3,406 research outputs found
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
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
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
The Born-Oppenheimer Approach to the Matter-Gravity System and Unitarity
The Born-Oppenheimer approach to the matter-gravity system is illustrated and
the unitary evolution for matter, in the absence of phenomena such as
tunnelling or other instabilities, verified. The Born-Oppenheimer approach to
the matter-gravity system is illustrated in a simple minisuperspace model and
the corrections to quantum field theory on a semiclassical background
exhibited. Within such a context the unitary evolution for matter, in the
absence of phenomena such as tunnelling or other instabilities, is verified and
compared with the results of other approaches. Lastly the simplifications
associated with the use of adiabatic invariants to obtain the solution of the
explicitly time dependent evolution equation for matter are evidenced.Comment: Latex, 12 pages. Revised version as accepted for publication by
Class. and Quant. Grav. Some points explained and misprints correcte
Adiabatic and Isocurvature Perturbations for Multifield Generalized Einstein Models
Low energy effective field theories motivated by string theory will likely
contain several scalar moduli fields which will be relevant to early Universe
cosmology. Some of these fields are expected to couple with non-standard
kinetic terms to gravity. In this paper, we study the splitting into adiabatic
and isocurvature perturbations for a model with two scalar fields, one of which
has a non-standard kinetic term in the Einstein-frame action. Such actions can
arise, e.g., in the Pre-Big-Bang and Ekpyrotic scenarios. The presence of a
non-standard kinetic term induces a new coupling between adiabatic and
isocurvature perturbations which is non-vanishing when the potential for the
matter fields is nonzero. This coupling is un-suppressed in the long wavelength
limit and thus can lead to an important transfer of power from the entropy to
the adiabatic mode on super-Hubble scales. We apply the formalism to the case
of a previously found exact solution with an exponential potential and study
the resulting mixing of adiabatic and isocurvature fluctuations in this
example. We also discuss the possible relevance of the extra coupling in the
perturbation equations for the process of generating an adiabatic component of
the fluctuations spectrum from isocurvature perturbations without considering a
later decay of the isocurvature component.Comment: 11 pages, 3 figures, one equation corrected, typos fixed, conclusions
unchange
Quantum Fields in an Expanding Universe
We extend our analysis for scalar fields in a Robertson-Walker metric to the
electromagnetic field and Dirac fields by the method of invariants. The issue
of the relation between conformal properties and particle production is
re-examined and it is verified that the electromagnetic and massless spinor
actions are conformal invariant, while the massless conformally coupled scalar
field is not. For the scalar field case it is pointed out that the violation of
conformal simmetry due to surface terms, although ininfluential for the
equation of motion, does lead to effects in the quantized theory.Comment: 15 pp, no figures, accepted for publication in Class. Quantum Gra
Cosmology with positive and negative exponential potentials
We present a phase-plane analysis of cosmologies containing a scalar field
with an exponential potential
where and may be positive or negative. We show that
power-law kinetic-potential scaling solutions only exist for sufficiently flat
() negative
potentials. The latter correspond to a class of ever-expanding cosmologies with
negative potential. However we show that these expanding solutions with a
negative potential are to unstable in the presence of ordinary matter, spatial
curvature or anisotropic shear, and generic solutions always recollapse to a
singularity. Power-law kinetic-potential scaling solutions are the late-time
attractor in a collapsing universe for steep negative potentials (the ekpyrotic
scenario) and stable against matter, curvature or shear perturbations.
Otherwise kinetic-dominated solutions are the attractor during collapse (the
pre big bang scenario) and are only marginally stable with respect to
anisotropic shear.Comment: 8 pages, latex with revtex, 9 figure
Energy-Momentum Tensor of Field Fluctuations in Massive Chaotic Inflation
We study the renormalized energy-momentum tensor (EMT) of the inflaton
fluctuations in rigid space-times during the slow-rollover regime for chaotic
inflation with a mass term. We use dimensional regularization with adiabatic
subtraction and introduce a novel analytic approximation for the inflaton
fluctuations which is valid during the slow-rollover regime. Using this
approximation we find a scale invariant spectrum for the inflaton fluctuations
in a rigid space-time, and we confirm this result by numerical methods. The
resulting renormalized EMT is covariantly conserved and agrees with the
Allen-Folacci result in the de Sitter limit, when the expansion is exactly
linearly exponential in time. We analytically show that the EMT tensor of the
inflaton fluctuations grows initially in time, but saturates to the value H^2
H(0)^2, where H is the Hubble parameter and H(0) is its value when inflation
has started. This result also implies that the quantum production of light
scalar fields (with mass smaller or equal to the inflaton mass) in this model
of chaotic inflation depends on the duration of inflation and is larger than
the usual result extrapolated from the de Sitter result.Comment: revtex style, 24 pages, 6 eps figures Numerical checks added and
moduli section improve
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