Geometric equations of state in Friedmann-Lema\^{i}tre universes admitting matter and Ricci Collineations

As a rule in General Relativity the spacetime metric fixes the Einstein tensor and through the Field Equations (FE) the energy-momentum tensor. However one cannot write the FE explicitly until a class of observers has been considered. Every class of observers defines a decomposition of the energy-momentum tensor in terms of the dynamical variables energy density ($\mu$), the isotropic pressure ($p$), the heat flux $q^a$ and the traceless anisotropic pressure tensor $\pi_{ab}$. The solution of the FE requires additional assumptions among the dynamical variables known with the generic name equations of state. These imply that the properties of the matter for a given class of observers depends not only on the energy-momentum tensor but on extra a priori assumptions which are relevant to that particular class of observers. This makes difficult the comparison of the Physics observed by different classes of observers for the {\it same} spacetime metric. One way to overcome this unsatisfactory situation is to define the extra condition required among the dynamical variables by a geometric condition, which will be based on the metric and not to the observers. Among the possible and multiple conditions one could use the consideration of collineations. We examine this possibility for the Friedmann-Lema\^{i}tre-Robertson-Walker models admitting matter and Ricci collineations and determine the equations of state for the comoving observers. We find linear and non-linear equations of state, which lead to solutions satisfying the energy conditions, therefore describing physically viable cosmological models.Comment: 14 pages, Latex; to appear in General Relativity and Gravitatio

Note on Matter Collineations in Kantowski-Sachs, Bianchi Types I and III Spacetimes

We show that the classification of Kantowski-Sachs, Bianchi Types I and III spacetimes admitting Matter Collineations (MCs) presented in a recent paper by Camci et al. [Camci, U., and Sharif, M. {Matter Collineations in Kantowski-Sachs, Bianchi Types I and III Spacetimes}, 2003 Gen. Relativ. Grav. vol. 35, 97-109] is incomplete. Furthermore for these spacetimes and when the Einstein tensor is non-degenerate, we give the complete Lie Algebra of MCs and the algebraic constraints on the spatial components of the Einstein tensor.Comment: 10 pages, Latex. Accepted for publication in General Relativity and Gravitatio

Acute visual loss and intraocular hemorrhages associated with endoscopic spinal surgery

Marilita M Moschos, Alexandros Rouvas, Alexios Papaspirou, Michael ApostolopoulosDepartment of Ophthalmology, University of Athens, GreecePurpose: To report a case of acute visual loss after endoscopic spinal surgery.Methods: A patient was immediately referred to our department after epidural endoscopy with severe bilateral visual impairment and underwent complete ophthalmologic examination.Results: Visual acuity was decreased in both eyes. Fundus examination revealed the presence of retinal and vitreous hemorrhages bilaterally. Four months later, visual acuity increased and the hemorrhages were remarkably resolved.Conclusion: The present case represents an extremely rare incidence of intraocular hemorrhage following epiduroscopy.Keywords: visual acuity, hemorrhage, intraocular, BCV