On four-dimensional Einsteinian gravity, quasitopological gravity, cosmology and black holes

We show that the combination of cubic invariants defining five-dimensional quasitopological gravity, when written in four dimensions, reduce to the version of four-dimensional Einsteinian gravity recently proposed by Arciniega, Edelstein & Jaime, that produces second order equations of motion in a FLRW ansatz, with a purely geometrical inflationary period. We introduce a quartic version of the four-dimensional Einsteinian theory with similar properties, and study its consequences. In particular we found that there exists a region on the space of parameters which allows for thermodynamically stable black holes, as well as a well-defined cosmology with geometrically driven inflation. We briefly discuss the cosmological inhomogeneities in this setup. We also provide a combination of quintic invariants with those properties.Comment: Minor corrections, 14 pages, 1 figur

Inteligencia Artificial, algoritmos y libertad de expresión

Artificial Intelligence can be presented as an ally when moderating violent content or apparent news, but its use without human intervention that contextualizes and adequately translates the expression leaves open the risk of prior censorship. At present this is under debate within the international arena given that, since Artificial Intelligence lacks the ability to contextualize what it moderates, it is presented more as a tool for indiscriminate prior censorship, than as a moderation in order to protect the freedom of expression. Therefore, after analyzing international legislation, reports from international organizations and the terms and conditions of Twitter and Facebook, we suggest five proposals aimed at improving algorithmic content moderation. In the first place, we propose that the States reconcile their internal laws while respecting international standards of freedom of expression. We also urge that they develop public policies consistent with implementing legislation that protects the working conditions of human supervisors on automated content removal decisions. For its part, we understand that social networks must present clear and consistent terms and conditions, adopt internal policies of transparency and accountability about how AI operates in the dissemination and removal of online content and, finally, they must carry out prior evaluations impact of your AI on human rights.La Inteligencia Artificial puede presentarse como un aliado al momento de moderar contenidos violentoso de noticias aparentes, pero su utilización sin intervención humana que contextualice y traduzca adecuadamente la expresión deja abierto el riesgo de que se genere censura previa En la actualidad esto se encuentra en debate dentro del ámbito internacional dado que, al carecer la Inteligencia Artificial de la capacidad para contextualizar lo que modera, se ésta presentando más como una herramienta de censura previa indiscriminada, que como una moderación en busca de proteger la libertad de expresión.Por ello luego de analizar la legislación internacional, informes de organismos internacionales y lostérminos y condiciones de Twitter y Facebook, sugerimos cinco propuesta tendientes a mejorar la moderación algorítmica de contenidos.En primer término proponemos que los Estados compatibilicen sus legislaciones internas respetando losestándares internacionales de libertad de expresión. También instamos a que desarrollo en políticas públicasconsistentes en implementar legislaciones protectoras de las condiciones laborales de supervisores humanos sobre las decisiones automatizadas de remoción de contenido. Por su parte, entendemos que las redes sociales deben presentar términos y condiciones claros y consistentes, adoptar políticas internas de transparencia y rendición de cuentas acerca de cómo opera la IA en la difusión y remoción de contenido en línea y, finalmente, deben realizar evaluaciones previas de impacto de su IA a los derechos humanos

Wormhole solutions to Horava gravity

We present wormhole solutions to Horava non-relativistic gravity theory in vacuum. We show that, if the parameter $\lambda$ is set to one, transversable wormholes connecting two asymptotically de Sitter or anti-de Sitter regions exist. In the case of arbitrary $\lambda$, the asymptotic regions have a more complicated metric with constant curvature. We also show that, when the detailed balance condition is violated softly, tranversable and asymptotically Minkowski, de Sitter or anti-de Sitter wormholes exist.Comment: 17 pages, 1 figure. Replaced by revised version. Includes an appendix with the proof of the matching condition. References added. Accepted for publication in Phys.Rev.

Holographic thermalization of charged operators

We study a light-like charged collapsing shell in AdS-Reissner-Nordstrom spacetime, investigating whether the corresponding Vaidya metric is supported by matter that satisfies the null energy condition. We find that, if the absolute value of the charge decreases during the collapse, energy conditions are fulfilled everywhere in spacetime. On the other hand, if the absolute value of the charge increases, the metric does not satisfy energy conditions in the IR region. Therefore, from the gauge/gravity perspective, this last case is only useful to study the thermalization of the UV degrees of freedom. For all these geometries, we probe the thermalization process with two point correlators of charged operators, finding that the thermalization time grows with the charge of the operator, as well as with the dimension of space.Comment: 28 pages, 19 figure

Exact oscillations and chaos on a non-Abelian coil

We construct new exact solutions of the Georgi-Glashow model in $3+1$ dimensions. These configurations are periodic in time but lead to a stationary energy density and no energy flux. Nevertheless, they possess a characteristic frequency which manifests itself through non-trivial resonances on test fields. This allows us to interpret them as non-Abelian self sustained coils. We show that for larger energies a transition to chaotic behavior takes place, which we characterize by Poincar\'e sections, Fourier spectra and exponential growth of the geodesic deviation in an effective Jacobi metric, the latter triggered by parametric resonances.Comment: 23 pages, 5 figure

TRANSICIONES DE FASE EN SISTEMAS AUTOGRAVITANTES A TEMPERATURA FINITA EN EL CONTEXTO DE AdS/CFT

Se realiza un estudio sobre la existencia de transiciones de fase de materia fermiónica a temperatura finita en el contexto de AdS/CFT

XIPE: the X-ray Imaging Polarimetry Explorer

X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017 but not selected. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus and two additional GPDs filled with pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 % at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil).Comment: 49 pages, 14 figures, 6 tables. Paper published in Experimental Astronomy http://link.springer.com/journal/1068