215 research outputs found
An alternative to quintessence
We consider a FRW cosmological model with an exotic fluid known as Chaplygin
gas. We show that the resulting evolution of the universe is not in
disagreement with the current observation of cosmic acceleration. The model
predict an increasing value for the effective cosmological constant.Comment: 8 pages, latex. References and a new section adde
Tachyons, Scalar Fields and Cosmology
We study the role that tachyon fields may play in cosmology as compared to
the well-established use of minimally coupled scalar fields. We first elaborate
on a kind of correspondence existing between tachyons and minimally coupled
scalar fields; corresponding theories give rise to the same cosmological
evolution for a particular choice of the initial conditions but not for any
other. This leads us to study a specific one-parameter family of tachyonic
models based on a perfect fluid mixed with a positive cosmological constant.
For positive values of the parameter one needs to modify Sen's action and use
the sigma process of resolution of singularities. The physics described by this
model is dramatically different and much richer than that of the corresponding
scalar field. For particular choices of the initial conditions the universe,
that does mimick for a long time a de Sitter-like expansion, ends up in a
finite time in a special type of singularity that we call a "big brake". This
singularity is characterized by an infinite deceleration.Comment: 7 figures. Enlarged discussion of the big brake cosmology.
Continuation of the model clarified. References adde
Do supernovae favor tachyonic Big Brake instead de Sitter?
We investigate whether a tachyonic scalar field, encompassing both dark
energy and dark matter-like features will drive our universe towards a Big
Brake singularity or a de Sitter expansion. In doing this it is crucial to
establish the parameter domain of the model, which is compatible with type Ia
supernovae data. We find the 1-sigma contours and evolve the tachyonic sytem
into the future. We conclude, that both future evolutions are allowed by
observations, Big Brake becoming increasingly likely with the increase of the
positive model parameter k.Comment: 8 pages, 6 figures, to be published in the Proceedings of the
Invisible Universe International Conference, Paris, 2009, Ed. J. M. Alimi;
v2: reference
Towards a Generalized Distribution Formalism for Gauge Quantum Fields
We prove that the distributions defined on the Gelfand-Shilov spaces, and
hence more singular than hyperfunctions, retain the angular localizability
property. Specifically, they have uniquely determined support cones. This
result enables one to develop a distribution-theoretic techniques suitable for
the consistent treatment of quantum fields with arbitrarily singular
ultraviolet and infrared behavior. The proofs covering the most general case
are based on the use of the theory of plurisubharmonic functions and
Hormander's estimates.Comment: 12 p., Department of Theoretical Physics, P.N.Lebedev Physical
Institute, Leninsky prosp. 53, Moscow 117924, Russi
Conservation laws and scattering for de Sitter classical particles
Starting from an intrinsic geometric characterization of de Sitter timelike
and lightlike geodesics we give a new description of the conserved quantities
associated with classical free particles on the de Sitter manifold. These
quantities allow for a natural discussion of classical pointlike scattering and
decay processes. We also provide an intrinsic definition of energy of a
classical de Sitter particle and discuss its different expressions in various
local coordinate systems and their relations with earlier definitions found in
the literature.Comment: 25 pages, 1 figur
Gravity of a static massless scalar field and a limiting Schwarzschild-like geometry
We study a set of static solutions of the Einstein equations in presence of a
massless scalar field and establish their connection to the Kantowski-Sachs
cosmological solutions based on some kind of duality transformations. The
physical properties of the limiting case of an empty hyperbolic spacetime
(pseudo-Schwarzschild geometry) are analyzed in some detail.Comment: 13 pages, 4 figure
Clinical neurophysiology and imaging of nerve injuries: preoperative diagnostic work-up and postoperative monitoring
Peripheral nerve injuries are a heterogeneous group of lesions that may occurs secondary to
various causes. Several different classifications have been used to describe the pathophysiological
mechanisms leading to the clinical deficit, from simple and reversible compressionâinduced
demyelination, to complete transection of nerve axons. Neurophysiological data localize, quantify,
and qualify (demyelination vs. axonal loss) the clinical and subclinical deficits. Highâresolution
ultrasound can demonstrate the morphological extent of nerve damage, fascicular echotexture
(epineurium vs. perineurium, focal alteration of the crossâsection of the nerve, any neuromas, etc.),
and the surrounding tissues. High field magnetic resonance imaging provides high contrast
neurography by fat suppression sequences and shows structural connectivity through the use
of diffusionâweighted sequences. The aim of this review is to provide clinical guidelines for the
diagnosis of nerve injuries, and the rationale for instrumental evaluation in the preoperative and
postoperative periods. While history and clinical approach guide neurophysiological examination,
nerve conduction and electromyography studies provide functional information on conduction
slowing and denervation to assist in monitoring the onset of reâinnervation. Highâresolution nerve
imaging complements neurophysiological data and allows direct visualization of the nerve injury
while providing insight into its cause and facilitating surgical treatment planning. Indications and
limits of each instrumental examination are discussed
More about the Tolman-Oppenheimer-Volkoff equations for the generalized Chaplygin gas
We investigate the Tolman-Oppenheimer-Volkoff equations for the generalized
Chaplygin gas with the aim of extending the findings of V. Gorini, U.
Moschella, A. Y. Kamenshchik, V. Pasquier, and A. A. Starobinsky [Phys. Rev. D
{\bf 78}, 064064 (2008)]. We study both the standard case, where we reproduce
some previous results, and the phantom case. In the phantom case we show that
even a superluminal group velocity arising for cannot prevent the
divergence of the pressure at a finite radial distance. Finally, we investigate
how a modification of the generalized Chaplygin gas equation of state, required
by causality arguments at densities very close to , affects the
results found so far.Comment: 8 pages, revtex style, minor stylistic changes, typos corrected. It
matches the published versio
Tolman-Oppenheimer-Volkoff equations in presence of the Chaplygin gas: stars and wormhole-like solutions
We study static solutions of the Tolman--Oppenheimer--Volkoff equations for
spherically symmetric objects (stars) living in a space filled with the
Chaplygin gas. Two cases are considered. In the normal case all solutions
(excluding the de Sitter one) realize a three-dimensional spheroidal geometry
because the radial coordinate achieves a maximal value (the "equator"). After
crossing the equator, three scenarios are possible: a closed spheroid having a
Schwarzschild-type singularity with infinite blue-shift at the "south pole", a
regular spheroid, and a truncated spheroid having a scalar curvature
singularity at a finite value of the radial coordinate. The second case arises
when the modulus of the pressure exceeds the energy density (the phantom
Chaplygin gas). There is no more equator and all solutions have the geometry of
a truncated spheroid with the same type of singularity. We consider also static
spherically symmetric configurations existing in a universe filled with the
phantom Chaplygin gas only. In this case two classes of solutions exist:
truncated spheroids and solutions of the wormhole type with a throat. However,
the latter are not asymptotically flat and possess curvature singularities at
finite values of the radial coordinate. Thus, they may not be used as models of
observable compact astrophysical objects.Comment: A reference added, matches the version published in Physical Review
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