438 research outputs found
Mode decomposition and renormalization in semiclassical gravity
We compute the influence action for a system perturbatively coupled to a
linear scalar field acting as the environment. Subtleties related to
divergences that appear when summing over all the modes are made explicit and
clarified. Being closely connected with models used in the literature, we show
how to completely reconcile the results obtained in the context of stochastic
semiclassical gravity when using mode decomposition with those obtained by
other standard functional techniques.Comment: 4 pages, RevTeX, no figure
FRW cosmologies between chaos and integrability
A recent paper by Castagnino, Giacomini and Lara concludes that there is no
chaos in a conformally coupled closed Friedmann-Robertson-Walker universe,
which is in apparent contradiction with previous works. We point out that
although nonchaotic the quoted system is nonintegrable.Comment: Revtex, 2 pages, no figure
Covariant approach to equilibration in effective field theories
The equilibration of two coupled reservoirs is studied using a Green function
approach which is suitable for future development with the closed time path
method. The problem is solved in two parameterizations, in order to demonstrate
the non-trivial issues of parameterization in both the intermediate steps and
the interpretation of physical quantities. We use a covariant approach to find
self-consistent solutions for the statistical distributions as functions of
time. We show that by formally introducing covariant connections, one can
rescale a slowly varying non-equilibrium theory so that it appears to be an
equilibrium one, for the purposes of calculation. We emphasize the importance
of properly tracking variable redefinitions in order to correctly interpret
physical quantities.Comment: 11 pages, Late
Quantum Fields in Nonstatic background: A Histories Perspective
For a quantum field living on a non - static spacetime no instantaneous
Hamiltonian is definable, for this generically necessitates a choice of
inequivalent representation of the canonical commutation relations at each
instant of time. This fact suggests a description in terms of time - dependent
Hilbert spaces, a concept that fits naturally in a (consistent) histories
framework. Our primary tool for the construction of the quantum theory in a
continuous -time histories format is the recently developed formalism based on
the notion of the history group . This we employ to study a model system
involving a 1+1 scalar field in a cavity with moving boundaries.
The instantaneous (smeared) Hamiltonian and a decoherence functional are then
rigorously defined so that finite values for the time - averaged particle
creation rate are obtainable through the study of energy histories. We also
construct the Schwinger - Keldysh closed- time - path generating functional as
a ``Fourier transform'' of the decoherence functional and evaluate the
corresponding n - point functions.Comment: 27 pages, LATEX; minor changes and corrections; version to appear in
JM
Semiclassical Effects and the Onset of Inflation
We present a class of exact solutions to the constraint equations of General
Relativity coupled to a Klein - Gordon field, these solutions being isotropic
but not homogeneous. We analyze the subsequent evolution of the consistent
Cauchy data represented by those solutions, showing that only certain special
initial conditions eventually lead to successfull Inflationary cosmologies. We
argue, however, that these initial conditions are precisely the likely outcomes
of quantum events occurred before the inflationary era.Comment: 22 pages, file written in RevTe
Noise induced transitions in semiclassical cosmology
A semiclassical cosmological model is considered which consists of a closed
Friedmann-Robertson-Walker in the presence of a cosmological constant, which
mimics the effect of an inflaton field, and a massless, non-conformally coupled
quantum scalar field. We show that the back-reaction of the quantum field,
which consists basically of a non local term due to gravitational particle
creation and a noise term induced by the quantum fluctuations of the field, are
able to drive the cosmological scale factor over the barrier of the classical
potential so that if the universe starts near zero scale factor (initial
singularity) it can make the transition to an exponentially expanding de Sitter
phase. We compute the probability of this transition and it turns out to be
comparable with the probability that the universe tunnels from "nothing" into
an inflationary stage in quantum cosmology. This suggests that in the presence
of matter fields the back-reaction on the spacetime should not be neglected in
quantum cosmology.Comment: LaTex, 33.tex pages, no figure
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