formula, with experiments that are becoming gradually more accurate. In particular, the finite temperature correction is considered, in view of the recent discussion in the literature on this point. A special attention is given to the case where the difference between the Casimir pressures at two different temperatures, T=300 K and T=350 K, is involved. This seems to be a case that will be experimentally attainable in the near future, and it will be a critical test of the temperature correction.

quant-ph↗
arXiv2005

Dark Energy and Viscous Cosmology

Singularities in the dark energy universe are discussed, assuming that there is a bulk viscosity in the cosmic fluid. In particular, it is shown how the physically natural assumption of letting the bulk viscosity be proportional to the scalar expansion in a spatially flat FRW universe can drive the fluid into the phantom region (w < -1), even if lies in the quintessence region (w > -1) in the non-viscous case.

gr-qc↗
arXiv2005

Viscous FRW Cosmology in Modified Gravity

We discuss a modified form of gravity implying that the action contains a power αof the scalar curvature. Coupling with the cosmic fluid is assumed. As equation of state for the fluid, we take the simplest version where the pressure is proportional to the density. Based upon a natural ansatz for the time variation of the scale factor, we show that the equations of motion are satisfied for general α. Also the condition of conservation of energy and momentum is satisfied. Moreover, we investigate the case where the fluid is allowed to possess a bulk viscosity, and find the noteworthy fact that consistency of the formalism requires the bulk viscosity to be proportional to the power (2α-1) of the scalar expansion. In Einstein's gravity, where α=1, this means that the bulk viscosity is proportional to the scalar expansion. This mathematical result is of physical interest; as discussed recently by the present authors, there exists in principle a viscosity-driven transition of the fluid from the quintessence region into the phantom region, implying a future Big Rip singularity.

gr-qc↗
arXiv2005

On the Temperature Dependence of the Casimir Effect

The temperature dependence of the Casimir force between a real metallic plate and a metallic sphere is analyzed on the basis of optical data concerning the dispersion relation of metals such as gold and copper. Realistic permittivities imply, together with basic thermodynamic considerations, that the transverse electric zero mode does not contribute. This results in observable differences with the conventional prediction, which does not take this physical requirement into account. The results are shown to be consistent with the third law of thermodynamics, as well as being consistent with current experiments. However, the predicted temperature dependence should be detectable in future experiments. The inadequacies of approaches based on {\it ad hoc} assumptions, such as the plasma dispersion relation and the use of surface impedance without transverse momentum dependence, are discussed.

quant-ph↗
arXiv2004

Expanding AdS5 Branes: Time Dependent Eigenvalue Problem and Production of Particles

An analysis is first given of the situation where a scalar field is contained between two fixed, spatially flat, branes. The usual fine-tuning (RS) condition is relaxed, and the branes are allowed to possess a positive effective cosmological constant λ. We first analyze the eigenvalue problem for the Kaluza-Klein masses when the metric is time dependent, and consider in detail the case when λis small. Thereafter we consider, in the case of one single brane, the opposite limit in which λis large, acting in a brief period of time T, and present a "sudden approximation" calculation of the energy produced on the brane by the rapidly expanding de Sitter space during this period.

hep-th↗
arXiv2004

Two-Brane Randall-Sundrum Model in AdS_5 and dS_5

Two flat Randall - Sundrum three-branes are analyzed, at fixed mutual distance, in the case where each brane contains an ideal isotropic fluid. Both fluids are to begin with assumed to obey the equation of state p=(γ-1)ρ, where γis a constant. Thereafter, we impose the condition that there is zero energy flux from the branes into the bulk, and assume that the tension on either brane is zero. It then follows that constant values of the fluid energies at the branes are obtained only if the value of γis equal to zero (i.e., a `vacuum' fluid). The fluids on the branes are related: if one brane is a dS_4 brane (the effective four-dimensional constant being positive), then the other brane is dS_4 also, and if the fluid energy density on one brane is positive, the energy density on the other brane is larger in magnitude but negative. This is a non-acceptable result, which sheds some light on how far it is possible to give a physical interpretation of the two-brane scenario. Also, we discuss the graviton localization problem in the two-brane setting, generalizing prior works.

gr-qc↗
arXiv2003

Temperature Dependence of the Casimir Force for Metals

Starting from the Lifshitz formula for the Casimir force between parallel plates we calculate the difference between the forces at two different settings, one in which the temperature is $T_1=350$ K, the other when $T_2=300$ K. As material we choose gold, and make use of the Drude dispersion relation. Our results, which are shown graphically, should be directly comparable to experiment. As an analogous calculation based upon the plasma dispersion relation leads to a different theoretical force difference, an experiment of this kind would be a decisive test. We also present an analogous calculation for the case when the two plates are replaced with a sphere-plate system, still with gold as material in both bodies. The sphere is assumed so large that the proximity theorem holds. Discussion of the consistency with the third law of thermodynamics and the validity of the surface impedance approach is provided.

quant-ph↗
arXiv2003

Randall-Sundrum Model in the Presence of a Brane Bulk Viscosity

The presence of a bulk viscosity for the cosmic fluid on a single Randall-Sundrum brane is considered. The spatial curvature is assumed to be zero. The five-dimensional Friedmann equation is derived, together with the energy conservation equation for the viscous fluid. These governing equations are solved for some special cases: (i) in the low-energy limit when the matter energy density is small compared with brane tension; (ii) for a matter-dominated universe, and (iii) for a radiation-dominated universe. Rough numerical estimates, for the extreme case when the universe is at its Planck time, indicate that the viscous effect can be significant.

gr-qc↗