Dynamical variables in Gauge-Translational Gravity

Assuming that the natural gauge group of gravity is given by the group of isometries of a given space, for a maximally symmetric space we derive a model in which gravity is essentially a gauge theory of translations. Starting from first principles we verify that a nonlinear realization of the symmetry provides the general structure of this gauge theory, leading to a simple choice of dynamical variables of the gravity field corresponding, at first order, to a diagonal matrix, whereas the non-diagonal elements contribute only to higher orders.Comment: 15 page

Is General Relativity a simpler theory?

Gravity is understood as a geometrization of spacetime. But spacetime is also the manifold of the boundary values of the spinless point particle in a variational approach. Since all known matter, baryons, leptons and gauge bosons are spinning objects, it means that the manifold, which we call the kinematical space, where we play the game of the variational formalism of an elementary particle is greater than spacetime. This manifold for any mechanical system is a Finsler metric space such that the variational formalism can always be interpreted as a geodesic problem on this space. This manifold is just the flat Minkowski space for the free spinless particle. Any interaction modifies its flat Finsler metric as gravitation does. The same thing happens for the spinning objects but now the Finsler metric space has more dimensions and its metric is modified by any interaction, so that to reduce gravity to the modification only of the spacetime metric is to make a simpler theory, the gravitational theory of spinless matter. Even the usual assumption that the modification of the metric only involves dependence of the metric coefficients on the spacetime variables is also a restriction because in general these coefficients are dependent on the velocities. In the spirit of unification of all forces, gravity cannot produce, in principle, a different and simpler geometrization than any other interaction.Comment: 10 pages 1 figure, several Finsler metric examples and a conclusion section added. Minor correction

Interacting dark matter and modified holographic Ricci dark energy induce a relaxed Chaplygin gas

We investigate a model of interacting dark matter and dark energy given by a modified holographic Ricci cutoff in a spatially flat Friedmann-Robertson-Walker (FRW) space-time. We consider a nonlinear interaction consisting of a significant rational function of the total energy density and its first derivative homogeneous of degree one and show that the effective one-fluid obeys the equation of state of a relaxed Chaplygin gas. So that, the universe is dominated by pressureless dark matter at early times and undergoes an accelerated expansion in the far future driven by a strong negative pressure. We apply the $\chi^{2}$-statistical method to the observational Hubble data and the Union2 compilation of SNe Ia for constraining the cosmological parameters and analyze the feasibility of the modified holographic Ricci cutoff. By using the new $Om$ diagnostic method, we find that the effective model differs substantially from the $\Lambda$--CDM one, because it gets the accelerated expansion faster than the $\Lambda$--CDM model. Finally, a new model with a third component decoupled from the interacting dark sector is presented for studying bounds on the dark energy at early times.Comment: 8 pages, 7 figures. Revtex4 Style. Accepted for its publication in PR

Resolved Hubble Space spectroscopy of ultracool binary systems

Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the \emph{Hubble Space Telescope} (HST), we have obtained spatially resolved spectra of 20 ultracool dwarfs. 18 of them belong to 9 known very low-mass binary systems with angular separations in the range 0.37-0.098 arcseconds. We have derived spectral types in the range dM7.5 to dL6 from the PC3 index, and by comparing our STIS spectra with ground-based spectra of similar spectral resolution from Mart{\'\i}n et al. (1999). We have searched for H$_\alpha$ emission in each object but it was clearly detected in only 2 of them. We find that the distribution of H$_\alpha$ emission in our sample is statistically different from that of single field dwarfs, suggesting an intriguing anticorrelation between chromospheric activity and binarity for M7--M9.5 dwarfs. We provide measuments of the strength of the main photospheric features and the PC3 index, and we derive calibrations of spectral subclasses versus F814W and K-band absolute magnitudes for a subset of 10 dwarfs in 5 binaries that have known trigonometric parallaxes.Comment: accepted for publication in A&

Stationary axisymmetric exteriors for perturbations of isolated bodies in general relativity, to second order

Perturbed stationary axisymmetric isolated bodies, e.g. stars, represented by a matter-filled interior and an asymptotically flat vacuum exterior joined at a surface where the Darmois matching conditions are satisfied, are considered. The initial state is assumed to be static. The perturbations of the matching conditions are derived and used as boundary conditions for the perturbed Ernst equations in the exterior region. The perturbations are calculated to second order. The boundary conditions are overdetermined: necessary and sufficient conditions for their compatibility are derived. The special case of perturbations of spherical bodies is given in detail.Comment: RevTeX; 32 pp. Accepted by Phys. Rev. D. Added references and extra comments in introductio

Yukawa terms in noncommutative SO(10) and E6 GUTs

We propose a method for constructing Yukawa terms for noncommutative SO(10) and E6 GUTs, when these GUTs are formulated within the enveloping-algebra formalism. The most general noncommutative Yukawa term that we propose contains, at first order in thetamunu, the most general BRS invariant Yukawa contribution whose only dimensionful parameter is the noncommutativity parameter. This noncommutative Yukawa interaction is thus renormalisable at first order in thetamunu.Comment: 14 pages, no figure

Multiplicity of very low-mass objects in the Upper Scorpius OB association: a possible wide binary population

We report the initial results of a VLT/NACO high spatial resolution imaging survey for multiple systems among 58 M-type members of the nearby Upper Scorpius OB association. Nine pairs with separations below 100 have been resolved. Their small angular separations and the similarity in the brightness of the components (DMagK <1 for all of them), indicate there is a reasonable likelihood several of them are true binaries rather than chance projections. Follow-up imaging observations with WHT/LIRIS of the two widest binaries confirm that their near-infrared colours are consistent with physical very low mass binaries. For one of these two binaries, WHT/LIRIS spectra of each component were obtained. We find that the two components have similar M6-M7 spectral types and signatures of low-gravity, as expected for a young brown dwarf binary in this association. Our preliminary results indicate a possible population of very low-mass binaries with semimajor axis in the range 100 AU 150 AU, which has not been seen in the Pleiades open cluster. If these candidates are confirmed (one is confirmed by this work), these results would indicate that the binary properties of very low-mass stars and brown dwarfs may depend on the environment where they form.Comment: 11 pages, 1 table, 7 figures, request high resolution copies to [email protected]