3,206 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
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
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
Scale Invariance without Inflation?
We propose a new alternative mechanism to seed a scale invariant spectrum of
primordial density perturbations that does not rely on inflation. In our
scenario, a perfect fluid dominates the early stages of an expanding,
non-inflating universe. Because the speed of sound of the fluid decays,
perturbations are left frozen behind the sound horizon, with a spectral index
that depends on the fluid equation of state. We explore here a toy model that
realizes this idea. Although the model can explain an adiabatic, Gaussian,
scale invariant spectrum of primordial perturbations, it turns out that in its
simplest form it cannot account for the observed amplitude of the primordial
density perturbations.Comment: 6 two-column pages, 1 figure. Uses RevTeX4. v2: References added and
number of required e-folds refine
From heaviness to lightness during inflation
We study the quantum fluctuations of scalar fields with a variable effective
mass during an inflationary phase. We consider the situation where the
effective mass depends on a background scalar field, which evolves during
inflation from being frozen into a damped oscillatory phase when the Hubble
parameter decreases below its mass. We find power spectra with suppressed
amplitude on large scales, similar to the standard massless spectrum on small
scales, and affected by modulations on intermediate scales. We stress the
analogies and differences with the parametric resonance in the preheating
scenario. We also discuss some potentially observable consequences when the
scalar field behaves like a curvaton.Comment: 23 pages; 8 figures; published versio
Energy-Momentum Tensor of Cosmological Fluctuations during Inflation
We study the renormalized energy-momentum tensor (EMT) of cosmological scalar
fluctuations during the slow-rollover regime for chaotic inflation with a
quadratic potential and find that it is characterized by a negative energy
density which grows during slow-rollover. We also approach the back-reaction
problem as a second-order calculation in perturbation theory finding no
evidence that the back-reaction of cosmological fluctuations is a gauge
artifact. In agreement with the results on the EMT, the average expansion rate
is decreased by the back-reaction of cosmological fluctuations.Comment: 19 pages, no figures.An appendix and references added, conclusions
unchanged, version accepted for publication in PR
The Primordial Perturbation Spectrum from Various Expanding and Contracting Phases
In this paper, focusing on the case of single scalar field, we discuss
various expanding and contracting phases generating primordial perturbations,
and study the relation between the primordial perturbation spectrum from these
phases and the parameter w of state equation in details. Furthermore, we offer
an interesting classification for the primordial perturbation spectrum from
various phases, which may have important implications for building an early
universe scenario embedded in possible high energy theories.Comment: 5 pages, 3 eps figure
Quantum gravitational fluctuations and the semi-classical limit
An attempt is made to go beyond the standard semi-classical approximation for
gravity in the Born-Oppenheimer decomposition of the wave-function in
minisuperspace. New terms are included which correspond to quantum
gravitational fluctuations on the background metric. Their existence renders
the definition of the semi-classical limit rather delicate and can lead to the
avoidance of the singularities the classical theory predicts in cosmology and
in the gravitational collapse of compact objects.Comment: LaTeX, 15 pages, no figure
Parametric amplification of metric fluctuations through a bouncing phase
We clarify the properties of the behavior of classical cosmological
perturbations when the Universe experiences a bounce. This is done in the
simplest possible case for which gravity is described by general relativity and
the matter content has a single component, namely a scalar field in a closed
geometry. We show in particular that the spectrum of scalar perturbations can
be affected by the bounce in a way that may depend on the wave number, even in
the large scale limit. This may have important implications for string
motivated models of the early Universe.Comment: 17 pages, 12 figures, LaTeX-ReVTeX format, version to match Phys.
Rev.
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