Einstein constraints on a characteristic cone

We analyse the Cauchy problem on a characteristic cone, including its vertex, for the Einstein equations in arbitrary dimensions. We use a wave map gauge, solve the obtained constraints and show gauge conservation.Comment: 10 pages, to be published in the Proceedings of the 15th International Conference on Waves and Stability in Continuous Media, held in Palermo, 28th June to 1st July 200

Further Improvements in the Understanding of Isotropic Loop Quantum Cosmology

The flat, homogeneous, and isotropic universe with a massless scalar field is a paradigmatic model in Loop Quantum Cosmology. In spite of the prominent role that the model has played in the development of this branch of physics, there still remain some aspects of its quantization which deserve a more detailed discussion. These aspects include the kinematical resolution of the cosmological singularity, the precise relation between the solutions of the densitized and non-densitized versions of the quantum Hamiltonian constraint, the possibility of identifying superselection sectors which are as simple as possible, and a clear comprehension of the Wheeler-DeWitt (WDW) limit associated with the theory in those sectors. We propose an alternative operator to represent the Hamiltonian constraint which is specially suitable to deal with these issues in a satisfactory way. In particular, with our constraint operator, the singularity decouples in the kinematical Hilbert space and can be removed already at this level. Thanks to this fact, we can densitize the quantum Hamiltonian constraint in a rigorous manner. Besides, together with the physical observables, this constraint superselects simple sectors for the universe volume, with a support contained in a single semiaxis of the real line and for which the basic functions that encode the information about the geometry possess optimal physical properties. Namely, they provide a no-boundary description around the cosmological singularity and admit a well-defined WDW limit in terms of standing waves. Both properties explain the presence of a generic quantum bounce replacing the singularity at a fundamental level, in contrast with previous studies where the bounce was proved in concrete regimes and focusing on states with a marked semiclassical behavior.Comment: 13 pages, version accepted for publication in Physical Review

Discovery of very nearby ultracool dwarfs from DENIS

We report new spectroscopic results, obtained with UKIRT/CGS4, of a sample of 14 candidate ultracool dwarfs selected from the DENIS (Deep Near-Infrared Survey of the Southern Sky) database. A further object, selected from the 2MASS Second Incremental Release, was observed at a later epoch with the same instrument. Six objects are already known in the literature; we re-derive their properties. A further four prove to be very nearby (~10 pc) mid-to-late L-dwarfs, three unknown hitherto, two of which are almost certainly substellar. These findings increase the number of L-dwarfs known within ~10 pc by ~25%. The remainder of the objects discussed here are early L or very late M-type dwarfs lying between ~45 and 15 pc and are also new to the literature. Spectral types have been derived by direct comparison with J-,H- and K- band spectra of known template ultracool dwarfs given by Leggett et al. (ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dL.spectra/) For the known objects, we generally find agreement to within ~1 subclass with previously derived spectral types. Distances are determined from the most recent M_J vs. spectral type calibrations, and together with our derived proper motions yield kinematics for most targets consistent with that expected for the disk population; for three probable late M-dwarfs, membership of a dynamically older population is postulated. The very nearby L-type objects discussed here are of great interest for future studies of binarity and parallaxes.Comment: 4 pages, 2 figures, accepted in A&A Letter

On The spectrum of a Noncommutative Formulation of the D=11 Supermembrane with Winding

A regularized model of the double compactified D=11 supermembrane with nontrivial winding in terms of SU(N) valued maps is obtained. The condition of nontrivial winding is described in terms of a nontrivial line bundle introduced in the formulation of the compactified supermembrane. The multivalued geometrical objects of the model related to the nontrivial wrapping are described in terms of a SU(N) geometrical object which in the $N\to \infty$ limit, converges to the symplectic connection related to the area preserving diffeomorphisms of the recently obtained non-commutative description of the compactified D=11 supermembrane.(I. Martin, J.Ovalle, A. Restuccia. 2000,2001) The SU(N) regularized canonical lagrangian is explicitly obtained. In the $N\to \infty$ limit it converges to the lagrangian in (I.Martin, J.Ovalle, A.Restuccia. 2000,2001) subject to the nontrivial winding condition. The spectrum of the hamiltonian of the double compactified D=11 supermembrane is discussed. Generically, it contains local string like spikes with zero energy. However the sector of the theory corresponding to a principle bundle characterized by the winding number $n \neq 0$, described by the SU(N) model we propose, is shown to have no local string-like spikes and hence the spectrum of this sector should be discrete.Comment: 16 pages.Latex2

Teaching Style in Physical Education and Changes of Daily Physical Activity after One Academic Year in Adolescents: GEOS Study

Increased Moderate-Vigorous physical activity (MVPA) and reduced sedentary time (ST) are key factors for a healthy lifestyle during childhood and adolescence. Studies have suggested that schools may be effective resources to promote healthy habits (Sallis, McKenzie et al. 2012). Therefore, in PE, is important to assess how teachers use strategies and provide students tools to engage in PA with the purpose of reduce the risk of sedentary behavior and contribute to promotion MVPA habits for a healthy lifestyle (Lonsdale, C. et al., 2013). Many factors may be involved in the successful PE class to promote healthy out-school behaviors, as teaching styles (TS), learning styles, learning time, motivation and so on (Mosston, M. 1966). Regarding TS, there is a lack of knowledge about influence of the teaching style (TS) in the promotion of daily MVPA. It was our aim to observe the differences of total daily PA dimensions between two groups of adolescents who were taught during a whole academic year using reproducing (RK) or producing knowledge (PK) TSs.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech