226 research outputs found
Noncommutative Geometry and Cosmology
We study some consequences of noncommutativity to homogeneous cosmologies by
introducing a deformation of the commutation relation between the
minisuperspace variables. The investigation is carried out for the
Kantowski-Sachs model by means of a comparative study of the universe evolution
in four different scenarios: the classical commutative, classical
noncommutative, quantum commutative, and quantum noncommutative. The comparison
is rendered transparent by the use of the Bohmian formalism of quantum
trajectories. As a result of our analysis, we found that noncommutativity can
modify significantly the universe evolution, but cannot alter its singular
behavior in the classical context. Quantum effects, on the other hand, can
originate non-singular periodic universes in both commutative and
noncommutative cases. The quantum noncommutative model is shown to present
interesting properties, as the capability to give rise to non-trivial dynamics
in situations where its commutative counterpart is necessarily static.Comment: 22 pages, 5 figures, substantial changes in the presentation, results
are the same, to appear in Physical Review
Noncommutative Conformally Coupled Scalar Field Cosmology and its Commutative Counterpart
We study the implications of a noncommutative geometry of the minisuperspace
variables for the FRW universe with a conformally coupled scalar field. The
investigation is carried out by means of a comparative study of the universe
evolution in four different scenarios: classical commutative, classical
noncommutative, quantum commutative, and quantum noncommutative, the last two
employing the Bohmian formalism of quantum trajectories. The role of
noncommutativity is discussed by drawing a parallel between its realizations in
two possible frameworks for physical interpretation: the NC-frame, where it is
manifest in the universe degrees of freedom, and in the C-frame, where it is
manifest through theta-dependent terms in the Hamiltonian. As a result of our
comparative analysis, we find that noncommutative geometry can remove
singularities in the classical context for sufficiently large values of theta.
Moreover, under special conditions, the classical noncommutative model can
admit bouncing solutions characteristic of the commutative quantum FRW
universe. In the quantum context, we find non-singular universe solutions
containing bounces or being periodic in the quantum commutative model. When
noncommutativity effects are turned on in the quantum scenario, they can
introduce significant modifications that change the singular behavior of the
universe solutions or that render them dynamical whenever they are static in
the commutative case. The effects of noncommutativity are completely specified
only when one of the frames for its realization is adopted as the physical one.
Non-singular solutions in the NC-frame can be mapped into singular ones in the
C-frame.Comment: explanations added, references include
Noncommutative quantum mechanics and Bohm's ontological interpretation
We carry out an investigation into the possibility of developing a Bohmian
interpretation based on the continuous motion of point particles for
noncommutative quantum mechanics. The conditions for such an interpretation to
be consistent are determined, and the implications of its adoption for
noncommutativity are discussed. A Bohmian analysis of the noncommutative
harmonic oscillator is carried out in detail. By studying the particle motion
in the oscillator orbits, we show that small-scale physics can have influence
at large scales, something similar to the IR-UV mixing
On the consistency of a repulsive gravity phase in the early Universe
We exploit the possibility of existence of a repulsive gravity phase in the
evolution of the Universe. A toy model with a free scalar field minimally
coupled to gravity, but with the "wrong sign" for the energy and negative
curvature for the spatial section, is studied in detail. The background
solutions display a bouncing, non-singular Universe. The model is well-behaved
with respect to tensor perturbations. But, it exhibits growing models with
respect to scalar perturbations whose maximum occurs in the bouncing. Hence,
large inhomogeneties are produced. At least for this case, a repulsive phase
may destroy homogeneity, and in this sense it may be unstable. A newtonian
analogous model is worked out; it displays qualitatively the same behaviour.
The generality of this result is discussed. In particular, it is shown that the
addition of an attractive radiative fluid does not change essentially the
results. We discuss also a quantum version of the classical repulsive phase,
through the Wheeler-de Witt equation in mini-superspace, and we show that it
displays essentially the same scenario as the corresponding attractive phase.Comment: Latex file, 15 pages, 7 figures. There is a new figure, a new section
and some other minor correction
Anisotropy and chemical composition of ultra-high energy cosmic rays using arrival directions measured by the Pierre Auger Observatory
The Pierre Auger Collaboration has reported evidence for anisotropy in the
distribution of arrival directions of the cosmic rays with energies
eV. These show a correlation with the distribution
of nearby extragalactic objects, including an apparent excess around the
direction of Centaurus A. If the particles responsible for these excesses at
are heavy nuclei with charge , the proton component of the
sources should lead to excesses in the same regions at energies . We here
report the lack of anisotropies in these directions at energies above
(for illustrative values of ). If the anisotropies
above are due to nuclei with charge , and under reasonable
assumptions about the acceleration process, these observations imply stringent
constraints on the allowed proton fraction at the lower energies
The Fluorescence Detector of the Pierre Auger Observatory
The Pierre Auger Observatory is a hybrid detector for ultra-high energy
cosmic rays. It combines a surface array to measure secondary particles at
ground level together with a fluorescence detector to measure the development
of air showers in the atmosphere above the array. The fluorescence detector
comprises 24 large telescopes specialized for measuring the nitrogen
fluorescence caused by charged particles of cosmic ray air showers. In this
paper we describe the components of the fluorescence detector including its
optical system, the design of the camera, the electronics, and the systems for
relative and absolute calibration. We also discuss the operation and the
monitoring of the detector. Finally, we evaluate the detector performance and
precision of shower reconstructions.Comment: 53 pages. Submitted to Nuclear Instruments and Methods in Physics
Research Section
The Pierre Auger Observatory III: Other Astrophysical Observations
Astrophysical observations of ultra-high-energy cosmic rays with the Pierre
Auger ObservatoryComment: Contributions to the 32nd International Cosmic Ray Conference,
Beijing, China, August 201
Operations of and Future Plans for the Pierre Auger Observatory
Technical reports on operations and features of the Pierre Auger Observatory,
including ongoing and planned enhancements and the status of the future
northern hemisphere portion of the Observatory. Contributions to the 31st
International Cosmic Ray Conference, Lodz, Poland, July 2009.Comment: Contributions to the 31st ICRC, Lodz, Poland, July 200
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