1,118 research outputs found
Equivalence of Darmois-Israel and Distributional-Methods for Thin Shells in General Relativity
A distributional method to solve the Einstein's field equations for thin
shells is formulated. The familiar field equations and jump conditions of
Darmois-Israel formalism are derived. A carefull analysis of the Bianchi
identities shows that, for cases under consideration, they make sense as
distributions and lead to jump conditions of Darmois-Israel formalism.Comment: 17 pages Latex, no figures, to be published in Journ. Math. Phy
On cosymplectic Lie Algebras
We give some properties of cosymplectic Lie algebras, we show, in particular,
that they support a left symmetric product. We also give some constructions of
cosymplectic Lie algebras, as well as a classification in three and
five-dimensional cosymplectic Lie algebras.Comment: 20 page
Eight-dimensional symplectic non-solvable Lie algebras
In this paper, we classify eight-dimensional non-solvable Lie algebras that
support a symplectic structure. As well as a complete classification is given,
up to symplectomorphism, of eight-dimensional symplectic non-solvable Lie
algebras.Comment: 29 pages. Comments welcom
Investigation of phonon behavior in Pr2NiMnO6 by micro-Raman spectroscopy
The temperature dependence of phonon excitations and the presence of spin
phonon coupling in polycrystalline Pr2NiMnO6 samples were studied using
micro-Raman spectroscopy and magnetometry. Magnetic properties show a single
ferromagnetic-to-paramagnetic transition at 228 K and a saturation
magnetization close to 4.95 \muB/f.u.. Three distinct Raman modes at 657, 642,
and 511 cm-1 are observed. The phonon excitations show a clear hardening due to
anharmonicity from 300 K down to 10 K. Further, temperature dependence of the
657 cm-1 mode shows only a small softening. This reflects the presence of a
relatively weak spin-phonon coupling in Pr2NiMnO6 contrary to other double
perovskites previously studied.Comment: 10 pages, 4 fig
Linearization of Poisson–Lie Structures on the 2d Euclidean and (1 + 1) Poincaré Groups
The paper deals with linearization problem of Poisson-Lie structures on the (1+1) Poincaré and 2D Euclidean groups. We construct the explicit form of linearizing coordinates of all these Poisson-Lie structures. For this, we calculate all Poisson-Lie structures on these two groups mentioned above, through the correspondence with Lie Bialgebra structures on their Lie algebras which we first determine.We thank Professor Morad EL OUALI for interesting and helpful discussions. Also we thank th ereferees for their time and comments
Noncommutative gravity: fuzzy sphere and others
Gravity on noncommutative analogues of compact spaces can give a finite mode
truncation of ordinary commutative gravity. We obtain the actions for gravity
on the noncommutative two-sphere and on the noncommutative in
terms of finite dimensional -matrices. The commutative large
limit is also discussed.Comment: LaTeX, 13 pages, section on CP^2 added + minor change
Quantum-gravity-motivated Lorentz-symmetry tests with laser interferometers
We consider the implications for laser interferometry of the
quantum-gravity-motivated modifications in the laws of particle propagation,
which are presently being considered in attempts to explain puzzling
observations of ultra-high-energy cosmic rays. We show that there are
interferometric setups in which the Planck-scale effect on propagation leads to
a characteristic signature. A naive estimate is encouraging with respect to the
possibility of achieving Planck-scale sensitivity, but we also point out some
severe technological challenges which would have to be overcome in order to
achieve this sensitivity.Comment: 17 pages, 3 figure
Power-law Parameterized Quintessence Model
We introduce a power-law parameterized quintessence model for the dark energy
which accelerate universe at the low redshifts while behaves as an ordinary
matter for the early universe. We construct a unique scalar potential for this
parameterized quintessence model. As the observational test, the Supernova Type
Ia (SNIa) Gold sample data, size of baryonic acoustic peak from Sloan Digital
Sky Survey (SDSS), the position of the acoustic peak from the CMB observations
and structure formation from the 2dFGRS survey are used to constrain the
parameters of the quintessence model. The best fit parameters indicates that
the equation of state of this model at the present time is less than one
which violates the energy condition in General Relativity. Finally
we compare the age of old objects with age of universe in this model.Comment: 11 pages, 17 figures, submitted to Phys. Rev.
Casimir stress on parallel plates in de Sitter space
The Casimir stress on two parallel plates in de Sitter background for
massless scalar field satisfying Robin boundary conditions on the plates is
calculated. The metric is written in conformally flat form to make maximum use
of the Minkowski space calculations. Different cosmological constants are
assumed for the space between and outside of the plates to have general results
applicable to the case of domain wall formations in the early universe.Comment: 6 page
Light Propagation and Large-Scale Inhomogeneities
We consider the effect on the propagation of light of inhomogeneities with
sizes of order 10 Mpc or larger. The Universe is approximated through a
variation of the Swiss-cheese model. The spherical inhomogeneities are
void-like, with central underdensities surrounded by compensating overdense
shells. We study the propagation of light in this background, assuming that the
source and the observer occupy random positions, so that each beam travels
through several inhomogeneities at random angles. The distribution of
luminosity distances for sources with the same redshift is asymmetric, with a
peak at a value larger than the average one. The width of the distribution and
the location of the maximum increase with increasing redshift and length scale
of the inhomogeneities. We compute the induced dispersion and bias on
cosmological parameters derived from the supernova data. They are too small to
explain the perceived acceleration without dark energy, even when the length
scale of the inhomogeneities is comparable to the horizon distance. Moreover,
the dispersion and bias induced by gravitational lensing at the scales of
galaxies or clusters of galaxies are larger by at least an order of magnitude.Comment: 27 pages, 9 figures, revised version to appear in JCAP, analytical
estimate included, typos correcte
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