6,451 research outputs found

### Decoherence and entropy of primordial fluctuations. I: Formalism and interpretation

We propose an operational definition of the entropy of cosmological
perturbations based on a truncation of the hierarchy of Green functions. The
value of the entropy is unambiguous despite gauge invariance and the
renormalization procedure. At the first level of truncation, the reduced
density matrices are Gaussian and the entropy is the only intrinsic quantity.
In this case, the quantum-to-classical transition concerns the entanglement of
modes of opposite wave-vectors, and the threshold of classicality is that of
separability. The relations to other criteria of classicality are established.
We explain why, during inflation, most of these criteria are not intrinsic. We
complete our analysis by showing that all reduced density matrices can be
written as statistical mixtures of minimal states, the squeezed properties of
which are less constrained as the entropy increases. Pointer states therefore
appear not to be relevant to the discussion. The entropy is calculated for
various models in paper II.Comment: 23 page

### Non Singular Origin of the Universe and the Cosmological Constant Problem (CCP)

We consider a non singular origin for the Universe starting from an Einstein
static Universe in the framework of a theory which uses two volume elements
$\sqrt{-{g}}d^{4}x$ and $\Phi d^{4}x$, where $\Phi$ is a metric independent
density, also curvature, curvature square terms, first order formalism and for
scale invariance a dilaton field $\phi$ are considered in the action. In the
Einstein frame we also add a cosmological term that parametrizes the zero point
fluctuations. The resulting effective potential for the dilaton contains two
flat regions, for $\phi \rightarrow \infty$ relevant for the non singular
origin of the Universe and $\phi \rightarrow -\infty$, describing our present
Universe. Surprisingly, avoidance of singularities and stability as $\phi
\rightarrow \infty$ imply a positive but small vacuum energy as $\phi
\rightarrow -\infty$. Zero vacuum energy density for the present universe is
the "threshold" for universe creation.Comment: awarded an honorable mention in the Gravity Research Foundation 2011
Awards for Essays in Gravitation for 201

### Non Singular Origin of the Universe and its Present Vacuum Energy Density

We consider a non singular origin for the Universe starting from an Einstein
static Universe, the so called "emergent universe" scenario, in the framework
of a theory which uses two volume elements $\sqrt{-{g}}d^{4}x$ and $\Phi
d^{4}x$, where $\Phi$ is a metric independent density, used as an additional
measure of integration. Also curvature, curvature square terms and for scale
invariance a dilaton field $\phi$ are considered in the action. The first order
formalism is applied. The integration of the equations of motion associated
with the new measure gives rise to the spontaneous symmetry breaking (S.S.B) of
scale invariance (S.I.). After S.S.B. of S.I., it is found that a non trivial
potential for the dilaton is generated. In the Einstein frame we also add a
cosmological term that parametrizes the zero point fluctuations. The resulting
effective potential for the dilaton contains two flat regions, for $\phi
\rightarrow \infty$ relevant for the non singular origin of the Universe,
followed by an inflationary phase and $\phi \rightarrow -\infty$, describing
our present Universe. The dynamics of the scalar field becomes non linear and
these non linearities are instrumental in the stability of some of the emergent
universe solutions, which exists for a parameter range of values of the vacuum
energy in $\phi \rightarrow -\infty$, which must be positive but not very big,
avoiding the extreme fine tuning required to keep the vacuum energy density of
the present universe small. Zero vacuum energy density for the present universe
defines the threshold for the creation of the universe.Comment: 28 pages, short version of this paper awarded an honorable mention by
the Gravity Research Foundation, 2011, accepted for publication in
International Journal of Modern Physics

### Decoherence and entropy of primordial fluctuations II. The entropy budget

We calculate the entropy of adiabatic perturbations associated with a
truncation of the hierarchy of Green functions at the first non trivial level,
i.e. in a self-consistent Gaussian approximation. We give the equation
governing the entropy growth and discuss its phenomenology. It is parameterized
by two model-dependent kernels. We then examine two particular inflationary
models, one with isocurvature perturbations, the other with corrections due to
loops of matter fields. In the first model the entropy grows rapidely, while in
the second the state remains pure (at one loop).Comment: 28 page

### Atom laser dynamics in a tight-waveguide

We study the transient dynamics that arise during the formation of an atom
laser beam in a tight waveguide. During the time evolution the density profile
develops a series of wiggles which are related to the diffraction in time
phenomenon. The apodization of matter waves, which relies on the use of smooth
aperture functions, allows to suppress such oscillations in a time interval,
after which there is a revival of the diffraction in time. The revival time
scale is directly related to the inverse of the harmonic trap frequency for the
atom reservoir.Comment: 6 pages, 5 figures, to be published in the Proceedings of the 395th
WE-Heraeus Seminar on "Time Dependent Phenomena in Quantum Mechanics ",
organized by T. Kramer and M. Kleber (Blaubeuren, Germany, September 2007

### Quantum Simulation of Dissipative Processes without Reservoir Engineering

We present a quantum algorithm to simulate general finite dimensional
Lindblad master equations without the requirement of engineering the
system-environment interactions. The proposed method is able to simulate both
Markovian and non-Markovian quantum dynamics. It consists in the quantum
computation of the dissipative corrections to the unitary evolution of the
system of interest, via the reconstruction of the response functions associated
with the Lindblad operators. Our approach is equally applicable to dynamics
generated by effectively non-Hermitian Hamiltonians. We confirm the quality of
our method providing specific error bounds that quantify itss accuracy.Comment: 7 pages + Supplemental Material (6 pages

### Quantum dynamics and entanglement of a 1D Fermi gas released from a trap

We investigate the entanglement properties of the nonequilibrium dynamics of
one-dimensional noninteracting Fermi gases released from a trap. The gas of N
particles is initially in the ground state within hard-wall or harmonic traps,
then it expands after dropping the trap. We compute the time dependence of the
von Neumann and Renyi entanglement entropies and the particle fluctuations of
spatial intervals around the original trap, in the limit of a large number N of
particles. The results for these observables apply to one-dimensional gases of
impenetrable bosons as well.
We identify different dynamical regimes at small and large times, depending
also on the initial condition, whether it is that of a hard-wall or harmonic
trap. In particular, we analytically show that the expansion from hard-wall
traps is characterized by the asymptotic small-time behavior $S \approx
(1/3)\ln(1/t)$ of the von Neumann entanglement entropy, and the relation
$S\approx \pi^2 V/3$ where V is the particle variance, which are analogous to
the equilibrium behaviors whose leading logarithms are essentially determined
by the corresponding conformal field theory with central charge $c=1$. The time
dependence of the entanglement entropy of extended regions during the expansion
from harmonic traps shows the remarkable property that it can be expressed as a
global time-dependent rescaling of the space dependence of the initial
equilibrium entanglement entropy.Comment: 19 pages, 18 fig

### Prestrain relaxation in non-covalently modified ethylene-vinyl acetate | PyChol | multiwall carbon nanotube nanocomposites

Effects of aging on chemical structure and molecular dynamic behaviour of strained thermally active ethylene-vinyl acetate | multiwall carbon nanotube (EVA|MWCNT) composites were investigated by spectroscopy and microscopy techniques. Aged composites showed spatial inhomogeneity due to system relaxation. Inhomogeneity is attributed to segregation of non-covalently linked cholestryl 1-pyrenecarboxylate, acting as MWCNT dispersant and polymer compatibilizer. Analysis of molecular interplay between filler and matrix upon in situ temperature variation showed a lack of synchronicity, which had been observed in fresh composites. Reduced synchronous interplay allowed quantification of degraded π-π interactions, promoting PyChol unlatching as a result of both sonication and strained-derived π-π degradation

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