arXiv subjects

Iver Brevik

Publications and source records attributed to Iver Brevik.

At least 73 records · Page 4Linked to original sources

arXiv2016

General Bulk-Viscous Solutions and Estimates of Bulk Viscosity in the Cosmic Fluid

We derive a general formalism for bulk viscous solutions of the energy-conservation-equation for $ρ(a,ζ)$, both for a single-component and a multicomponent fluid in the Friedmann universe. For our purposes these general solutions become valuable in estimating order of magnitude of the phenomenological viscosity in the cosmic fluid at present. $H(z)$ observations are found to put an upper limit on the magnitude of the modulus of the present day bulk viscosity. It is found to be $ζ_0\sim 10^6~$Pa s, in agreement with previous works. We point out that this magnitude is acceptable from a hydrodynamic point of view. Finally, we bring new insight by using our estimates of $ζ$ to analyse the fate of the future universe. Of special interest is the case $ζ\propto \sqrtρ$ for which the fluid, originally situated in the quintessence region, may slide through the phantom barrier and inevitably be driven into a big rip. Typical rip times are found to be a few hundred Gy.

gr-qc↗

arXiv2016

Casimir Force between a Half-Space and a Plate of Finite Thickness

Zero-frequency Casimir theory is analyzed from different viewpoints, focusing on the Drude-plasma issue that turns up when one considers thermal corrections to the Casimir force. The problem is that the plasma model, although leaving out dissipation in the material, apparently gives the best agreement with recent experiments. We consider a dielectric plate separated from a dielectric half-space by a vacuum gap, both media being similar. We consider the following categories: (1) Making use of the statistical mechanical method developed by Høye and Brevik (1998), implying that the quantized electromagnetic field is replaced by interaction between dipole moments oscillating in harmonic potentials, we first verify that the Casimir force is in agreement with the Drude prediction. No use of Fresnel's reflection coefficients is made at this stage. (2) Then turning to the field theoretical description implying use of the reflection coefficients, we derive results in agreement with the forgoing when first setting the frequency equal to zero, before letting the permittivity becoming large. With the plasma relation the reflection coefficient for TE zero frequency modes depend on the component of the wave vector parallel to the surfaces and lies between 0 and 1. This contradicts basic electrostatic theory. (3) Turning to high permeability magnetic materials the TE zero frequency mode describes the static magnetic field in the same way as the TM zero frequency modes describe the static electric fields in electrostatics. With the plasma model magnetic fields, except for a small part, can not pass through metals. i.e.~metals effectively become superconductors. However, recent experimental results clearly favor the plasma model. We shortly discuss a possible explanation for this apparent conflict with electrostatics.

quant-ph↗

arXiv2016

Temperature Variation in the Dark Cosmic Fluid in the Late Universe

A one-component dark energy fluid model of the late universe is considered ($w<-1$) when the fluid, initially assumed laminar, makes a transition into a turbulent state of motion. Spatial isotropy is assumed so that only the bulk viscosities are included ($ζ$ in the laminar epoch and $ζ_{\rm turb}$ in the turbulent epoch). Both viscosities are assumed to be constants. We derive a formula, new as far as we know, for the time dependence of the temperature $T(t)$ in the laminar case when viscosity is included. Assuming that the laminar/turbulent transition takes place at some time $t_s$ before the big rip is reached, we then analyze the positive temperature jump experienced by the fluid at $t=t_*$ if $ζ_{\rm turb} > ζ$. This is just as one would expect physically. The corresponding entropy production is also considered. A special point emphasized in the paper is the analogy that exists between the cosmic fluid and a so-called Maxwell fluid in viscoelasticity.

gr-qc↗

arXiv2015

Null-Field Radiationless Sources: Comment

We elaborate upon a recent paper of E. Hurwitz and G. Gbur [Opt. Lett. {\bf 39}, 6529 (2014)], dealing with null-field sources. After commenting on their theory in Minkowski space, we show how the theory can be conveniently generalized to the case of curvilinear space.

physics.optics↗

arXiv2015

Viscosity-Induced Crossing of the Phantom Barrier

We show explicitly, by using astrophysical data plus reasonable assumptions for the bulk viscosity in the cosmic fluid, how the magnitude of this viscosity may be high enough to drive the fluid from its position in the quintessence region at present time $t=0$ across the barrier $w=-1$ into the phantom region in the late universe. The phantom barrier is accordingly not a sharp mathematical divide, but rather a fuzzy concept. We also calculate the limiting forms of various thermodynamical quantities, including the rate of entropy production, for a dark energy fluid near the future Big Rip singularity.

gr-qc↗

arXiv2015

Anisotropic contribution to the van der Waals and the Casimir-Polder energies for CO$_2$ and CH$_4$ molecules near surfaces and thin films

In order to understand why carbon dioxide (CO$_2$) and methane (CH$_4$) molecules interact differently with surfaces, we investigate the Casimir-Polder energy of a linearly polarizable CO$_2$ molecule and an isotropically polarizable CH$_4$ molecule in front of an atomically thin gold film and an amorphous silica slab. We quantitatively analyze how the anisotropy in the polarizability of the molecule influences the van der Waals contribution to the binding energy of the molecule.

cond-mat.mes-hall↗

arXiv2015

Inhomogeneous Dark Fluid and Dark Matter, Leading to a Bounce Cosmology

The purpose of this short review is to describe cosmological models with a linear inhomogeneous time-dependent equation of state (EoS) for the dark energy, when the dark fluid is coupled with dark matter. This may lead to a bounce cosmology. We consider equivalent descriptions in terms of the EoS parameters for an exponential, a power-law, or a double-exponential law for the scale factor $a$. Stability issues are discussed by considering small perturbations around the critical points for the bounce, in the early as well as in the late, universe. The latter part of the paper is concerned with dark energy coupled with dark matter in viscous fluid cosmology. We allow the bulk viscosity $ζ=ζ(H,t)$ to be a function of the Hubble parameter and the time, and consider the Little Rip, the Pseudo Rip, and the bounce universe. Analytic expressions for characteristic properties of these cosmological models are obtained.

gr-qc↗

arXiv2015

Casimir-Polder repulsion: Three-body effects

In this paper we study an archetypical scenario in which repulsive Casimir-Polder forces between an atom or molecule and two macroscopic bodies can be achieved. This is an extension of previous studies of the interaction between a polarizable atom and a wedge, in which repulsion occurs if the atom is sufficiently anisotropic and close enough to the symmetry plane of the wedge. A similar repulsion occurs if such an atom passes a thin cylinder or a wire. An obvious extension is to compute the interaction between such an atom and two facing wedges, which includes as a special case the interaction of an atom with a conducting screen possessing a slit, or between two parallel wires. To this end we further extend the electromagnetic multiple-scattering formalism for three-body interactions. To test this machinery we reinvestigate the interaction of a polarizable atom between two parallel conducting plates. In that case, three-body effects are shown to be small, and are dominated by three- and four-scattering terms. The atom-wedge calculation is illustrated by an analogous scalar situation, described in the Appendix. The wedge-wedge-atom geometry is difficult to analyze because this is a scale-free problem. But it is not so hard to investigate the three-body corrections to the interaction between an anisotropic atom or nanoparticle and a pair of parallel conducting cylinders, and show that the three-body effects are very small and do not affect the Casimir-Polder repulsion at large distances between the cylinders. Finally, we consider whether such highly anisotropic atoms needed for repulsion are practically realizable. Since this appears rather difficult to accomplish, it may be more feasible to observe such effects with highly anisotropic nano particles.

quant-ph↗

arXiv2014

Brane Viscous Cosmology in the Plasma Era

We consider how the five-dimensional Randall-Sundrum (one-brane) theory becomes modified when account is taken of the bulk viscosity of the cosmic fluid on the brane. We focus on the plasma era between $10^{12}$K (muon pair annihilation) to about $5\times 10^9$K (electron-positron annihilation), which includes the first order quark-hadron transition beginning at an energy density of about $5\times 10^9\rm MeV^4$. Various possibilities are examined for modeling the bulk viscosity, preference being at the end given to the results calculated from relativistic kinetic theory. According to this, the viscosity is negligible at the highest temperatures, but may amount to a few per cent corrections in the later stages of the plasma era. We also briefly consider anisotropic universes where the shear viscosity comes into play, and show that in the case of the Kasner model the influences from bulk viscosity and shear viscosity become comparable when the anisotropy parameter of the universe is of order $A \sim 10^{-11} $ in the beginning of the plasma era, and $A \sim 10^{-2}$ in its later region.

gr-qc↗

arXiv2014

Relativistic Viscous Universe Models

The research on relativistic universe models with viscous fluids is reviewed. Viscosity may have been of significance during the early inflationary era, and may also be of importance for the late time evolution of the Universe. Bulk viscosity and shear viscosity cause exponential decay of anisotropy, while nonlinear viscosity causes power-law decay of anisotropy. We consider also the influence from turbulence, in connection with future singularities of the universe (Big Rip and Little Rip). Finally, we review some recent developments of causal cosmology theories.

gr-qc↗

arXiv2014

Casimir friction: Relative motion more generally

This paper extends our recent study on Casimir friction forces for dielectric plates moving parallel to each other [J. S. Høye and I. Brevik, Eur. Phys. J. D {\bf 68}, 61 (2014)], to the case where the plates are no longer restricted to rectilinear motion. Part of the mathematical formalism thereby becomes more cumbersome, but reduces in the end to the form that we could expect to be the natural one in advance. As an example, we calculate the Casimir torque on a planar disc rotating with constant angular velocity around its vertical symmetry axis next to another plate.

quant-ph↗

arXiv2014

Casimir forces in a Plasma: Possible Connections to Yukawa Potentials

We present theoretical and numerical results for the screened Casimir effect between perfect metal surfaces in a plasma. We show how the Casimir effect in an electron-positron plasma can provide an important contribution to nuclear interactions. Our results suggest that there is a connection between Casimir forces and nucleon forces mediated by mesons. Correct nuclear energies and meson masses appear to emerge naturally from the screened Casimir-Lifshitz effect.

quant-ph↗

arXiv2014

Casimir friction at zero and finite temperatures

The Casimir friction problem for dielectric plates that move parallel to each other is treated by assuming one of the plates to be at rest. The other performs a closed loop motion in the longitudinal direction. Therewith by use of energy dissipation the formalism becomes more manageable and transparent than in the conventional setting where uniform sliding motion is assumed from $t=-\infty$ to $t=+\infty$. One avoids separating off a reversible interparticle force (independent of friction) from the total force. Moreover, the cases of temperatures $T=0$ and finite $T$ are treated on the same footing. For metal plates we find the friction force to be proportional to $v^3$ at $T=0$ while at finite $T$ it is proportional to $v$ for small $v$ as found earlier. Comparisons with earlier results of Pendry (1997, 2010) and Barton (2011) are made.

quant-ph↗

arXiv2014

Explanation for the Transverse Radiation Force Observed on a Vertically Hanging Fiber

As shown in the experiment of She {\it et al.} [Phys. Rev. Lett. {\bf 101}, 243601 (2008)], a weak laser beam sent through a vertically hanging fiber exerts a transverse force and produces a lateral displacement of the fiber's lower end. The experiment is of obvious theoretical interest in connection with the electromagnetic theory of media. Suggested explanations given for this effect in the past include the famous Abraham-Minkowski issue concerning the "correct" photon momentum in matter. In our opinion such an explanation can hardly be right. Instead, we propose instead a very simple description of the effect implying that the sideways deflection is caused by the radiation force on the {\it obliquely cut} lower end face of the fiber. From a calculation based upon geometrical optics, we find quite good agreement with the observations. We present also, as an alternative approach, a calculation involving wave optics instead of geometrical optics, and find comparable results.

physics.optics↗

arXiv2014

How linear surface waves are affected by a current with constant vorticity

The interaction of surface waves with Couette-type current with uniform vorticity is a well suited problem for students approaching the theory of surface waves. The problem, although mathematically simple, contains rich physics, and is moreover important in several situations from oceanography and marine technology to microfluidics. We here lay out a simple two-dimensional theory of waves propagating upon a basic flow of uniform vorticity of constant depth. The dispersion relation is found, showing how the shearing current introduces different phase velocities for upstream and downstream propagating waves. The role of surface tension is discussed and applied to the case of a wave pattern created by a moving source, stationary as seen by the source. We conclude by discussing how the average potential and kinetic energies are no longer equal in the presence of shear.

physics.flu-dyn↗

arXiv2013

Temperature Dependence of the Casimir Force

The Casimir force - at first a rather unexpected consequence of quantum electrodynamics - was discovered by Hendrik Casimir in Eindhoven in 1948. It predicts that two uncharged metal plates experience an attractive force because of the zero-point fluctuations of the electromagnetic field. The idea was tested experimentally in the 1950's and 1960's, but the results were not so accurate that one could make a definite conclusion regarding the existence of the effect. Evgeny Lifshitz expanded the theory in 1955 so as to deal with general dielectric media. Much experimental work has later been done to test the theory's predictions, especially with regards to the temperature dependence of the effect. The existence of the effect itself was verified beyond doubt by Sabisky and Anderson in 1973. Another quarter century had to pass before Lamoreaux and collaborators were able to confirm - or at least make plausible - the temperature dependence predicted by Lifshitz formula in combination with reasonable input data for the material's dispersive properties. The situation is not yet clear-cut, however; there are recent experiments indicating results in disagreement with those of Lamoreaux. In the present paper a brief review is given of the status of this research field.

quant-ph↗

arXiv2013

Viscosity-Induced Crossing of the Phantom Divide in the Dark Cosmic Fluid

Choosing various natural forms for the equation-of-state parameter w and the bulk viscosity ζ, we discuss how it is possible for a dark energy fluid to slide from the quintessence region across the divide w=-1 into the phantom region, and thus into a Big Rip future singularity. Different analytic forms for ζ, as powers of the scalar expansion, are suggested and compared with experiments.

gr-qc↗

arXiv2013

Casimir Friction for Media of Finite Density

This work is a continuation of our papers from the last couple of years on the Casimir friction for a pair of particles at low relative velocity. The new element in the present analysis is to allow the media to be dense. Then the situation becomes more complex due to induced dipolar correlations, both within the two planes, and between the planes. We show that the structure of the problem can be simplified by regarding the two plates to be a generalized version of a pair of particles. The force is predicted to be very small, far beyond what is practically measurable.

quant-ph↗