3,785 research outputs found
Energy-Momentum Tensor and Particle Creation in the de Sitter Universe
For a quantum field in a curved background the choice of the vacuum state is
crucial. We exhibit a vacuum state in which the expectation values of the
energy and pressure allow an intuitive interpretation. We apply this general
result to the de Sitter universe.Comment: 3 pages, 1 figure, revtex. To appear in the proceedings of the 8th
Marcel Grossmann Meetin
Quantum Gravity Without Ghosts
An outline is given of a recently discovered technique for building a quantum
effective action that is completely independent of gauge-fixing choices and
ghost determinants. One makes maximum use of the geometry and fibre-bundle
structure of the space of field histories and introduces a set of nonlocal
composite fields: the geodesic normal fields based on Vilkovisky's connection
on the space of histories. The closed-time-path formalism of Schwinger, Bakshi,
Mahantappa {\it et al} can be adapted for these fields, and a set of
gauge-fixing-independent dynamical equations for their expectation values
(starting from given initial conditions) can be computed. An obvious
application for such equations is to the study of the formation and radiative
decay of black holes, and to other back-reaction problems.Comment: This is a paper submitted to the Gravity Research Foundation Essay
Competition of 1998 that received Honorable Mentio
Large Scale Features of Rotating Forced Turbulence
Large scale features of a randomly isotropically forced incompressible and
unbounded rotating fluid are examined in perturbation theory. At first order in
both the random force amplitude and the angular velocity we find two types of
modifications to the fluid equation of motion. The first correction transforms
the molecular shear viscosity into a (rotation independent) effective
viscosity. The second perturbative correction leads to a new large scale
non-dissipative force proportional to the fluid angular velocity in the slow
rotation regime. This effective force does no net work and alters the
dispersion relation of inertial waves propagating in the fluid. Both
dynamically generated corrections can be identified with certain components of
the most general axisymmetric ``viscosity tensor'' for a Newtonian fluid.Comment: 12 pages, 2 figures, RevTeX, and accepted for publication in Phys.
Rev.
Instrumented activity and semiotic mediation: two frames to describe the conjecture construction process as curricular organizer
We document part of the process through which conjectures produced by students, with the aid of the dynamic geometry software Cabri, when they solve proposed geometric problems, become a curriculum organizer in the classroom. We first focus on characterizing students’ instrumented activity recurring to utilization schema (Rabardel, 1995, in Bartolini Bussi and Mariotti, 2008), and then describe the teacher’s content management through which the ideas produced by the students become key elements of knowledge construction
Analyzing the proving activity of a group of three students
We present an analysis and outline an evaluation of the proving activity of a group of three university level students when solving a geometrical problem whose solution required the formulation of a conjecture and its justification within a specific theoretical system. To carry out the analysis, we used the model presented by Boero, Douek, Morselli and Pedemonte (2010) that centers on the arguments and rational behavior. Our analysis indicates that the student‘s proving activity is close to the one we used as a reference
Cosmological Horizon Modes and Linear Response in de Sitter Spacetime
Linearized fluctuations of quantized matter fields and the spacetime geometry
around de Sitter space are considered in the case that the matter fields are
conformally invariant. Taking the unperturbed state of the matter to be the de
Sitter invariant Bunch-Davies state, the linear variation of the stress tensor
about its self-consistent mean value serves as a source for fluctuations in the
geometry through the semi-classical Einstein equations. This linear response
framework is used to investigate both the importance of quantum backreaction
and the validity of the semi-classical approximation in cosmology. The full
variation of the stress tensor, delta T^a_b contains two kinds of terms: (1)
those that depend explicitly upon the linearized metric variation delta g_{cd}
through the [T^a_b, T^{cd}] causal response function; and (2) state dependent
variations, independent of delta g_{cd}. For perturbations of the first kind,
the criterion for the validity of the semi-classical approximation in de Sitter
space is satisfied for fluctuations on all scales well below the Planck scale.
The perturbations of the second kind contain additional massless scalar degrees
of freedom associated with changes of state of the fields on the cosmological
horizon scale. These scalar degrees of freedom arise necessarily from the local
auxiliary field form of the effective action associated with the trace anomaly,
are potentially large on the horizon scale, and therefore can lead to
substantial non-linear quantum backreaction effects in cosmology.Comment: 62 pages, 4 figures v.2 is amended to match the published version in
Phys. Rev. D: Eqs. (6.13)-(6.14) for the quadratic action added, two
references added, several minor typos correcte
Dynamic geometry, implication and abduction: a case study
In this paper we illustrate the role of dynamic geometry as an environment that propitiates the use of empirical explorations to favor learning to prove. This is possible thanks to abductive processes, related to the establishment of implications that university students of a plane geometry course carry out when, supported by a dynamic geometry program, they solve a problem in which they must discover a geometric fact, formulate a conjecture and prove it
La función cultural de los museos en San Salvador
La función cultural de los museos en San Salvador. Los resultados. Referencias BibliografÃcas.En Kóot, enero 2010, año 1, N°1, p. 25-32Universidad Tecnológica de El Salvado
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