7,641 research outputs found
Oscillatons revisited
In this paper, we study some interesting properties of a spherically
symmetric oscillating soliton star made of a real time-dependent scalar field
which is called an oscillaton. The known final configuration of an oscillaton
consists of a stationary stage in which the scalar field and the metric
coefficients oscillate in time if the scalar potential is quadratic. The
differential equations that arise in the simplest approximation, that of
coherent scalar oscillations, are presented for a quadratic scalar potential.
This allows us to take a closer look at the interesting properties of these
oscillating objects. The leading terms of the solutions considering a quartic
and a cosh scalar potentials are worked in the so called stationary limit
procedure. This procedure reveals the form in which oscillatons and boson stars
may be related and useful information about oscillatons is obtained from the
known results of boson stars. Oscillatons could compete with boson stars as
interesting astrophysical objects, since they would be predicted by scalar
field dark matter models.Comment: 10 pages REVTeX, 10 eps figures. Updated files to match version
published in Classical and Quantum Gravit
Decoherence and the quantum-classical limit in the presence of chaos
We investigate how decoherence affects the short-time separation between
quantum and classical dynamics for classically chaotic systems, within the
framework of a specific model. For a wide range of parameters, the distance
between the corresponding phase-space distributions depends on a single
parameter that relates an effective Planck constant ,
the Lyapunov coeffficient, and the diffusion constant. This distance peaks at a
time that depends logarithmically on , in agreement with
previous estimations of the separation time for Hamiltonian systems. However,
for , the separation remains small, going down with , so the concept of separation time loses its meaning.Comment: 5 pages, 4 figures (in 6 postscript files) two of them are color
figure
Stock market series analysis using self-organizing maps
In this work a new clustering technique is implemented and tested. The proposed approach is based on the application of a SOM (self-organizing map) neural network and provides means to cluster U-MAT aggregated data. It relies on a flooding algorithm operating on the U-MAT and resorts to the Calinski and Harabask index to assess the depth of flooding, providing an adequate number of clusters. The method is tuned for the analysis of stock market series. Results obtained are promising although limited in scope. Neste trabalho Ă© implementada e testada uma nova tĂ©cnica de agrupamento. A abordagem proposta baseia-se na aplicação de uma rede neuronal SOM (mapa autoorganizado) e permite agrupar dados sobre a matriz de distancias (U-MAT). É utilizado um algoritmo de alagamento ("flooding") sobre a U-MAT e o Ăndice de Calinski e Harabasz avalia a profundidade do alagamento determinando-se, assim, o nĂşmero de grupos mais adequado. O mĂ©todo Ă© desenhado especificamente para a análise de sĂ©ries temporais da bolsa de valores. Os resultados obtidos sĂŁo promissores, embora se registem ainda limitações
Spherical Scalar Field Halo in Galaxies
We study a spherically symmetric fluctuation of scalar dark matter in the
cosmos and show that it could be the dark matter in galaxies, provided that the
scalar field has an exponential potential whose overall sign is negative and
whose exponent is constrained observationally by the rotation velocities of
galaxies. The local space-time of the fluctuation contains a three dimensional
space-like hypersurface with surplus of angle.Comment: 5 REVTeX pages, no figures. Contains important suggestions provided
by the referee. Final version, to appear in Phys. Rev.
Ground-state proton decay of 69Br and implications for the rp-process 68Se waiting-point
The first direct measurement of the proton separation energy, Sp, for the
proton-unbound nucleus 69Br is reported. Of interest is the exponential
dependence of the 2p-capture rate on Sp which can bypass the 68Se waiting-point
in the astrophysical rp process. An analysis of the observed proton decay
spectrum is given in terms of the 69Se mirror nucleus and the influence of Sp
is explored within the context of a single-zone X-ray burst model.Comment: 6 pages, 6 figures, INPC 2010 conference proceeding
New Features of Extended Wormhole Solutions in the Scalar Field Gravity Theories
The present paper reports interesting new features that wormhole solutions in
the scalar field gravity theory have. To demonstrate these, we obtain, by using
a slightly modified form of the Matos-Nunez algorithm, an extended class of
asymptotically flat wormhole solutions belonging to Einstein minimally coupled
scalar field theory. Generally, solutions in these theories do not represent
traversable wormholes due to the occurrence of curvature singularities.
However, the Ellis I solution of the Einstein minimally coupled theory, when
Wick rotated, yields Ellis class III solution, the latter representing a
singularity-free traversable wormhole. We see that Ellis I and III are not
essentially independent solutions. The Wick rotated seed solutions, extended by
the algorithm, contain two new parameters a and \delta;. The effect of the
parameter a on the geodesic motion of test particles reveals some remarkable
features. By arguing for Sagnac effect in the extended Wick rotated solution,
we find that the parameter a can indeed be interpreted as a rotation parameter
of the wormhole. The analyses reported here have wider applicability in that
they can very well be adopted in other theories, including in the string
theory.Comment: 19 page
Numerical evidence for `multi-scalar stars'
We present a class of general relativistic soliton-like solutions composed of
multiple minimally coupled, massive, real scalar fields which interact only
through the gravitational field. We describe a two-parameter family of
solutions we call ``phase-shifted boson stars'' (parameterized by central
density rho_0 and phase delta), which are obtained by solving the ordinary
differential equations associated with boson stars and then altering the phase
between the real and imaginary parts of the field. These solutions are similar
to boson stars as well as the oscillating soliton stars found by Seidel and
Suen [E. Seidel and W.M. Suen, Phys. Rev. Lett. 66, 1659 (1991)]; in
particular, long-time numerical evolutions suggest that phase-shifted boson
stars are stable. Our results indicate that scalar soliton-like solutions are
perhaps more generic than has been previously thought.Comment: Revtex. 4 pages with 4 figures. Submitted to Phys. Rev.
Environmental effects in the quantum-classical transition for the delta-kicked harmonic oscillator
We discuss the roles of the macroscopic limit and of different
system-environment interactions in the quantum-classical transition for a
chaotic system. We consider the kicked harmonic oscillator subject to
reservoirs that correspond in the classical case to purely dissipative or
purely diffusive behavior, in a situation that can be implemented in ion trap
experiments. In the dissipative case, we derive an expression for the time at
which quantum and classical predictions become different (breaking time) and
show that a complete quantum-classical correspondence is not possible in the
chaotic regime. For the diffusive environment we estimate the minimum value of
the diffusion coefficient necessary to retrieve the classical limit and also
show numerical evidence that, for diffusion below this threshold, the breaking
time behaves, essentially, as in the case of the system without a reservoir.Comment: 16 pages, 13 figures. Accepted for publication in Phys. Rev.
Quantum-state synthesis of multi-mode bosonic fields: Preparation of arbitrary states of 2-D vibrational motion of trapped ions
We present a universal algorithm for an efficient deterministic preparation
of an arbitrary two--mode bosonic state. In particular, we discuss in detail
preparation of entangled states of a two-dimensional vibrational motion of a
trapped ion via a sequence of laser stimulated Raman transitions. Our formalism
can be generalized for multi-mode bosonic fields. We examine stability of our
algorithm with respect to a technical noise.Comment: 8 pages, revtex, including 2 ps-figures, section about physical
implementation added, references updated, submitted to Phys. Rev. A, computer
program available at http://www.savba.sk/sav/inst/fyzi/qo
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