Iver Brevik | Norwegian University of Science and Technology (original) (raw)

Papers by Iver Brevik

Entropy, Jul 29, 2013

The present paper-a continuation of our recent series of papers on Casimir friction for a pair of... more The present paper-a continuation of our recent series of papers on Casimir friction for a pair of particles at low relative particle velocity-extends the analysis, so as to include dense media. The situation becomes, in this case, more complex, due to induced dipolar correlations, both within planes and between planes. We show that the structure of the problem can be simplified by regarding the two half-planes as a generalized version of a pair of particles. It turns out that macroscopic parameters, such as permittivity, suffice to describe the friction, also in the finite density case. The expression for the friction force per unit surface area becomes mathematically well-defined and finite at finite temperature. We give numerical estimates and compare them with those obtained earlier by Pendry (1997) and by Volokitin and Persson (2007). We also show in an appendix how the statistical methods that we are using correspond to the field theoretical methods more commonly in use.

Physical Review E, May 3, 2005

The temperature dependence of the Casimir force between a real metallic plate and a metallic sphe... more 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 not inconsistent with current experiments. However, the predicted temperature dependence should be detectable in future experiments. The inadequacies of approaches based on ad hoc assumptions, such as the plasma dispersion relation and the use of surface impedance without transverse momentum dependence, are discussed.

Physical Review E, Jul 6, 2009

The statistical mechanical approach to Casimir problems for dielectrics separated by a vacuum gap... more The statistical mechanical approach to Casimir problems for dielectrics separated by a vacuum gap turns out to be compact and effective. A central ingredient of this method is the effect of interacting fluctuating dipole moments of the polarizable particles. At arbitrary temperature the path integral formulation of quantized particles, developed by Høye-Stell and others, is needed. At high temperature-the limit considered in the present paper-the classical theory is however sufficient. Our present theory is related to an idea put forward earlier by Jancovici andŠamaj (2004), namely to evaluate the Casimir force between parallel plates invoking an electronic plasma model and the Debye-Hückel theory for electrolytes. Their result was recently recovered by Høye (2008), using a related statistical mechanical method. In the present paper we generalize this by including a constant permittivity in the description. The present paper generalizes our earlier theory for parallel plates (1998), as well as for spherical dielectrics (2001). We also consider the Casimir force between a polarizable particle and a conductor with a small density of charges, finding agreement with the result recently derived by Pitaevskii (2008).

Physica Scripta, Apr 9, 2013

In the increasingly active field of optofluidics, a series of experiments involving near-critical... more In the increasingly active field of optofluidics, a series of experiments involving near-critical two-fluid interfaces have shown a number of interesting non-linear effects. We here offer, for the first time to our knowledge, an explanation for one such feature, observed in experiments by Casner and Delville [Phys. Rev. Lett. 90, 144503 (2003)], namely the sudden formation of "shoulder"-like shapes in a laser-induced deformation of the liquid-liquid interface at high laser power. Two candidate explanations are the following: firstly, that the shape can be explained by balancing forces of buoyancy, laser pull and surface tension only, and that the observed change of deformation shape is the sudden jump from one solution of the strongly nonlinear governing differential equation to another. Secondly, it might be that the nontrivial shape observed could be the result of temperature gradients due to local absorptive heating of the liquid. We report that a systematic search for solutions of the governing equation in the first case yields no trace of solutions containing such features. By contrast, an investigation of the second option shows that the narrow shape of the tip of the deformation can be explained by a slight heating of the liquids. The local heating amounts to a few kelvins, with the parameters given, although there are uncertainties here. Our investigations suggest that local temperature variations are the crucial element behind the instability and the shoulder-like deformation.

Physics of Fluids, Sep 1, 2011

Recent times have seen surge of research activity on systems combining fluid mechanics and electr... more Recent times have seen surge of research activity on systems combining fluid mechanics and electromagnetic fields. In radiation optics, whenever information about the distribution of pressure in a dielectric fluid is required, the contribution from electrostriction becomes important. In the present paper we calculate how the local pressure varies with position and time when a laser beam is imposed in a uniform fluid. A Gaussian intensity profile of arbitrary time dependence is assumed for the beam, and general results are derived in this case. For demonstration we analyze two different cases: first, that the beam is imposed suddenly (mathematically in the form of a step function); secondly, that the beam is switched on in a soft way. In both cases, simple analytical expressions for the pressure distribution are found.

arXiv (Cornell University), Sep 30, 2021

The Casimir-Lifhitz force acts between neutral material bodies and is due to the fluctuations (ar... more The Casimir-Lifhitz force acts between neutral material bodies and is due to the fluctuations (around zero) of the electrical polarizations of the bodies. This force is a macroscopic manifestation of the van der Waals forces between atoms and molecules. In addition to being of fundamental interest, the Casimir-Lifshitz force plays an important role in surface physics, nanotechnology and biophysics. There are two different approaches in the theory of this force. One is centered on the fluctuations inside the bodies, as the source of the fluctuational electromagnetic fields and forces. The second approach is based on finding the eigenmodes of the field, while the material bodies are assumed to be passive and non-fluctuating. In spite of the fact that both approaches have a long history, there are still some misconceptions in the literature. In particular, there are claims that (hypothetical) materials with a strictly real dielectric function ε(ω) can give rise to fluctuational Casimir-Lifshitz forces. We review and compare the two approaches, using the simple example of the force in the absence of retardation. We point out that also in the second (the "field-oriented") approach one cannot avoid introducing an infinitesimal imaginary part into the dielectric function, i.e. introducing some dissipation. Furthermore, we emphasize that the requirement of analyticity of ε(ω) in the upper half of the complex ω plane is not the only one for a viable dielectric function. There are other requirements as well. In particular, models that use a strictly real ε(ω) (for all real positive ω) are inadmissible and lead to various contradictions and inconsistencies. Specifically, we present a critical discussion of the "dissipation-less plasma model". Our emphasis is not on the most recent developments in the field but on some conceptual, not fully resolved issues.

arXiv (Cornell University), Dec 23, 2020

Theoretical models and experimental observations suggest that gamma-ray bursts (GRB) and high-ene... more Theoretical models and experimental observations suggest that gamma-ray bursts (GRB) and high-energy neutrino bursts travelling through the interstellar space may reach the Earth at different speeds. We propose and study in details the mechanism i), which always exists, where GRB are slowed down due to the dispersion of light in the interstellar medium. In addition to the standard media such as electrons and photons as CMB, we consider the medium with invisible axions. The amount of GRB delays in different media are calculated in details utilizing a novel technique in QFT by using the hitherto known or estimated densities of particles in the space without introducing any arbitrary parameter. Previously, the GRB delays have been interpreted as a sign of Lorentz invariance violation by modifying the dispersion relation of Special Relativity, which relates the energy, the momentum and the mass of a particle, based on different mechanisms ii), such as a stringy spacetime foam, coming from a quantum gravity effect and using an adjustable parameter. Obviously, all the above-mentioned mechanisms i) and ii) are induced (seeming) Lorentz invariance violations but not an intrinsic (genuine) one. The amount of GRB delay due to the two aforementioned interpretations can be distinguished by observing the time of arrival of light with different frequencies. Namely, dispersion of light i) predicts that the higher energy GRB arrive the Earth earlier, while in the other interpretations ii), they arrive later. We notice that the needed amount for delay due to the dispersion of light shall have the potential power to shed additional light on the microstructure of interstellar media with respect to the densities of constituent particles and the origins of their sources. Finally, we indicate the ways to detect the intrinsic Lorentz invariance violation and to interpret them theoretically.

arXiv (Cornell University), Oct 18, 2013

This work is a continuation of our papers from the last couple of years on the Casimir friction f... more 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.

arXiv (Cornell University), Dec 10, 2018

We consider an open string with ends laying on the two different solid beams (rods). This setup i... more We consider an open string with ends laying on the two different solid beams (rods). This setup is equivalent to two scalar fields with a set of constraints at their end-points. We calculate the zero-point energy and the Casimir energy in three different ways: (1) by use of the Hurwitz zeta function, (2) by employing the contour integration method in the complex frequency plane, and (3) by constructing the Green's function for the system. In the case of contour integration we also present a finite temperature expression for the Casimir energy, along with a convenient analytic approximation for high temperatures. The Casimir energy at zero temperature is found to be a sum of the Lüscher potential energy and a term depending on the angle between the beams. The relationship of this model to an analogous open string model with charges fixed at its ends, moving in an electromagnetic field, is discussed.

arXiv (Cornell University), Jan 17, 2012

The main part of this paper is to present an updated review of the Casimir energy at zero and fin... more The main part of this paper is to present an updated review of the Casimir energy at zero and finite temperature for the transverse oscillations of a piecewise uniform closed string. We make use of three different regularizations: the cutoff method, the complex contour integration method, and the zeta-function method. The string model is relativistic, in the sense that the velocity of sound is for each string piece set equal to the velocity of light. In this sense the theory is analogous to the electromagnetic theory in a dielectric medium in which the product of permittivity and permeability is equal to unity (an isorefractive medium). We demonstrate how the formalism works for a two-piece string, and for a 2N-piece string, and show how in the latter case a compact recursion relation serves to facilitate the formalism considerably. The Casimir energy turns out to be negative, and the more so the larger the number of pieces in the string. The two-piece string is quantized in D-dimensional spacetime, in the limit when the ratio between the two tensions is very small. We calculate the free energy and other thermodynamic quantities, demonstrate scaling properties, and comment finally on the meaning of the Hagedorn critical temperature for the two-piece string.Thereafter, as a novel development we present a scalar field theory for a real field in three-dimensional space in a potential rising linearly with a longitudinal coordinate z in the interval 0 < z < 1, and which is thereafter held constant on a horizontal plateau. The potential is taken as a rough

arXiv (Cornell University), Nov 7, 2008

Analytic properties of physical quantities in the cosmic fluid such as energy density ρ(t) and Hu... more 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 α with α a constant. If ζ is 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.

Journal of Mathematical Physics, Mar 1, 2003

The thermodynamic free energy F (β) is calculated for a gas consisting of the transverse oscillat... more The thermodynamic free energy F (β) is calculated for a gas consisting of the transverse oscillations of a piecewise uniform bosonic string. The string consists of 2N parts of equal length, of alternating type I and type II material, and is relativistic in the sense that the velocity of sound everywhere equals the velocity of light. The present paper is a continuation of two earlier papers, one dealing with the Casimir energy of a 2N-piece string [I. Brevik and R. Sollie (1997)], and another dealing with the thermodynamic properties of a string divided into two (unequal) parts [I. Brevik, A. A. Bytsenko and H. B. Nielsen (1998)]. Making use of the Meinardus theorem we calculate the asymptotics of the level state density, and show that the critical temperatures in the individual parts are equal, for arbitrary spacetime dimension D. If D = 26, we find β = (2/N) 2π/T II , T II being the tension in part II. Thermodynamic interactions of parts related to high genus g is also considered.

arXiv (Cornell University), Mar 27, 1999

The Casimir energy for a compact dielectric sphere is considered in a novel way, using the quantu... more The Casimir energy for a compact dielectric sphere is considered in a novel way, using the quantum statistical method introduced by Høye-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 G.

Physical review, Jun 23, 1998

We revisit two-point function approaches used to study vacuum fluctuation in wedge-shaped regions... more We revisit two-point function approaches used to study vacuum fluctuation in wedge-shaped regions and conical backgrounds. Appearance of divergent integrals is discussed and circumvented. The issue is considered in the context of a massless scalar field in cosmic string spacetime.

arXiv (Cornell University), Aug 16, 2001

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calcul... more 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 and the critical Hagedorn temperature is calculated for a two-piece string. Mass and decay spectra are calculated for quantum massive exitations and the physical meaning of the critical temperatures characterising the radiation in the decay of a massive microstate is discussed. Date: August, 2001. We would like to thank Profs. A.E. Gonçalves and H.B. Nielsen for useful discussions and suggestions. A.A.

arXiv (Cornell University), Feb 8, 2000

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calcul... more 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.

arXiv (Cornell University), Apr 21, 2004

The holographic principle in a radiation dominated universe, as discussed first by Verlinde [hep-... more The holographic principle in a radiation dominated universe, as discussed first by Verlinde [hep-th/008140], is extended so as to incorporate the case of a bulk-viscous cosmic fluid. This corresponds to a non-conformally invariant theory. Generalization of the Cardy-Verlinde entropy formula to the viscous case appears to be formally possible, although on physical grounds one may question some elements in this type of theory, especially the manner in which the Casimir energy is evaluated. Also, we consider the observation made by Youm [Phys. Lett. B531, 276 (2002)], namely that the entropy of the universe is no longer expressible in the conventional Cardy-Verlinde form if one relaxes the radiation dominance equation of state for the fluid and instead merely assumes that the pressure is proportional to the energy density. We show that Youm's generalized entropy formula remains valid when the cosmic fluid is no longer ideal, but endowed with a constant bulk viscosity. In the introductory part of this article, we take a rather general point of view and survey the essence of cosmological theory applied to a fluid containing both a constant shear viscosity and a constant bulk viscosity.

Journal of Experimental and Theoretical Physics, Apr 1, 2016

We consider the inflation produced by two coupled fluids in the flat Friedmann-Robertson-Walker u... more We consider the inflation produced by two coupled fluids in the flat Friedmann-Robertson-Walker universe. Different cosmological models for describing inflation by use of an inhomogeneous equation of state for the fluid are investigated. The gravitational equations for energy and matter are solved, and analytic representations for the Hubble parameter and the energy density are obtained. Corrections in the energy density for matter inducing the inflation and the coupling with energy are discussed. We analyze the description of inflation induced by non-constant equation-of-state parameters from fluid viscosity. The correspondence between the spectral index and the tensor-to-scalar ratio recently observed by the Planck satellite is considered.

Physical review, Jan 29, 2004

The presence of a bulk viscosity for the cosmic fluid on a single Randall-Sundrum brane is consid... more 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.

European Physical Journal D, Mar 1, 2014

The Casimir friction problem for dielectric plates that move parallell to each other is treated b... more The Casimir friction problem for dielectric plates that move parallell 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 = −∞ to t = +∞. 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.

Entropy, Jul 29, 2013

The present paper-a continuation of our recent series of papers on Casimir friction for a pair of... more The present paper-a continuation of our recent series of papers on Casimir friction for a pair of particles at low relative particle velocity-extends the analysis, so as to include dense media. The situation becomes, in this case, more complex, due to induced dipolar correlations, both within planes and between planes. We show that the structure of the problem can be simplified by regarding the two half-planes as a generalized version of a pair of particles. It turns out that macroscopic parameters, such as permittivity, suffice to describe the friction, also in the finite density case. The expression for the friction force per unit surface area becomes mathematically well-defined and finite at finite temperature. We give numerical estimates and compare them with those obtained earlier by Pendry (1997) and by Volokitin and Persson (2007). We also show in an appendix how the statistical methods that we are using correspond to the field theoretical methods more commonly in use.

Physical Review E, May 3, 2005

The temperature dependence of the Casimir force between a real metallic plate and a metallic sphe... more 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 not inconsistent with current experiments. However, the predicted temperature dependence should be detectable in future experiments. The inadequacies of approaches based on ad hoc assumptions, such as the plasma dispersion relation and the use of surface impedance without transverse momentum dependence, are discussed.

Physical Review E, Jul 6, 2009

The statistical mechanical approach to Casimir problems for dielectrics separated by a vacuum gap... more The statistical mechanical approach to Casimir problems for dielectrics separated by a vacuum gap turns out to be compact and effective. A central ingredient of this method is the effect of interacting fluctuating dipole moments of the polarizable particles. At arbitrary temperature the path integral formulation of quantized particles, developed by Høye-Stell and others, is needed. At high temperature-the limit considered in the present paper-the classical theory is however sufficient. Our present theory is related to an idea put forward earlier by Jancovici andŠamaj (2004), namely to evaluate the Casimir force between parallel plates invoking an electronic plasma model and the Debye-Hückel theory for electrolytes. Their result was recently recovered by Høye (2008), using a related statistical mechanical method. In the present paper we generalize this by including a constant permittivity in the description. The present paper generalizes our earlier theory for parallel plates (1998), as well as for spherical dielectrics (2001). We also consider the Casimir force between a polarizable particle and a conductor with a small density of charges, finding agreement with the result recently derived by Pitaevskii (2008).

Physica Scripta, Apr 9, 2013

In the increasingly active field of optofluidics, a series of experiments involving near-critical... more In the increasingly active field of optofluidics, a series of experiments involving near-critical two-fluid interfaces have shown a number of interesting non-linear effects. We here offer, for the first time to our knowledge, an explanation for one such feature, observed in experiments by Casner and Delville [Phys. Rev. Lett. 90, 144503 (2003)], namely the sudden formation of "shoulder"-like shapes in a laser-induced deformation of the liquid-liquid interface at high laser power. Two candidate explanations are the following: firstly, that the shape can be explained by balancing forces of buoyancy, laser pull and surface tension only, and that the observed change of deformation shape is the sudden jump from one solution of the strongly nonlinear governing differential equation to another. Secondly, it might be that the nontrivial shape observed could be the result of temperature gradients due to local absorptive heating of the liquid. We report that a systematic search for solutions of the governing equation in the first case yields no trace of solutions containing such features. By contrast, an investigation of the second option shows that the narrow shape of the tip of the deformation can be explained by a slight heating of the liquids. The local heating amounts to a few kelvins, with the parameters given, although there are uncertainties here. Our investigations suggest that local temperature variations are the crucial element behind the instability and the shoulder-like deformation.

Physics of Fluids, Sep 1, 2011

Recent times have seen surge of research activity on systems combining fluid mechanics and electr... more Recent times have seen surge of research activity on systems combining fluid mechanics and electromagnetic fields. In radiation optics, whenever information about the distribution of pressure in a dielectric fluid is required, the contribution from electrostriction becomes important. In the present paper we calculate how the local pressure varies with position and time when a laser beam is imposed in a uniform fluid. A Gaussian intensity profile of arbitrary time dependence is assumed for the beam, and general results are derived in this case. For demonstration we analyze two different cases: first, that the beam is imposed suddenly (mathematically in the form of a step function); secondly, that the beam is switched on in a soft way. In both cases, simple analytical expressions for the pressure distribution are found.

arXiv (Cornell University), Sep 30, 2021

The Casimir-Lifhitz force acts between neutral material bodies and is due to the fluctuations (ar... more The Casimir-Lifhitz force acts between neutral material bodies and is due to the fluctuations (around zero) of the electrical polarizations of the bodies. This force is a macroscopic manifestation of the van der Waals forces between atoms and molecules. In addition to being of fundamental interest, the Casimir-Lifshitz force plays an important role in surface physics, nanotechnology and biophysics. There are two different approaches in the theory of this force. One is centered on the fluctuations inside the bodies, as the source of the fluctuational electromagnetic fields and forces. The second approach is based on finding the eigenmodes of the field, while the material bodies are assumed to be passive and non-fluctuating. In spite of the fact that both approaches have a long history, there are still some misconceptions in the literature. In particular, there are claims that (hypothetical) materials with a strictly real dielectric function ε(ω) can give rise to fluctuational Casimir-Lifshitz forces. We review and compare the two approaches, using the simple example of the force in the absence of retardation. We point out that also in the second (the "field-oriented") approach one cannot avoid introducing an infinitesimal imaginary part into the dielectric function, i.e. introducing some dissipation. Furthermore, we emphasize that the requirement of analyticity of ε(ω) in the upper half of the complex ω plane is not the only one for a viable dielectric function. There are other requirements as well. In particular, models that use a strictly real ε(ω) (for all real positive ω) are inadmissible and lead to various contradictions and inconsistencies. Specifically, we present a critical discussion of the "dissipation-less plasma model". Our emphasis is not on the most recent developments in the field but on some conceptual, not fully resolved issues.

arXiv (Cornell University), Dec 23, 2020

Theoretical models and experimental observations suggest that gamma-ray bursts (GRB) and high-ene... more Theoretical models and experimental observations suggest that gamma-ray bursts (GRB) and high-energy neutrino bursts travelling through the interstellar space may reach the Earth at different speeds. We propose and study in details the mechanism i), which always exists, where GRB are slowed down due to the dispersion of light in the interstellar medium. In addition to the standard media such as electrons and photons as CMB, we consider the medium with invisible axions. The amount of GRB delays in different media are calculated in details utilizing a novel technique in QFT by using the hitherto known or estimated densities of particles in the space without introducing any arbitrary parameter. Previously, the GRB delays have been interpreted as a sign of Lorentz invariance violation by modifying the dispersion relation of Special Relativity, which relates the energy, the momentum and the mass of a particle, based on different mechanisms ii), such as a stringy spacetime foam, coming from a quantum gravity effect and using an adjustable parameter. Obviously, all the above-mentioned mechanisms i) and ii) are induced (seeming) Lorentz invariance violations but not an intrinsic (genuine) one. The amount of GRB delay due to the two aforementioned interpretations can be distinguished by observing the time of arrival of light with different frequencies. Namely, dispersion of light i) predicts that the higher energy GRB arrive the Earth earlier, while in the other interpretations ii), they arrive later. We notice that the needed amount for delay due to the dispersion of light shall have the potential power to shed additional light on the microstructure of interstellar media with respect to the densities of constituent particles and the origins of their sources. Finally, we indicate the ways to detect the intrinsic Lorentz invariance violation and to interpret them theoretically.

arXiv (Cornell University), Oct 18, 2013

This work is a continuation of our papers from the last couple of years on the Casimir friction f... more 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.

arXiv (Cornell University), Dec 10, 2018

We consider an open string with ends laying on the two different solid beams (rods). This setup i... more We consider an open string with ends laying on the two different solid beams (rods). This setup is equivalent to two scalar fields with a set of constraints at their end-points. We calculate the zero-point energy and the Casimir energy in three different ways: (1) by use of the Hurwitz zeta function, (2) by employing the contour integration method in the complex frequency plane, and (3) by constructing the Green's function for the system. In the case of contour integration we also present a finite temperature expression for the Casimir energy, along with a convenient analytic approximation for high temperatures. The Casimir energy at zero temperature is found to be a sum of the Lüscher potential energy and a term depending on the angle between the beams. The relationship of this model to an analogous open string model with charges fixed at its ends, moving in an electromagnetic field, is discussed.

arXiv (Cornell University), Jan 17, 2012

The main part of this paper is to present an updated review of the Casimir energy at zero and fin... more The main part of this paper is to present an updated review of the Casimir energy at zero and finite temperature for the transverse oscillations of a piecewise uniform closed string. We make use of three different regularizations: the cutoff method, the complex contour integration method, and the zeta-function method. The string model is relativistic, in the sense that the velocity of sound is for each string piece set equal to the velocity of light. In this sense the theory is analogous to the electromagnetic theory in a dielectric medium in which the product of permittivity and permeability is equal to unity (an isorefractive medium). We demonstrate how the formalism works for a two-piece string, and for a 2N-piece string, and show how in the latter case a compact recursion relation serves to facilitate the formalism considerably. The Casimir energy turns out to be negative, and the more so the larger the number of pieces in the string. The two-piece string is quantized in D-dimensional spacetime, in the limit when the ratio between the two tensions is very small. We calculate the free energy and other thermodynamic quantities, demonstrate scaling properties, and comment finally on the meaning of the Hagedorn critical temperature for the two-piece string.Thereafter, as a novel development we present a scalar field theory for a real field in three-dimensional space in a potential rising linearly with a longitudinal coordinate z in the interval 0 < z < 1, and which is thereafter held constant on a horizontal plateau. The potential is taken as a rough

arXiv (Cornell University), Nov 7, 2008

Analytic properties of physical quantities in the cosmic fluid such as energy density ρ(t) and Hu... more 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 α with α a constant. If ζ is 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.

Journal of Mathematical Physics, Mar 1, 2003

The thermodynamic free energy F (β) is calculated for a gas consisting of the transverse oscillat... more The thermodynamic free energy F (β) is calculated for a gas consisting of the transverse oscillations of a piecewise uniform bosonic string. The string consists of 2N parts of equal length, of alternating type I and type II material, and is relativistic in the sense that the velocity of sound everywhere equals the velocity of light. The present paper is a continuation of two earlier papers, one dealing with the Casimir energy of a 2N-piece string [I. Brevik and R. Sollie (1997)], and another dealing with the thermodynamic properties of a string divided into two (unequal) parts [I. Brevik, A. A. Bytsenko and H. B. Nielsen (1998)]. Making use of the Meinardus theorem we calculate the asymptotics of the level state density, and show that the critical temperatures in the individual parts are equal, for arbitrary spacetime dimension D. If D = 26, we find β = (2/N) 2π/T II , T II being the tension in part II. Thermodynamic interactions of parts related to high genus g is also considered.

arXiv (Cornell University), Mar 27, 1999

The Casimir energy for a compact dielectric sphere is considered in a novel way, using the quantu... more The Casimir energy for a compact dielectric sphere is considered in a novel way, using the quantum statistical method introduced by Høye-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 G.

Physical review, Jun 23, 1998

We revisit two-point function approaches used to study vacuum fluctuation in wedge-shaped regions... more We revisit two-point function approaches used to study vacuum fluctuation in wedge-shaped regions and conical backgrounds. Appearance of divergent integrals is discussed and circumvented. The issue is considered in the context of a massless scalar field in cosmic string spacetime.

arXiv (Cornell University), Aug 16, 2001

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calcul... more 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 and the critical Hagedorn temperature is calculated for a two-piece string. Mass and decay spectra are calculated for quantum massive exitations and the physical meaning of the critical temperatures characterising the radiation in the decay of a massive microstate is discussed. Date: August, 2001. We would like to thank Profs. A.E. Gonçalves and H.B. Nielsen for useful discussions and suggestions. A.A.

arXiv (Cornell University), Feb 8, 2000

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calcul... more 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.

arXiv (Cornell University), Apr 21, 2004

The holographic principle in a radiation dominated universe, as discussed first by Verlinde [hep-... more The holographic principle in a radiation dominated universe, as discussed first by Verlinde [hep-th/008140], is extended so as to incorporate the case of a bulk-viscous cosmic fluid. This corresponds to a non-conformally invariant theory. Generalization of the Cardy-Verlinde entropy formula to the viscous case appears to be formally possible, although on physical grounds one may question some elements in this type of theory, especially the manner in which the Casimir energy is evaluated. Also, we consider the observation made by Youm [Phys. Lett. B531, 276 (2002)], namely that the entropy of the universe is no longer expressible in the conventional Cardy-Verlinde form if one relaxes the radiation dominance equation of state for the fluid and instead merely assumes that the pressure is proportional to the energy density. We show that Youm's generalized entropy formula remains valid when the cosmic fluid is no longer ideal, but endowed with a constant bulk viscosity. In the introductory part of this article, we take a rather general point of view and survey the essence of cosmological theory applied to a fluid containing both a constant shear viscosity and a constant bulk viscosity.

Journal of Experimental and Theoretical Physics, Apr 1, 2016

We consider the inflation produced by two coupled fluids in the flat Friedmann-Robertson-Walker u... more We consider the inflation produced by two coupled fluids in the flat Friedmann-Robertson-Walker universe. Different cosmological models for describing inflation by use of an inhomogeneous equation of state for the fluid are investigated. The gravitational equations for energy and matter are solved, and analytic representations for the Hubble parameter and the energy density are obtained. Corrections in the energy density for matter inducing the inflation and the coupling with energy are discussed. We analyze the description of inflation induced by non-constant equation-of-state parameters from fluid viscosity. The correspondence between the spectral index and the tensor-to-scalar ratio recently observed by the Planck satellite is considered.

Physical review, Jan 29, 2004

The presence of a bulk viscosity for the cosmic fluid on a single Randall-Sundrum brane is consid... more 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.

European Physical Journal D, Mar 1, 2014

The Casimir friction problem for dielectric plates that move parallell to each other is treated b... more The Casimir friction problem for dielectric plates that move parallell 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 = −∞ to t = +∞. 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.