524 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
Infinite Infrared Regularization and a State Space for the Heisenberg Algebra
We present a method for the construction of a Krein space completion for
spaces of test functions, equipped with an indefinite inner product induced by
a kernel which is more singular than a distribution of finite order. This
generalizes a regularization method for infrared singularities in quantum field
theory, introduced by G. Morchio and F. Strocchi, to the case of singularites
of infinite order. We give conditions for the possibility of this procedure in
terms of local differential operators and the Gelfand- Shilov test function
spaces, as well as an abstract sufficient condition. As a model case we
construct a maximally positive definite state space for the Heisenberg algebra
in the presence of an infinite infrared singularity.Comment: 18 pages, typos corrected, journal-ref added, reference adde
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
A propeller flap for single-stage nose reconstruction in selected patients: supratrochlear artery axial propeller flap
The paramedian forehead flap is the gold standard technique for nose reconstruction. It requires two different surgical operations which prolonged the postoperative dressing and care. We present our 5-year experience with a propeller flap based on the supratrochlear artery, which allows one-stage transfer of the forehead skin to the nose without the need for pedicle division. This technique is indicated in a selected group of patients who are not suitable for multiple-stage reconstructions because they have concurrent medical conditions, reduced mobility, or live far away from specialized medical centers. We have renamed this procedure as supratrochlear artery axial propeller flap, from the acronym STAAP flap, to stress the axial, well known and constant, vascularization of the flap. In the past 5 years, we have been performing 25 STAAP flaps; full-thickness nasal reconstruction was performed in 11 cases. The patients were 16 males and 9 females, with a mean age of 79.5 years. All patients had multiple comorbidities. Complete flap survival was observed in 23 cases and healing was complete in 7 days. In two cases, there was a partial distal necrosis of the flap treated conservatively. Cosmetic results were good and the patient's satisfaction was significant. These results indicate that the STAAP flap is a reliable and useful technique in selected cases, as old or noncompliant patients who benefit from a one-stage technique of nose reconstructio
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
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