Revisiting the Stability of Quadratic Poincar\'e Gauge Gravity

Poincar\'e gauge theories provide an approach to gravity based on the gauging of the Poincar\'e group, whose homogeneous part generates curvature while the translational sector gives rise to torsion. In this note we revisit the stability of the widely studied quadratic theories within this framework. We analyse the presence of ghosts without fixing any background by obtaining the relevant interactions in an exact post-Riemannian expansion. We find that the axial sector of the theory exhibits ghostly couplings to the graviton sector that render the theory unstable. Remarkably, imposing the absence of these pathological couplings results in a theory where either the axial sector or the torsion trace becomes a ghost. We conclude that imposing ghost-freedom generically leads to a non-dynamical torsion. We analyse however two special choices of parameters that allow a dynamical scalar in the torsion and obtain the corresponding effective action where the dynamics of the scalar is apparent. These special cases are shown to be equivalent to a generalised Brans-Dicke theory and a Holst Lagrangian with a dynamical Barbero-Immirzi pseudoscalar field respectively. The two sectors can co-exist giving a bi-scalar theory. Finally, we discuss how the ghost nature of the vector sector can be avoided by including additional dimension four operators.Comment: 25 pages, 1 figure. More insight on the bi-scalar theory has been added, including its possible extensions and the coupling with fermions. A clarifying footnote on the Holst term has been introduced. Extended discussion. It matches the version published in EPJ

Cosmology with moving dark energy and the CMB quadrupole

We study the consequences of a homogeneous dark energy fluid having a non-vanishing velocity with respect to the matter and radiation large-scale rest frames. We consider homogeneous anisotropic cosmological models with four fluids (baryons, radiation, dark matter and dark energy) whose velocities can differ from each other. Performing a perturbative calculation up to second order in the velocities, we obtain the contribution of the anisotropies generated by the fluids motion to the CMB quadrupole and compare with observations. We also consider the exact problem for arbitrary velocities and solve the corresponding equations numerically for different dark energy models. We find that models whose equation of state is initially stiffer than radiation, as for instance some tracking models, are unstable against velocity perturbations, thus spoiling the late-time predictions for the energy densities. In the case of scaling models, the contributions to the quadrupole can be non-negligible for a wide range of initial conditions. We also consider fluids moving at the speed of light (null fluids) with positive energy and show that, without assuming any particular equation of state, they generically act as a cosmological constant at late times. We find the parameter region for which the models considered could be compatible with the measured (low) quadrupole.Comment: 23 pages, 6 figures. Confidence intervals calculated from WMAP data, new references and comments included. Final version to appear in PR

Effects of biasing on the galaxy power spectrum at large scales

n this paper we study the effect of biasing on the power spectrum at large scales. We show that even though non-linear biasing does introduce a white noise contribution on large scales, the $P(k)\propto k^n$ behavior of the matter power spectrum on large scales may still be visible and above the white noise for about one decade. We show, that the Kaiser biasing scheme which leads to linear bias of the correlation function on {\em large} scales, also generates a linear bias of the {\rm power spectrum} on rather small scales. This is a consequence of the divergence on small scales of the pure Harrison-Zeldovich spectrum. However, biasing becomes k-dependent when we damp the underlying power spectrum on small scales. We also discuss the effect of biasing on the baryon acoustic oscillations.Comment: 9 pages, 4 figures. One figure and comments clarifying the linear biasing on small scales and references added. V3 version accepted in PR

Whole Core Pin-by-Pin Coupled Neutronic-Thermal-hydraulic Steady state and Transient Calculations using COBAYA3 code

Nowadays, coupled 3D neutron-kinetics and thermal-hydraulic core calculations are performed by applying a radial average channel approach using a meshing of one quarter of assembly in the best case. This approach does not take into account the subchannels effects due to the averaging of the physical fields and the loose of heterogeneity in the thermal-hydraulic modelization. Therefore the models do not have enough resolution to predict those subchannels effects which are important for the fuel design safety margins, because it is in the local scale, where we can search the hottest pellet or the maximum heat flux. The UPM advanced multi-scale neutron-kinetics and thermal-hydraulics methodologies being implemented in COBAYA3 include domain decomposition by alternate core dissections for the local 3D fine-mesh scale problems (pin cells/subchannels) and an analytical nodal diffusion solver for the coarse mesh scale coupled with the thermal-hydraulic using a modelization of one channel per assembly or per quarter of assembly. In this work, we address the domain decomposition by the alternate core dissections methodology applied to solve coupled 3D neutronic/thermal-hydraulic (N-TH) problems at the fine-mesh scale. The N-TH coupling at the cell-subchannel scale allows the treatment of the effects of the detailed TH feedbacks on cross-sections, thus resulting in better estimates of the local safety margins at the pin level

Cosmological evolution in vector-tensor theories of gravity

We present a detailed study of the cosmological evolution in general vector-tensor theories of gravity without potential terms. We consider the evolution of the vector field throughout the expansion history of the universe and carry out a classification of models according to the behavior of the vector field in each cosmological epoch. We also analyze the case in which the universe is dominated by the vector field, performing a complete analysis of the system phase map and identifying those attracting solutions which give rise to accelerated expansion. Moreover, we consider the evolution in a universe filled with a pressureless fluid in addition to the vector field and study the existence of attractors in which we can have a transition from matter-domination to vector-domination with accelerated expansion so that the vector field may play the role of dark energy. We find that the existence of solutions with late-time accelerated expansion is a generic prediction of vector-tensor theories and that such solutions typically lead to the presence of future singularities. Finally, limits from local gravity tests are used to get constraints on the value of the vector field at small (Solar System) scales.Comment: 35 pages, 8 figure

Optimización del envasado con atmósferas modificadas innovadoras en pescado fresco

La naturaleza altamente perecedera de los productos de la pesca y la tendencia actual del mercado a consumir productos frescos y de calidad, implica la necesidad de desarrollar nuevas tecnologías que alarguen la vida útil de dichos productos y cumplan con las expectativas de los consumidores. En los últimos años y como respuesta a todas estas exigencias, dentro del campo de la tecnología de los alimentos, se ha producido un importante desarrollo de nuevas tecnologías de conservación relacionadas con el envasado, de las cuales, la más utilizada y extendida actualmente es el envasado en atmósferas protectoras combinado con la refrigeración. El objetivo del presente estudio es realizar una estimación de la vida útil de muestras frescas en distintas especies frescas de pescado, envasadas en atmósfera modificada en diferentes envases y atmósferas de gases, a refrigeración en cámara a 3-4ºC, analizando diferentes parámetros de calidad y optimizando la proporción óptima de gases para la mejor conservación de cada especie. Para la realización del presente estudio, se utilizaron muestras de merluza, perca, y bonito envasadas en distintas composiciones de gases. Las especies merluza y perca se envasaron en dos tipos de atmósferas modificadas 50 % CO2 / 50 % N2 y 50 % CO2 / 40 % N2 / 10 % O2 en envase formato macro. A rasgos generales, la atmósfera 50 % CO2 / 50 % N2 ha mostrado ligeramente mejores resultados que la atmósfera 50 % CO2 / 40 % N2 / 10 % O2. Este hecho queda especialmente reflejado en oxidación y carga microbiológica. Por último se realizó un último estudio con bonito en donde se comparó tres tipos de composición de gases en dos tipos de envases. Dos atmósferas en formato macro 50 % CO2 y 50 % N2 y 40 % CO2 / 30 % O2 / 30 % N2. añadido a dos más en formato supermercado . 40 % CO2 / 30 % O2 / 30 % N2 y 40 % CO2 / 30 % O2 / 30 % Ar.En general podemos decir que ambos tipos de envasado, en las diferentes atmósferas estudiadas, han sido efectivas para mantener la calidad organoléptica del producto a lo largo de los 10 días del experimento. Por tanto, en general el formato macro ha resultado ser algo más efectivo que el formato supermercado, siendo cierto que las condiciones de almacenamiento del primero (cámara fría a oscuridad, y por tanto sin afección de la luz) no son las mismas que en el formato súper ni representan las condiciones de mercado

Caracterización geoquímica del sistema hidrotermal de Luchon (Pirineo Central, Francia)

Se ha realizado una caracterización geoquímica del sistema hidrotermal de Luchon mediante modelización geoquímica de los procesos secundarios y cálculos geotermométricos. Las características geoquímicas del sistema están controladas por la actuación de tres procesos: 1) mezcla de aguas entre un polo termal y un polo frío, 2) Enfriamiento conductivo y 3) Transferencia de CO2. Se han determinado temperaturas en el reservorio de aproximadamente 115 ºC. <br /

Characterization of Poly(ε-caprolactone)-Based Nanocomposites Containing Hydroxytyrosol for Active Food Packaging

Antioxidant nano-biocomposites based on poly(ε-caprolactone) (PCL) were prepared by incorporating hydroxytyrosol (HT) and a commercial montmorillonite, Cloisite®30B (C30B), at different concentrations. A full structural, thermal, mechanical and functional characterization of the developed nano-biocomposites was carried out. The presence of the nanoclay and HT increased PCL crystallinity, whereas some decrease in thermal stability was observed. TEM analyses corroborated the good dispersion of C30B into the PCL macromolecular structure as already asserted by XRD tests, since no large aggregates were observed. A reduction in oxygen permeability and increase in elastic modulus were obtained for films containing the nanoclay. Finally, the presence of the nanoclay produced a decrease in the HT release from films due to some interaction between HT and C30B. Results proved that these nano-biocomposites can be an interesting and environmentally-friendly alternative for active food packaging applications with antioxidant performance.Financial support by the Spanish Ministry of Economy and Competitiveness (Ref. MAT2011-28468-C02-01) and University of Alicante (UAUSTI12-04)

Physical education and school bullying: a systematic review

Objectives: To evaluate the associations of physical education (PE) with school violence and bullying. Design: Systematic review. Method: Using a systematic search in Medline, PsycINFO, SPORTDiscus, Web of Science, and Scopus, relevant studies with a quantitative and qualitative design were identified that met previously established eligibility criteria. Quality was assessed (bias risk analysis) and data were extracted from a previously elaborated template. Results: The systematic review finally included 16 studies, of which 10 had a quantitative design (n = 12795), 5 a qualitative design (n = 79) and 1 a mixed design (n = 86). The high heterogeneity presented by the measures used in the included studies hindered the comparison of the outcomes and prevented meta-analysis of the data. Although there is insufficient evidence about the positive impact of PE on bullying prevention, the results of this review indicate that some aspects of PE programs could improve students’ skills to cope with these situations. Conclusions: The results of this review suggest the importance of PE in the prevention of bullying. Secondly, it is emphasized that bullying situations have a negative impact on students’ enjoyment of PE, leading to detrimental consequences for their physical and psychological health. Thirdly, the figure of the PE teacher as a key element to prevent and/or encourage bullying was obvious