876 research outputs found
Cosmological backreaction of a quantized massless scalar field
We consider the backreaction problem of a quantized minimally coupled
massless scalar field in cosmology. The adiabatically regularized stress-energy
tensor in a general Friedmann-Robertson-Walker background is approximately
evaluated by using the fact that subhorizon modes evolve adiabatically and
superhorizon modes are frozen. The vacuum energy density is verified to obey a
new first order differential equation depending on a dimensionless parameter of
order unity, which calibrates subhorizon/superhorizon division. We check the
validity of the approximation by calculating the corresponding vacuum energy
densities in fixed backgrounds, which are shown to agree with the known results
in de Sitter space and space-times undergoing power law expansions. We then
apply our findings to slow-roll inflationary models. Although backreaction
effects are found to be negligible during the near exponential expansion, the
vacuum energy density generated during this period might be important at later
stages since it decreases slower than radiation or dust.Comment: 20 pages, 2 figures, v2: comments and a reference added, to appear in
JCA
Ultrathin Oxide Films by Atomic Layer Deposition on Graphene
In this paper, a method is presented to create and characterize mechanically
robust, free standing, ultrathin, oxide films with controlled, nanometer-scale
thickness using Atomic Layer Deposition (ALD) on graphene. Aluminum oxide films
were deposited onto suspended graphene membranes using ALD. Subsequent etching
of the graphene left pure aluminum oxide films only a few atoms in thickness. A
pressurized blister test was used to determine that these ultrathin films have
a Young's modulus of 154 \pm 13 GPa. This Young's modulus is comparable to much
thicker alumina ALD films. This behavior indicates that these ultrathin
two-dimensional films have excellent mechanical integrity. The films are also
impermeable to standard gases suggesting they are pinhole-free. These
continuous ultrathin films are expected to enable new applications in fields
such as thin film coatings, membranes and flexible electronics.Comment: Nano Letters (just accepted
Energy-Momentum Tensor of Particles Created in an Expanding Universe
We present a general formulation of the time-dependent initial value problem
for a quantum scalar field of arbitrary mass and curvature coupling in a FRW
cosmological model. We introduce an adiabatic number basis which has the virtue
that the divergent parts of the quantum expectation value of the
energy-momentum tensor are isolated in the vacuum piece of , and
may be removed using adiabatic subtraction. The resulting renormalized
is conserved, independent of the cutoff, and has a physically transparent,
quasiclassical form in terms of the average number of created adiabatic
`particles'. By analyzing the evolution of the adiabatic particle number in de
Sitter spacetime we exhibit the time structure of the particle creation
process, which can be understood in terms of the time at which different
momentum scales enter the horizon. A numerical scheme to compute as a
function of time with arbitrary adiabatic initial states (not necessarily de
Sitter invariant) is described. For minimally coupled, massless fields, at late
times the renormalized goes asymptotically to the de Sitter invariant
state previously found by Allen and Folacci, and not to the zero mass limit of
the Bunch-Davies vacuum. If the mass m and the curvature coupling xi differ
from zero, but satisfy m^2+xi R=0, the energy density and pressure of the
scalar field grow linearly in cosmic time demonstrating that, at least in this
case, backreaction effects become significant and cannot be neglected in de
Sitter spacetime.Comment: 28 pages, Revtex, 11 embedded .ps figure
Rayleigh Imaging of Graphene and Graphene Layers
We investigate graphene and graphene layers on different substrates by
monochromatic and white-light confocal Rayleigh scattering microscopy. The
image contrast depends sensitively on the dielectric properties of the sample
as well as the substrate geometry and can be described quantitatively using the
complex refractive index of bulk graphite. For few layers (<6) the
monochromatic contrast increases linearly with thickness: the samples behave as
a superposition of single sheets which act as independent two dimensional
electron gases. Thus, Rayleigh imaging is a general, simple and quick tool to
identify graphene layers, that is readily combined with Raman scattering, which
provides structural identification.Comment: 8 pages, 9 figure
Renormalization of nonequilibrium dynamics in FRW cosmology
We derive the renormalized nonequilibrium equations of motion for a scalar
field and its quantum back reaction in a conformally flat
Friedmann-Robertson-Walker universe. We use a fully covariant formalism
proposed by us recently for handling numerically and analytically
nonequilibrium dynamics in one-loop approximation. The system is assumed to be
in a conformal vacuum state initially. We use dimensional regularization; we
find that the counter terms can be chosen independent of the initial conditions
though the divergent leading order graphs do depend on them.Comment: 14 pages, RevTeX, some obvious typos correcte
Renormalization of the nonequilibrium dynamics of fermions in a flat FRW universe
We derive the renormalized equations of motion and the renormalized
energy-momentum tensor for fermions coupled to a spatially homogeneous scalar
field (inflaton) in a flat FRW geometry. The fermion back reaction to the
metric and to the inflaton field is formulated in one-loop approximation.
Having determined the infinite counter terms in an scheme we
formulate the finite terms in a form suitable for numerical computation. We
comment on the trace anomaly which is inferred from the standard analysis. We
also address the problem of initial singularities and determine the Bogoliubov
transformation by which they are removed.Comment: 26 pages, LaTe
Out-of-equilibrium evolution of scalar fields in FRW cosmology: renormalization and numerical simulations
We present a renormalized computational framework for the evolution of a
self-interacting scalar field (inflaton) and its quantum fluctuations in an FRW
background geometry. We include a coupling of the field to the Ricci scalar
with a general coupling parameter . We take into account the classical and
quantum back reactions, i.e., we consider the the dynamical evolution of the
cosmic scale factor. We perform, in the one-loop and in the large-N
approximation, the renormalization of the equation of motion for the inflaton
field, and of its energy momentum tensor. Our formalism is based on a
perturbative expansion for the mode functions, and uses dimensional
regularization. The renormalization procedure is manifestly covariant and the
counter terms are independent of the initial state. Some shortcomings in the
renormalization of the energy-momentum tensor in an earlier publication are
corrected. We avoid the occurence of initial singularities by constructing a
suitable class of initial states. The formalism is implemented numerically and
we present some results for the evolution in the post-inflationary preheating
era.Comment: 44 pages, uses latexsym, 6 pages with 11 figures in a .ps fil
STOCHASTIC DYNAMICS OF LARGE-SCALE INFLATION IN DE~SITTER SPACE
In this paper we derive exact quantum Langevin equations for stochastic
dynamics of large-scale inflation in de~Sitter space. These quantum Langevin
equations are the equivalent of the Wigner equation and are described by a
system of stochastic differential equations. We present a formula for the
calculation of the expectation value of a quantum operator whose Weyl symbol is
a function of the large-scale inflation scalar field and its time derivative.
The unique solution is obtained for the Cauchy problem for the Wigner equation
for large-scale inflation. The stationary solution for the Wigner equation is
found for an arbitrary potential. It is shown that the large-scale inflation
scalar field in de Sitter space behaves as a quantum one-dimensional
dissipative system, which supports the earlier results. But the analogy with a
one-dimensional model of the quantum linearly damped anharmonic oscillator is
not complete: the difference arises from the new time dependent commutation
relation for the large-scale field and its time derivative. It is found that,
for the large-scale inflation scalar field the large time asymptotics is equal
to the `classical limit'. For the large time limit the quantum Langevin
equations are just the classical stochastic Langevin equations (only the
stationary state is defined by the quantum field theory).Comment: 21 pages RevTex preprint styl
Resonant decay of flat directions
We study preheating, i.e., non-perturbative resonant decay, of flat direction
fields, concentrating on MSSM flat directions and the right handed sneutrino.
The difference between inflaton preheating and flaton preheating, is that the
potential is more constraint in the latter case. The effects of a complex
driving field, quartic couplings in the potential, and the presence of a
thermal bath are important and cannot be neglected.
Preheating of MSSM flat directions is typically delayed due to out-of-phase
oscillations of the real and imaginary components and may be preceded by
perturbative decay or -ball formation. Particle production due to the
violation of adiabaticity is expected to be inefficient due to back reaction
effects. For a small initial sneutrino VEV, with
the mass of the right handed sneutrino and a yakawa coupling, there are
tachyonic instabilities. The -term quartic couplings do not generate an
effective mass for the tachyonic modes, making it an efficient decay channel.
It is unclear how thermal scattering affects the resonance.Comment: 20 pages, 4 figure
Cosmic Microwave Background Anisotropy with Cosine-Type Quintessence
We study the Cosmic Microwave Background (CMB) anisotropies produced by
cosine-type quintessence models. In our analysis, effects of the adiabatic and
isocurvature fluctuations are both taken into account. For purely adiabatic
fluctuations with scale invariant spectrum, we obtain a stringent constraint on
the model parameters using the CMB data from COBE, BOOMERanG and MAXIMA.
Furthermore, it is shown that isocurvature fluctuations have significant
effects on the CMB angular power spectrum at low multipoles in some parameter
space, which may be detectable in future satellite experiments. Such a signal
may be used to test the cosine-type quintessence models.Comment: 21 pages, 9 figure
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