12,922 research outputs found
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 -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 diagnostic method, we find that
the effective model differs substantially from the --CDM one, because
it gets the accelerated expansion faster than the --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 emission in each object but it was clearly detected in only 2 of
them. We find that the distribution of H 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]
- …