4,218 research outputs found
Stress response function of a granular layer: quantitative comparison between experiments and isotropic elasticity
We measured the vertical pressure response function of a layer of sand
submitted to a localized normal force at its surface. We found that this
response profile depends on the way the layer has been prepared: all profiles
show a single centered peak whose width scales with the thickness of the layer,
but a dense packing gives a wider peak than a loose one. We calculate the
prediction of isotropic elastic theory in presence of a bottom boundary and
compare it to the data. We found that the theory gives the right scaling and
the correct qualitative shape, but fails to really fit the data.Comment: 22 pages, 9 figures, submitted to Euro. Phys. J.
Black branes on the linear dilaton background
We show that the complete static black p-brane supergravity solution with a
single charge contains two and only two branches with respect to behavior at
infinity in the transverse space. One branch is the standard family of
asymptotically flat black branes, and another is the family of black branes
which asymptotically approach the linear dilaton background with antisymmetric
form flux (LDB). Such configurations were previously obtained in the
near-horizon near-extreme limit of the dilatonic asymptotically flat p-branes,
and used to describe the thermal phase of field theories involved in the DW/QFT
dualities and the thermodynamics of little string theory in the case of the
NS5-brane. Here we show by direct integration of the Einstein equations that
the asymptotically LDB p-branes are indeed exact supergravity solutions, and we
prove a new uniqueness theorem for static p-brane solutions satisfying cosmic
censorship. In the non-dilatonic case, our general non-asymptotically flat
p-branes are uncharged black branes on the background supported by the form flux. We develop the general formalism of
quasilocal quantities for non-asymptotically flat supergravity solutions with
antisymmetric form fields, and show that our solutions satisfy the first law of
theormodynamics. We also suggest a constructive procedure to derive rotating
asymptotically LDB brane solutions.Comment: 16 pages, revtex4, v2 - references added, "authors" metatag correcte
Hidden symmetry of the three-dimensional Einstein-Maxwell equations
It is shown how to generate three-dimensional Einstein-Maxwell fields from
known ones in the presence of a hypersurface-orthogonal non-null Killing vector
field. The continuous symmetry group is isomorphic to the Heisenberg group
including the Harrison-type transformation. The symmetry of the
Einstein-Maxwell-dilaton system is also studied and it is shown that there is
the transformation between the Maxwell and the dilaton fields.
This transformation is identified with the Geroch
transformation of the four-dimensional vacuum Einstein equation in terms of the
Ka{\l}uza-Klein mechanism.Comment: 5 page
Micro-Brillouin spectroscopy mapping of the residual density field induced by Vickers indentation in a soda-lime silicate glass
High-resolution Brillouin scattering is used to achieve 3-dimensional maps of
the longitudinal acoustic mode frequency shift in soda-lime silicate glasses
subject to Vickers indentations. Assuming that residual stress-induced effects
are simply proportional to density changes, residual densification fields are
obtained. The density gradient is nearly isotropic, confirming earlier optical
observations made on a similar glass. The results show that Brillouin
micro-spectroscopy opens the way to a fully quantitative comparison of
experimental data with predictions of mechanical models for the identification
of a constitutive law.Comment: 4 pages, 3 figures, revised version, to appear in Appl. Phys. Let
Elliptic flow in transport theory and hydrodynamics
We present a new direct simulation Monte-Carlo method for solving the
relativistic Boltzmann equation. We solve numerically the 2-dimensional
Boltzmann equation using this new algorithm. We find that elliptic flow from
this transport calculation smoothly converges towards the value from ideal
hydrodynamics as the number of collisions per particle increases, as expected
on general theoretical grounds, but in contrast with previous transport
calculations.Comment: 5 pages, 3 figures, revise
Density modulations in an elongated Bose-Einstein condensate released from a disordered potential
We observe large density modulations in time-of-flight images of elongated
Bose-Einstein condensates, initially confined in a harmonic trap and in the
presence of weak disorder. The development of these modulations during the
time-of-flight and their dependence with the disorder are investigated. We
render an account of this effect using numerical and analytical calculations.
We conclude that the observed large density modulations originate from the weak
initial density modulations induced by the disorder, and not from initial phase
fluctuations (thermal or quantum).Comment: Published version; 4+ pages; 4 figure
Multi-Black-Holes in Three Dimensions
We construct time-dependent multi-centre solutions to three-dimensional
general relativity with zero or negative cosmological constant. These solutions
correspond to dynamical systems of freely falling black holes and conical
singularities, with a multiply connected spacetime topology. Stationary
multi-black-hole solutions are possible only in the extreme black hole case.Comment: 8 pages, \LaTex, 4 figures (available on request), GCR 94/02/0
Wormhole cosmic strings
We construct regular multi-wormhole solutions to a gravitating model
in three space-time dimensions, and extend these solutions to cylindrical
traversable wormholes in four and five dimensions. We then discuss the
possibility of identifying wormhole mouths in pairs to give rise to Wheeler
wormholes. Such an identification is consistent with the original field
equations only in the absence of the -model source, but with possible
naked cosmic string sources. The resulting Wheeler wormhole space-times are
flat outside the sources and may be asymptotically Minkowskian.Comment: 17 pages, LaTeX, 4 figures (hard copy available on request
- âŠ