2,930 research outputs found
Second Order Gauge-Invariant Perturbations during Inflation
The evolution of gauge invariant second-order scalar perturbations in a
general single field inflationary scenario are presented. Different second
order gauge invariant expressions for the curvature are considered. We evaluate
perturbatively one of these second order curvature fluctuations and a second
order gauge invariant scalar field fluctuation during the slow-roll stage of a
massive chaotic inflationary scenario, taking into account the deviation from a
pure de Sitter evolution and considering only the contribution of super-Hubble
perturbations in mode-mode coupling. The spectra resulting from their
contribution to the second order quantum correlation function are nearly
scale-invariant, with additional logarithmic corrections to the first order
spectrum. For all scales of interest the amplitude of these spectra depend on
the total number of e-folds. We find, on comparing first and second order
perturbation results, an upper limit to the total number of e-folds beyond
which the two orders are comparable.Comment: 17 pages, 6 figures. Final version to appear in Phys. Rev.
Metric perturbations at reheating: the use of spherical symmetry
We consider decay of the inflaton with a quartic potential coupled to other
fields, including gravity, but restricted to spherical symmetry. We describe
analytically an early, quasilinear regime, during which inflaton fluctuations
and the metric functions are driven by nonlinear effects of the decay products.
We present a detailed study of the leading nonlinear effects in this regime.
Results of the quasilinear approximation, in its domain of applicability, are
found to be consistent with those of fully nonlinear lattice studies. We
discuss how these results may be promoted to the full three dimensions.Comment: 18 pages, revtex, 2 figure
Resonance enhancement of particle production during reheating
We found a consistent equation of reheating after inflation, which shows that
for small quantum fluctuations the frequencies of resonance are slighted
different from the standard ones. Quantum interference is taken into account
and we found that at large fluctuations the process mimics very well the usual
parametric resonance but proceed in a different dynamical way. The analysis is
made in a toy quantum mechanical model and we discuss further its extension to
quantum field theory.Comment: 4 pages, 4 figures(eps), using RevTe
Three-body spin-orbit forces from chiral two-pion exchange
Using chiral perturbation theory, we calculate the density-dependent
spin-orbit coupling generated by the two-pion exchange three-nucleon
interaction involving virtual -isobar excitation. From the
corresponding three-loop Hartree and Fock diagrams we obtain an isoscalar
spin-orbit strength which amounts at nuclear matter
saturation density to about half of the empirical value of MeVfm. The
associated isovector spin-orbit strength comes out about a
factor of 20 smaller. Interestingly, this three-body spin-orbit coupling is not
a relativistic effect but independent of the nucleon mass . Furthermore, we
calculate the three-body spin-orbit coupling generated by two-pion exchange on
the basis of the most general chiral -contact interaction. We find
similar (numerical) results for the isoscalar and isovector spin-orbit
strengths and with a strong dominance of
the p-wave part of the -contact interaction and the Hartree
contribution.Comment: 8 pages, 4figure, published in : Physical Review C68, 054001 (2003
Evolution of Large Scale Curvature Fluctuations During the Perturbative Decay of the Inflaton
We study the evolution of cosmological fluctuations during and after
inflation driven by a scalar field coupled to a perfect fluid through afriction
term. During the slow-roll regime for the scalar field, the perfect fluid is
also frozen and isocurvature perturbations are generated. After the end of
inflation, during the decay of the inflaton, we find that a change in the
observationally relevant large scale curvature fluctuations is possible.Comment: 9 pages, 2 figures; v2: version published in PR
Scalar-tensor cosmologies: fixed points of the Jordan frame scalar field
We study the evolution of homogeneous and isotropic, flat cosmological models
within the general scalar-tensor theory of gravity with arbitrary coupling
function and potential. After introducing the limit of general relativity we
describe the details of the phase space geometry. Using the methods of
dynamical systems for the decoupled equation of the Jordan frame scalar field
we find the fixed points of flows in two cases: potential domination and matter
domination. We present the conditions on the mathematical form of the coupling
function and potential which determine the nature of the fixed points
(attractor or other). There are two types of fixed points, both are
characterized by cosmological evolution mimicking general relativity, but only
one of the types is compatible with the Solar System PPN constraints. The phase
space structure should also carry over to the Einstein frame as long as the
transformation between the frames is regular which however is not the case for
the latter (PPN compatible) fixed point.Comment: 21 pages, 4 figures, some comments and references adde
Study of a Class of Four Dimensional Nonsingular Cosmological Bounces
We study a novel class of nonsingular time-symmetric cosmological bounces. In
this class of four dimensional models the bounce is induced by a perfect fluid
with a negative energy density. Metric perturbations are solved in an analytic
way all through the bounce. The conditions for generating a scale invariant
spectrum of tensor and scalar metric perturbations are discussed.Comment: 16 pages, 10 figure
Mean-field calculations of exotic nuclei ground states
We study the predictions of three mean-field theoretical approaches in the
description of the ground state properties of some spherical nuclei far from
the stability line. We compare binding energies, single particle spectra,
density distributions, charge and neutron radii obtained with non-relativistic
Hartree-Fock calculations carried out with both zero and finite-range
interactions, and with a relativistic Hartree approach which uses a
finite-range interaction. The agreement between the results obtained with the
three different approaches indicates that these results are more related to the
basic hypotheses of the mean-field approach rather than to its implementation
in actual calculations.Comment: 16 pages, 12 figures, 2 tables, accepted for publication in Physical
Review
Improved WKB analysis of Slow-Roll Inflation
We extend the WKB method for the computation of cosmological perturbations
during inflation beyond leading order and provide the power spectra of scalar
and tensor perturbations to second order in the slow-roll parameters. Our
method does not require that the slow-roll parameters be constant. Although
leading and next-to-leading results in the slow-roll parameters depend on the
approximation technique used in the computation, we find that the inflationary
theoretical predictions obtained may reach the accuracy required by planned
observations. In two technical appendices, we compare our techniques and
results with previous findings.Comment: REVTeX 4, 13 pages, no figures, final version to appear in Phys. Rev.
Density correlations in ultracold atomic Fermi gases
We investigate density fluctuations in a coherent ensemble of interacting
fermionic atoms. Adapting the concept of full counting statistics, well-known
from quantum optics and mesoscopic electron transport, we study second-order as
well as higher-order correlators of density fluctuations. Using the mean-field
BCS state to describe the whole interval between the BCS limit and the BEC
limit, we obtain an exact expression for the cumulant-generating function of
the density fluctuations of an atomic cloud. In the two-dimensional case, we
obtain a closed analytical expression. Poissonian fluctuations of a molecular
condensate on the BEC side are strongly suppressed on the BCS side. The size of
the fluctuations in the BCS limit is a direct measure of the pairing potential.
We also discuss the BEC-BCS crossover of the third cumulant and the temperature
dependence of the second cumulant.Comment: 4 pages, 4 figures. To appear in Phys. Rev. A. New calculation of the
bin statistics of a free Bose gas; updated and extended bibliograph
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