3,652 research outputs found
Large quantum gravity effects: Cylindrical waves in four dimensions
Linearly polarized cylindrical waves in four-dimensional vacuum gravity are
mathematically equivalent to rotationally symmetric gravity coupled to a
Maxwell (or Klein-Gordon) field in three dimensions. The quantization of this
latter system was performed by Ashtekar and Pierri in a recent work. Employing
that quantization, we obtain here a complete quantum theory which describes the
four-dimensional geometry of the Einstein-Rosen waves. In particular, we
construct regularized operators to represent the metric. It is shown that the
results achieved by Ashtekar about the existence of important quantum gravity
effects in the Einstein-Maxwell system at large distances from the symmetry
axis continue to be valid from a four-dimensional point of view. The only
significant difference is that, in order to admit an approximate classical
description in the asymptotic region, states that are coherent in the Maxwell
field need not contain a large number of photons anymore. We also analyze the
metric fluctuations on the symmetry axis and argue that they are generally
relevant for all of the coherent states.Comment: Version accepted for publication in Int. J. Mod. Phys.
On a Construction of Markov Models in Continuous Time
This paper studies a novel idea for constructing continuous-time stationary Markov models. The approach undertaken is based on a latent representation of the corresponding transition probabilities that conveys to appealing ways to study and simulate the dynamics of the constructed processes. Some well-known models are shown to fall within this construction shedding some light on both theoretical and applied properties. As an illustration of the capabilities of our proposal a simple estimation problem is posed.Gibbs sampler; Markov process; Stationary process
Thiemann transform for gravity with matter fields
The generalised Wick transform discovered by Thiemann provides a
well-established relation between the Euclidean and Lorentzian theories of
general relativity. We extend this Thiemann transform to the Ashtekar
formulation for gravity coupled with spin-1/2 fermions, a non-Abelian
Yang-Mills field, and a scalar field. It is proved that, on functions of the
gravitational and matter phase space variables, the Thiemann transform is
equivalent to the composition of an inverse Wick rotation and a constant
complex scale transformation of all fields. This result holds as well for
functions that depend on the shift vector, the lapse function, and the Lagrange
multipliers of the Yang-Mills and gravitational Gauss constraints, provided
that the Wick rotation is implemented by means of an analytic continuation of
the lapse. In this way, the Thiemann transform is furnished with a geometric
interpretation. Finally, we confirm the expectation that the generator of the
Thiemann transform can be determined just from the spin of the fields and give
a simple explanation for this fact.Comment: LaTeX 2.09, 14 pages, no figure
Quantum Cylindrical Waves and Sigma Models
We analyze cylindrical gravitational waves in vacuo with general polarization
and develop a viewpoint complementary to that presented recently by Niedermaier
showing that the auxiliary sigma model associated with this family of waves is
not renormalizable in the standard perturbative sense.Comment: 11 pages (DIN A4), accepted in International Journal of Modern
Physics
Hybrid Quantum Cosmology: Combining Loop and Fock Quantizations
As a necessary step towards the extraction of realistic results from Loop
Quantum Cosmology, we analyze the physical consequences of including
inhomogeneities. We consider in detail the quantization of a gravitational
model in vacuo which possesses local degrees of freedom, namely, the linearly
polarized Gowdy cosmologies with the spatial topology of a three-torus. We
carry out a hybrid quantization which combines loop and Fock techniques. We
discuss the main aspects and results of this hybrid quantization, which include
the resolution of the cosmological singularity, the polymeric quantization of
the internal time, a rigorous definition of the quantum constraints and the
construction of their solutions, the Hilbert structure of the physical states,
and the recovery of a conventional Fock quantization for the inhomogeneities.Comment: 24 pages, published in International Journal of Modern Physics A,
Special Issue: Proceedings of the Second Workshop on Quantum Gravity and
Noncommutative Geometry (Lisbon, Portugal
Geometric Stick-Breaking Processes for Continuous-Time Nonparametric Modeling
This paper is concerned with the construction of a continuous parameter sequence of random probability measures and its application for modeling random phenomena evolving in continuous time. At each time point we have a random probability measure which is generated by a Bayesian nonparametric hierarchical model, and the dependence structure is induced through a Wright-Fisher diffusion with mutation. The sequence is shown to be a stationary and reversible diffusion taking values on the space of probability measures. A simple estimation procedure for discretely observed data is presented and illustrated with simulated and real data sets.Bayesian non-parametric inference, continuous time dependent random measure, Markov process, measure-valued process, stationary process, stick-breaking process
ARES v2 - new features and improved performance
Aims: We present a new upgraded version of ARES. The new version includes a
series of interesting new features such as automatic radial velocity
correction, a fully automatic continuum determination, and an estimation of the
errors for the equivalent widths. Methods: The automatic correction of the
radial velocity is achieved with a simple cross-correlation function, and the
automatic continuum determination, as well as the estimation of the errors,
relies on a new approach to evaluating the spectral noise at the continuum
level. Results: ARES v2 is totally compatible with its predecessor. We show
that the fully automatic continuum determination is consistent with the
previous methods applied for this task. It also presents a significant
improvement on its performance thanks to the implementation of a parallel
computation using the OpenMP library.Comment: 4 pages, 2 Figures; accepted in A&A; ARES Webpage:
www.astro.up.pt/~sousasag/are
Needle-like subtarsal foreign body in a patient with no history of injury: case report
Subtarsal foreign bodies (FB) are a significant cause of ocular discomfort and are often missed on slit lamp examination of the everted upper eye lid. We report a patient with a needle-like subtarsal FB which was missed on repeated examinations over a three-month period. Staining with fluorescein and observation of scratch marks on epithelial surface of the cornea can give a clue to the presence of such a FB.
Key words: Subtarsal foreign bodies, ocular morbidity, fluorescein staining, case repor
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