Entre experiencia y estructura : sufrimiento social, identidades colectivas y justicia en Iris Marion Young

This paper analyzes the role of phenomenology in Iris Marion Young's model of critical theory through a discussion of the different strategies she mobilizes in articulating the notions of identity and social collectivities in Justice and the Politics of Difference (1990) and Inclusion and Democracy (2000). By reconstructing the debate Young had with Nancy Fraser during the 1990s, we seek to demonstrate that, although Fraser mischaracterizes Justice and Politics of Difference as representative of the “cultural turn” in social theory, her criticisms can illuminate some of the tensions and shortcomings of the text. Moreover, we argue that the emphasis in a structural-analytical strategy of argumentation, characteristic of Young's later work, can be traced back to the contentions formulated by Fraser. Nonetheless, it is sustained in a final step that Young never completely abdicated the phenomenological approach as a tool for social criticism. Although the argument of Inclusion and Democracy is developed primarily in a structural way, Young repeatedly mobilizes the experiences of social suffering and the demands for justice voiced by social movements as the basis of her large scale democratic proposals.Este artículo analiza el papel de la fenomenología en el modelo de la teoría crítica de Iris Marion Young a través de una discusión de las diferentes estrategias que utiliza para articular las nociones de identidad y colectividades sociales en La justicia y la política de la diferencia (1990) y en Inclusion and Democracy (2000). Al reconstruir el debate que Young tuvo con Nancy Fraser durante la década de los 90, buscamos demostrar que, aunque Fraser malinterpreta La justicia y la política de la diferencia como representante del “giro cultural” en la teoría social, sus críticas pueden iluminar algunas de las tensiones y defectos del texto. Además, sostenemos que el énfasis en una estrategia estructural-analítica de la argumentación, característica del trabajo posterior de Young, puede rastrearse hasta los argumentos formulados por Fraser. Sin embargo, se sostiene en un paso final que Young nunca abdicó completamente del enfoque fenomenológico como herramienta para la crítica social. Aunque el argumento de Inclusion and Democracy es desarrollado principalmente de manera estructural, Young utiliza repetidamente las experiencias de sufrimiento social y las demandas de justicia expresadas por movimientos sociales como base de sus propuestas democráticas a gran escala

On Useful Conformal Tranformations In General Relativity

Local conformal transformations are known as a useful tool in various applications of the gravitational theory, especially in cosmology. We describe some new aspects of these transformations, in particular using them for derivation of Einstein equations for the cosmological and Schwarzschild metrics. Furthermore, the conformal transformation is applied for the dimensional reduction of the Gauss-Bonnet topological invariant in $d=4$ to the spaces of lower dimensions.Comment: 17 pages, LaTeX. The paper is intended mainly for pedagogical purposes and represents a collection of exercises concerning local conformal transformations and dimensional reduction. To be published in "Gravitation and Cosmology

Dynamo in the Intra-Cluster Medium: Simulation of CGL-MHD Turbulent Dynamo

The standard magnetohydrodynamic (MHD) description of the plasma in the hot, magnetized gas of the intra-cluster (ICM) medium is not adequate because it is weakly collisional. In such collisionless magnetized gas, the microscopic velocity distribution of the particles is not isotropic, giving rise to kinetic effects on the dynamical scales. These kinetic effects could be important in understanding the turbulence, as so as the amplification and maintenance of the magnetic fields in the ICM. It is possible to formulate fluid models for collisonless or weakly collisional gas by introducing modifications in the MHD equations. These models are often referred as kinetic MHD (KMHD). Using a KMHD model based on the CGL-closure, which allows the adiabatic evolution of the two components of the pressure tensor (the parallel and perpendicular components with respect to the local magnetic field), we performed 3D numerical simulations of forced turbulence in order to study the amplification of an initially weak seed magnetic field. We found that the growth rate of the magnetic energy is comparable to that of the ordinary MHD turbulent dynamo, but the magnetic energy saturates in a level smaller than of the MHD case. We also found that a necessary condition for the dynamo works is to impose limits to the anisotropy of the pressure.Comment: 3 pages, 1 figure, 274 IAU Symposium: Advances in Plasma Astrophysic

Features of collisionless turbulence in the intracluster medium from simulated Faraday Rotation maps

Observations of the intracluster medium (ICM) in galaxy clusters suggest for the presence of turbulence and the magnetic fields existence has been proved through observations of Faraday Rotation and synchrotron emission. The ICM is also known to be filled by a rarefied weakly collisional plasma. In this work we study the possible signatures left on Faraday Rotation maps by collisionless instabilities. For this purpose we use a numerical approach to investigate the dynamics of the turbulence in collisionless plasmas based on an magnetohydrodynamical (MHD) formalism taking into account different levels of pressure anisotropy. We consider models covering the sub/super-Alfv\'enic and trans/supersonic regimes, one of them representing the fiducial conditions corresponding to the ICM. From the simulated models we compute Faraday Rotation maps and analyze several statistical indicators in order to characterize the magnetic field structure and compare the results obtained with the collisionless model to those obtained using standard collisional MHD framework. We find that important imprints of the pressure anisotropy prevails in the magnetic field and also manifest in the associated Faraday Rotation maps which evidence smaller correlation lengths in the collisionless MHD case. These points are remarkably noticeable for the case mimicking the conditions prevailing in ICM. Nevertheless, in this study we have neglected the decrease of pressure anisotropy due to the feedback of the instabilities that naturally arise in collisionless plasmas at small scales. This decrease may not affect the statistical imprint differences described above, but should be examined elsewhere.Comment: 24 pages, 15 figures, MNRAS accepte

Crystallographic groups and flat manifolds from surface braid groups

Let $M$ be a compact surface without boundary, and $n\geq 2$. We analyse the quotient group $B_n(M)/\Gamma_2(P_n(M))$ of the surface braid group $B_{n}(M)$ by the commutator subgroup $\Gamma_2(P_n(M))$ of the pure braid group $P_{n}(M)$. If $M$ is different from the $2$-sphere $\mathbb{S}^2$, we prove that $B_n(M)/\Gamma_2(P_n(M))$ is isomorphic rho $P_n(M)/\Gamma_2(P_n(M)) \rtimes_{\varphi} S_n$, and that $B_n(M)/\Gamma_2(P_n(M))$ is a crystallographic group if and only if $M$ is orientable. If $M$ is orientable, we prove a number of results regarding the structure of $B_n(M)/\Gamma_2(P_n(M))$. We characterise the finite-order elements of this group, and we determine the conjugacy classes of these elements. We also show that there is a single conjugacy class of finite subgroups of $B_n(M)/\Gamma_2(P_n(M))$ isomorphic either to $S_n$ or to certain Frobenius groups. We prove that crystallographic groups whose image by the projection $B_n(M)/\Gamma_2(P_n(M))\to S_n$ is a Frobenius group are not Bieberbach groups. Finally, we construct a family of Bieberbach subgroups $\tilde{G}_{n,g}$ of $B_n(M)/\Gamma_2(P_n(M))$ of dimension $2ng$ and whose holonomy group is the finite cyclic group of order $n$, and if $\mathcal{X}_{n,g}$ is a flat manifold whose fundamental group is $\tilde{G}_{n,g}$, we prove that it is an orientable K\"ahler manifold that admits Anosov diffeomorphisms

MHD turbulence-Star Formation Connection: from pc to kpc scales

The transport of magnetic flux to outside of collapsing molecular clouds is a required step to allow the formation of stars. Although ambipolar diffusion is often regarded as a key mechanism for that, it has been recently argued that it may not be efficient enough. In this review, we discuss the role that MHD turbulence plays in the transport of magnetic flux in star forming flows. In particular, based on recent advances in the theory of fast magnetic reconnection in turbulent flows, we will show results of three-dimensional numerical simulations that indicate that the diffusion of magnetic field induced by turbulent reconnection can be a very efficient mechanism, especially in the early stages of cloud collapse and star formation. To conclude, we will also briefly discuss the turbulence-star formation connection and feedback in different astrophysical environments: from galactic to cluster of galaxy scales.Comment: 6 pages, 5 figures, 274 IAU Symposium: Advances in Plasma Astrophysic

Modified kraske procedure with mid-sacrectomy and coccygectomy for en bloc excision of sacral giant cell tumors.

Sacral giant cell tumors are rare neoplasms, histologically benign but potentially very aggressive due to the difficulty in achieving a complete resection, their high recurrence rate, and metastization capability. Although many treatment options have been proposed, en bloc excision with tumor-free margins seems to be the most effective, being associated with long term tumor control, improved outcome, and potential cure. An exemplifying case of a 29-year-old female with progressive complaints of pain and paresthesias in the sacral and perianal regions, constipation, and weight loss for 6 months is presented. The surgical technique for en bloc excision of a large sacral giant cell tumor through a modified Kraske procedure with mid-sacrectomy and coccygectomy is described. Complete resection with wide tumor-free margins was achieved. At 5 years of follow-up the patient is neurologically intact, without evidence of local recurrence on imaging studies. A multidisciplinary surgical procedure is mandatory to completely remove sacral tumors. In the particular case of giant cell tumors, it allows minimizing local recurrence preserving neurovascular function, through a single dorsal and definitive approach