The Pseudo-continuum Bound-free Opacity of Hydrogen and its Importance in Cool White Dwarf Atmospheres

We investigate the importance of the pseudo-continuum bound-free opacity from hydrogen atoms in the atmospheres of cool white dwarfs. This source of absorption, when calculated by the occupation probability formalism applied in the modeling of white dwarf atmospheres with $T_{\rm eff}\rm <17000 K$, dominates all other sources of opacity at optical wavelengths. This is unrealistic and not observed. On the other hand, a significant flux suppression in the blue part of the spectra of cool white dwarfs has been reported, and mainly interpreted as a result of the pseudo-continuum absorption from atomic hydrogen. We investigate this problem by proposing a new, more realistic approach to calculating this source of opacity. We show that this absorption is orders of magnitude smaller than that predicted by current methods. Therefore, we rule out the pseudo-continuum opacity as a source of the flux deficiency observed in the spectra of cool white dwarfs.Comment: 11 pages, 5 gigures, accepted for publication in the Astrophysical Journa

Homogeneous geodesics of non-unimodular Lorentzian Lie groups and naturally reductive Lorentzian spaces in dimension three

We determine, for all three-dimensional non-unimodular Lie groups equipped with a Lorentzian metric, the set of homogeneous geodesics through a point. Together with the results of [C] and [CM2], this leads to the full classification of three-dimensional Lorentzian g.o. spaces and naturally reductive spaces

Coherent states for a particle on a sphere

The coherent states for a particle on a sphere are introduced. These states are labelled by points of the classical phase space, that is the position on the sphere and the angular momentum of a particle. As with the coherent states for a particle on a circle discussed in Kowalski K {\em et al} 1996 {\em J. Phys. A} {\bf 29} 4149, we deal with a deformation of the classical phase space related with quantum fluctuations. The expectation values of the position and the angular momentum in the coherent states are regarded as the best possible approximation of the classical phase space. The correctness of the introduced coherent states is illustrated by an example of the rotator.Comment: LaTeX, 16 pages, 2 figure

Twisted Covariance and Weyl Quantisation

In this letter we wish to clarify in which sense the tensor nature of the commutation relations [x^mu,x^nu]=i theta ^{mu nu} underlying Minkowski spacetime quantisation cannot be suppressed even in the twisted approach to Lorentz covariance. We then address the vexata quaestio "why theta"?Comment: 6 pages, pdf has active hyperlinks Slight change in title. Appendix added on more general coordinates for symbols. References added. To appear in the Proceedings of the XXV Max Born Symposium, Wroclaw, June 29-July 3, 200

Cosmological consequences of the NonCommutative Geometry Spectral Action

Cosmological consequences of the noncommutative geometry spectral action are presented. Neglecting the nonminimal coupling of the Higgs field to the curvature, background cosmology remains unchanged, and only the inhomogeneous perturbations will evolve differently from the equivalent classical system. However, considering the nonminimal coupling, corrections will be obtained even at the level of the background cosmologies. Finally, the Higgs field may act as an inflaton field, due to its nonminimal coupling with geometry.Comment: 7 pages, LaTeX. Invited talk at the XXV Max Bonn Symposium "Physics at the Planck Scale", in Wroclaw (Poland), 29th June-3rd July 2009. To appear in the Conference Proceeding

Fun from none: deformed symmetries and Fock space

We give a pedagogical introduction to the basics of deformations of relativistic symmetries and the Hilbert spaces of free quantum fields built as their representations. We focus in particular on the example of a $\kappa$-deformed scalar quantum field for which the generators of spatial translations that label the field modes act according to a deformed Leibnitz rule. We explore the richer structure of the $\kappa$-Fock space and point out possible physical consequences of the deformation.Comment: 7 pages, no figures. Invited talk at XXV Max Born Symposium, The Planck scale, Wroclaw (Poland), June 29 - July 3, 2009. To appear in the Proceeding

AdS--Maxwell superalgebra and supergravity

In this paper we derive the Anti de Sitter counterpart of the super-Maxwell algebra presented recently by Bonanos et.\ al. Then we gauge this algebra and derive the corresponding supergravity theory, which turns out to be described by the standard N=1 supergravity lagrangian, up to topological terms.Comment: 8 pages, in v2 reference adde

Quantum geometry and quantum dynamics at the Planck scale

Canonical quantum gravity provides insights into the quantum dynamics as well as quantum geometry of space-time by its implications for constraints. Loop quantum gravity in particular requires specific corrections due to its quantization procedure, which also results in a discrete picture of space. The corresponding changes compared to the classical behavior can most easily be analyzed in isotropic models, but perturbations around them are more involved. For one type of corrections, consistent equations have been found which shed light on the underlying space-time structure at the Planck scale: not just quantum dynamics but also the concept of space-time manifolds changes in quantum gravity. Effective line elements provide indications for possible relationships to other frameworks, such as non-commutative geometry.Comment: 10 pages, 2 figures, Proceedings of "The Planck Scale" (XXV Max Born Symposium, Wroclaw