Casimir Effects Near the Big Rip Singularity in Viscous Cosmology

Analytical properties of the scalar expansion in the cosmic fluid are investigated, especially near the future singularity, when the fluid possesses a constant bulk viscosity \zeta. In addition, we assume that there is a Casimir-induced term in the fluid's energy-momentum tensor, in such a way that the Casimir contributions to the energy density and pressure are both proportional to 1/a^4, 'a' being the scale factor. A series expansion is worked out for the scalar expansion under the condition that the Casimir influence is small. Close to the Big Rip singularity the Casimir term has however to fade away and we obtain the same singular behavior for the scalar expansion, the scale factor, and the energy density, as in the Casimir-free viscous case.Comment: 7 pages RevTeX, no figures. Minor changes in discussion, some references added. To appear in Gen. Rel. Gra

Casimir energy of a non-uniform string

The Casimir energy of a non-uniform string built up from two pieces with different speed of sound is calculated. A standard procedure of subtracting the energy of an infinite uniform string is applied, the subtraction being interpreted as the renormalization of the string tension. It is shown that in the case of a homogeneous string this method is completely equivalent to the zeta renormalization.Comment: 11 pages, REVTeX, no figures and table

Software development methods and usability: Perspectives from a survey in the software industry in Norway

This paper investigates the relationship between traditional software development methodologies and usability. The point of departure is the assumption that two important disciplines in software development, one of software development methods (SDMs) and one of usability work, are not integrated in industrial software projects. Building on previous research we investigate two questions; (1) Will software companies generally acknowledge the importance of usability, but not prioritise it in industrial projects? and (2) To what degree are software development methods and usability perceived by practitioners as being integrated? To this end a survey in the Norwegian IT industry was conducted. From a sample of 259 companies we received responses from 78 companies. In response to our first research question, our findings show that although there is a positive bias towards usability, the importance of usability testing is perceived to be much less than that of usability requirements. Given the strong time and cost pressures associated with the software industry, we believe that these results highlight that there is a gap between intention and reality. Regarding our second research question our survey revealed that companies perceive usability and software development methods to be integrated. This is in contrast to earlier research, which, somewhat pessimistically, has argued for the existence of two different cultures, one of software development and one of usability. The findings give hope for the future, in particular because the general use of system development methods are pragmatic and adaptable

Casimir Surface Force on a Dilute Dielectric Ball

The Casimir surface force density F on a dielectric dilute spherical ball of radius a, surrounded by a vacuum, is calculated at zero temperature. We treat (n-1) (n being the refractive index) as a small parameter. The dispersive properties of the material are taken into account by adopting a simple dispersion relation, involving a sharp high frequency cutoff at omega = omega_0. For a nondispersive medium there appears (after regularization) a finite, physical, force F^{nondisp} which is repulsive. By means of a uniform asymptotic expansion of the Riccati-Bessel functions we calculate F^{nondisp} up to the fourth order in 1/nu. For a dispersive medium the main part of the force F^{disp} is also repulsive. The dominant term in F^{disp} is proportional to (n-1)^2{omega_0}^3/a, and will under usual physical conditions outweigh F^{nondisp} by several orders of magnitude.Comment: 24 pages, latex, no figures, some additions to the Acknowledments sectio

Shear Viscosity of Yang-Mills Theory in the Confinement Phase

In terms of a simple holographic model, we study the absorption cross section and the shear viscosity of a pure Yang-Mills field at low temperature where the system is in the confinement phase. Then we expect that the glueball states are the dominant modes in this phase. In our holographic model an infrared cutoff r_m is introduced as a parameter which fixes the lowest mass of the glueball. As a result the critical temperature of gluon confinement T_c is estimated to be about 127 MeV. For T < T_c, we find that both the absorption cross section and the shear viscosity are independent of the temperature. Their values are frozen at the values corresponding to the critical point, for 0 < T < T_c. We discuss this behavior by considering the glueball mass and its temperature dependence.Comment: 11 pages latex, 2 figures; minor changes in the discussion, reference added. To appear in Int. J. Mod. Phys.

Casimir energy of a dilute dielectric ball in the mode summation method

In the $(\epsilon_1-\epsilon_2)^2$--approximation the Casimir energy of a dilute dielectric ball is derived using a simple and clear method of the mode summation. The addition theorem for the Bessel functions enables one to present in a closed form the sum over the angular momentum before the integration over the imaginary frequencies. The linear in $(\epsilon_1-\epsilon_2)$ contribution into the vacuum energy is removed by an appropriate subtraction. The role of the contact terms used in other approaches to this problem is elucidated.Comment: 14 pages, REVTeX, no figures, no tables; presentation is made better, new references are adde

Mode-by-mode summation for the zero point electromagnetic energy of an infinite cylinder

Using the mode-by-mode summation technique the zero point energy of the electromagnetic field is calculated for the boundary conditions given on the surface of an infinite solid cylinder. It is assumed that the dielectric and magnetic characteristics of the material which makes up the cylinder $(\epsilon_1, \mu_1)$ and of that which makes up the surroundings $(\epsilon_2, \mu_2)$ obey the relation $\epsilon_1\mu_1= \epsilon_2\mu_2$. With this assumption all the divergences cancel. The divergences are regulated by making use of zeta function techniques. Numerical calculations are carried out for a dilute dielectric cylinder and for a perfectly conducting cylindrical shell. The Casimir energy in the first case vanishes, and in the second is in complete agreement with that obtained by DeRaad and Milton who employed a Green's function technique with an ultraviolet regulator.Comment: REVTeX, 16 pages, no figures and tables; transcription error in previous version corrected, giving a zero Casimir energy for a tenuous cylinde