373 research outputs found
D=5 M-theory radion supermultiplet dynamics
We show how the bosonic sector of the radion supermultiplet plus d=4, N=1
supergravity emerge from a consistent braneworld Kaluza-Klein reduction of D=5
M--theory. The radion and its associated pseudoscalar form an SL(2,R)/U(1)
nonlinear sigma model. This braneworld system admits its own brane solution in
the form of a 2-supercharge supersymmetric string. Requiring this to be free of
singularities leads to an SL(2,Z) identification of the sigma model target
space. The resulting radion mode has a minimum length; we suggest that this
could be used to avoid the occurrence of singularities in brane-brane
collisions. We discuss possible supersymmetric potentials for the radion
supermultiplet and their relation to cosmological models such as the cyclic
universe or hybrid inflation.Comment: 18 pages, 4 figures, plain Late
`c' is the speed of light, isn't it?
Theories proposing a varying speed of light have recently been widely
promoted under the claim that they offer an alternative way of solving the
standard cosmological problems. Recent observational hints that the fine
structure constant may have varied during over cosmological scales also has
given impetus to these models. In theoretical physics the speed of light, ,
is hidden in almost all equations but with different facets that we try to
distinguish. Together with a reminder on scalar-tensor theories of gravity,
this sheds some light on these proposed varying speed of light theories.Comment: 14 pages, Late
Parallel computation of entries of A-1
In this paper, we are concerned about computing in parallel several entries of the inverse of a large sparse matrix. We assume that the matrix has already been factorized by a direct method and that the factors are distributed. Entries are efficiently computed by exploiting sparsity of the right-hand sides and the solution vectors in the triangular solution phase. We demonstrate that in this setting, parallelism and computational efficiency are two contrasting objectives. We develop an efficient approach and show its efficacy by runs using the MUMPS code that implements a parallel multifrontal method
Kaluza-Klein towers for real vector fields in flat space
We consider a free real vector field propagating in a five dimensional flat
space with its fifth dimension compactified either on a strip or on a circle
and perform a Kalaza Klein reduction which breaks SO(4,1) invariance while
reserving SO(3,1) invariance. Taking into account the Lorenz gauge condition,
we obtain from the most general hermiticity conditions for the relevant
operators all the allowed boundary conditions which have to be imposed on the
fields in the extra-dimension. The physical Kaluza-Klein mass towers, which
result in a four-dimensional brane, are determined in the different distinct
allowed cases. They depend on the bulk mass, on the parameters of the boundary
conditions and on the extra parameter present in the Lagrangian. In general,
they involve vector states together with accompanying scalar states.Comment: 28 pages, 4 independent table
The âcaged torch processionâ: Celebrities, protesters and the 2008 Olympic torch relay in London, Paris and San Francisco
Along with the opening and closing ceremonies, one of the major non-sports events associated with the modern Olympic Games is the torch relay. Although initiated in 1936, the relay has been subject to relatively little academic scrutiny. The events of April 2008 however will have cast a long shadow on the practice. This essay focuses primarily on one week (6â13 April) in the press coverage of the 2008 torch relay as the flame made its way from London to Paris in Europe and then to San Francisco in the USA. It discusses the interpretations offered in the mediated coverage about the relay, the Olympic movement, the host city and the locations where the relay was taking place, and critically analyses the role of agencies, both for and against the Olympics, that framed the ensuing debate
M-Theory on S^1/Z_2 : New Facts from a Careful Analysis
We carefully re-examine the issues of solving the modified Bianchi identity,
anomaly cancellations and flux quantization in the S^1/Z_2 orbifold of M-theory
using the boundary-free "upstairs" formalism, avoiding several misconceptions
present in earlier literature. While the solution for the four-form G to the
modified Bianchi identity appears to depend on an arbitrary parameter b, we
show that requiring G to be globally well-defined, i.e. invariant under small
and large gauge and local Lorentz transformations, fixes b=1. This value also
is necessary for a consistent reduction to the heterotic string in the
small-radius limit. Insisting on properly defining all fields on the circle, we
find that there is a previously unnoticed additional contribution to the
anomaly inflow from the eleven-dimensional topological term. Anomaly
cancellation then requires a quadratic relation between b and the combination
lambda^6/kappa^4 of the gauge and gravitational coupling constants lambda and
kappa. This contrasts with previous beliefs that anomaly cancellation would
give a cubic equation for b. We observe that our solution for G automatically
satisfies integer or half-integer flux quantization for the appropriate cycles.
We explicitly write out the anomaly cancelling terms of the heterotic string as
inherited from the M-theory approach. They differ from the usual ones by the
addition of a well-defined local counterterm. We also show how five-branes
enter our analysis.Comment: 32 pages, version to appear in Nucl. Phys. B, no figures, uses PHYZZ
DR1 doped sol-gel planar waveguides for nonlinear devices operating at telecommunications wavelengths
International audienceWe report on linear and nonlinear optical properties of Disperse Red 1-doped solgel waveguides. The refractive-index and optical-propagation losses of the guiding layer were measured between 0.756 mm and 1.64 mm. The spectral broadening of the chromophore charge-transfer transition in the visible is modeled with a Voigt-profile function. In the telecommunications window the attenuation is dominated by the overtones of the O--H bonds vibration bands. The nonlinear optical coefficients were measured at different poling strengths with the Maker-fringe method. The nonlinear coefficient d33 was found to be 4.5 pm V21 at 1.58 mm for a poling field of 60 Vmm21
Mesure de vitesse acoustique particulaire en champ libre par anémométrie laser Doppler
- Cette étude présente une méthode d'estimation de la vitesse particulaire acoustique et de la vitesse de convection par Anémométrie Laser à effet Doppler (ALD). Ces paramÚtres sont estimés grùce à l'utilisation conjointe d'une technique de démodulation synchrone en quadrature et d'un estimateur basé sur la dérivée de la phase du signal Doppler. Enfin, cette méthode est appliquée à la mesure des paramÚtres de la vitesse acoustique en champ libre pour une excitation sinusoïdale
Parallel computation of entries in A-1
International audienceIn this paper, we consider the computation in parallel of several entries of the inverseof a large sparse matrix. We assume that the matrix has already been factorized by a direct methodand that the factors are distributed. Entries are efficiently computed by exploiting sparsity of theright-hand sides and the solution vectors in the triangular solution phase. We demonstrate that inthis setting, parallelism and computational efficiency are two contrasting objectives. We develop anefficient approach and show its efficiency on a general purpose parallel multifrontal solver
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