6,525 research outputs found
Generation of Closed Timelike Curves with Rotating Superconductors
The spacetime metric around a rotating SuperConductive Ring (SCR) is deduced
from the gravitomagnetic London moment in rotating superconductors. It is shown
that theoretically it is possible to generate Closed Timelike Curves (CTC) with
rotating SCRs. The possibility to use these CTC's to travel in time as
initially idealized by G\"{o}del is investigated. It is shown however, that
from a technology and experimental point of view these ideas are impossible to
implement in the present context.Comment: 9 pages. Submitted to Classical and Quantum Gravit
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
An Alternative Interpretation for the Moduli Fields of the Cosmology Associated to Type IIB Supergravity with Fluxes
We start with a particular cosmological model derived from type IIB
supergravity theory with fluxes, where usually the dilaton is interpreted as a
Quintessence field. Instead of that, in this letter we interpret the dilaton as
the dark matter of the universe. With this alternative interpretation we find
that in this supergravity model gives a similar evolution and structure
formation of the universe compared with the CDM model in the linear
regime of fluctuations of the structure formation. Some free parameters of the
theory are fixed using the present cosmological observations. In the non-linear
regimen there are some differences between the type IIB supergravity theory
with the traditional CDM paradigm. The supergravity theory predicts the
formation of galaxies earlier than the CDM and there is no density cusp in the
center of galaxies. These differences can distinguish both models and can give
a distinctive feature to the phenomenology of the cosmology coming from
superstring theory with fluxes.Comment: 7 pages, 5 figures, references added, minor modifications, typos
corrected. Version accepted for publication in IJMP
Scalar-Tensor theories from Plebanski gravity
We study a modification of the Plebanski action, which generically
corresponds to a bi-metric theory of gravity, and identify a subclass which is
equivalent to the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories.
In this manner, scalar-tensor theories are displayed as constrained BF
theories. We find that in this subclass, there is no need to impose reality of
the Urbantke metrics, as also the theory with real bivectors is a scalar-tensor
theory with a real Lorentzian metric. Furthermore, while under the former
reality conditions instabilities can arise from a wrong sign of the scalar mode
kinetic term, we show that such problems do not appear if the bivectors are
required to be real. Finally, we discuss how matter can be coupled to these
theories. The phenomenology of scalar field dark matter arises naturally within
this framework.Comment: 21 page
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.
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
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
Finding co-solvers on Twitter, with a little help from Linked Data
In this paper we propose a method for suggesting potential collaborators for solving innovation challenges online, based on their competence, similarity of interests and social proximity with the user. We rely on Linked Data to derive a measure of semantic relatedness that we use to enrich both user profiles and innovation problems with additional relevant topics, thereby improving the performance of co-solver recommendation. We evaluate this approach against state of the art methods for query enrichment based on the distribution of topics in user profiles, and demonstrate its usefulness in recommending collaborators that are both complementary in competence and compatible with the user. Our experiments are grounded using data from the social networking service Twitter.com
Electromagnetic Pulse Driven Spin-dependent Currents in Semiconductor Quantum Rings
We investigate the non-equilibrium charge and spin-dependent currents in a
quantum ring with a Rashba spin orbit interaction (SOI) driven by two
asymmetric picosecond electromagnetic pulses. The equilibrium persistent charge
and persistent spin-dependent currents are investigated as well. It is shown
that the dynamical charge and the dynamical spin-dependent currents vary
smoothly with a static external magnetic flux and the SOI provides a SU(2)
effective flux that changes the phases of the dynamic charge and the dynamic
spin-dependent currents. The period of the oscillation of the total charge
current with the delay time between the pulses is larger in a quantum ring with
a larger radius. The parameters of the pulse fields control to a certain extent
the total charge and the total spin-dependent currents. The calculations are
applicable to nano-meter rings fabricated in heterojuctions of III-V and II-VI
semiconductors containing several hundreds electrons.Comment: 15pages, 5 figure
Mg(, )Na reaction study for spectroscopy of Na
The Mg(, )Na reaction was measured at the Holifield
Radioactive Ion Beam Facility at Oak Ridge National Laboratory in order to
better constrain spins and parities of energy levels in Na for the
astrophysically important F()Ne reaction rate
calculation. 31 MeV proton beams from the 25-MV tandem accelerator and enriched
Mg solid targets were used. Recoiling He particles from the
Mg(, )Na reaction were detected by a highly segmented
silicon detector array which measured the yields of He particles over a
range of angles simultaneously. A new level at 6661 5 keV was observed in
the present work. The extracted angular distributions for the first four levels
of Na and Distorted Wave Born Approximation (DWBA) calculations were
compared to verify and extract angular momentum transfer.Comment: 11 pages, 6 figures, proceedings of the 18th International Conference
on Accelerators and Beam Utilization (ICABU2014
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