2,792 research outputs found
Statistical analysis of direct-strike lightning data (1980 to 1982)
Electromagnetic measurements are being made during direct lightning strikes by NASA Langley Center using a specially instrumented F-106B aircraft. The research is to aid refinement, characterization, and understanding of the lightning-aircraft interaction process and the lightning hazards to aircraft. Statistical methods are applied to characterize some aspects of the lightning data obtained from 176 strikes to the aircraft. Specific attention is given to the problem of estimating the upper extreme quantiles of the distributions of peak-to-peak values for currents and rates of change in the magnetic and flux densities. A formal treatment via a general location-scale family of models allows the estimation method to be adapted to the realized shapes the distributions. The shapes are examined by probability plotting methods
An optimal estimator for the CMB-LSS angular power spectrum and its application to WMAP and NVSS data
We use a Quadratic Maximum Likelihood (QML) method to estimate the angular
power spectrum of the cross-correlation between cosmic microwave background and
large scale structure maps as well as their individual auto-spectra. We
describe our implementation of this method and demonstrate its accuracy on
simulated maps. We apply this optimal estimator to WMAP 7-year and NRAO VLA Sky
Survey (NVSS) data and explore the robustness of the angular power spectrum
estimates obtained by the QML method. With the correction of the declination
systematics in NVSS, we can safely use most of the information contained in
this survey. We then make use of the angular power spectrum estimates obtained
by the QML method to derive constraints on the dark energy critical density in
a flat CDM model by different likelihood prescriptions. When using
just the cross-correlation between WMAP 7 year and NVSS maps with 1.8
resolution, the best-fit model has a cosmological constant of approximatively
70% of the total energy density, disfavouring an Einstein-de Sitter Universe at
more than 2 CL (confidence level).Comment: 12 pages, 12 figure
An Early Universe Model with Stiff Matter and a Cosmological Constant
In the present work, we study the quantum cosmology description of a
Friedmann-Robertson-Walker model in the presence of a stiff matter perfect
fluid and a negative cosmological constant. We work in the Schutz's variational
formalism and the spatial sections have constant negative curvature. We
quantize the model and obtain the appropriate Wheeler-DeWitt equation. In this
model the states are bounded therefore we compute the discrete energy spectrum
and the corresponding eigenfunctions. In the present work, we consider only the
negative eigenvalues and their corresponding eigenfunctions. This choice
implies that the energy density of the perfect fluid is negative. A stiff
matter perfect fluid with this property produces a model with a bouncing
solution, at the classical level, free from an initial singularity. After that,
we use the eigenfunctions in order to construct wave packets and evaluate the
time-dependent expectation value of the scale factor. We find that it
oscillates between maximum and minimum values. Since the expectation value of
the scale factor never vanishes, we confirm that this model is free from an
initial singularity, also, at the quantum level.Comment: 12 Pages, 4 Figures. Final version. Accepted for publication in the
Proceedings of the 8th Friedmann Seminar, Rio de Janeiro, 2011. We restricted
our attention to treat the case where the stiff matter has negative energy
eigenvalues, following the referee's suggestio
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
Parametric phenomena of the particle dynamics in a periodic gravitational wave field
We establish exactly solvable models for the motion of neutral particles,
electrically charged point and spin particles (U(1) symmetry), isospin
particles (SU(2) symmetry), and particles with color charges (SU(3) symmetry)
in a gravitational wave background. Special attention is devoted to parametric
effects induced by the gravitational field. In particular, we discuss
parametric instabilities of the particle motion and parametric oscillations of
the vectors of spin, isospin, and color charge.Comment: 26 pages, to be published in J. Math. Phy
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
On Metric Preheating
We consider the generation of super-horizon metric fluctuations during an
epoch of preheating in the presence of a scalar field \chi quadratically
coupled to the inflaton. We find that the requirement of efficient broad
resonance is concomitant with a severe damping of super-horizon \delta\chi
quantum fluctuations during inflation. Employing perturbation theory with
backreaction included as spatial averages to second order in the scalar fields
and in the metric, we argue that the usual inflationary prediction for metric
perturbations on scales relevant for structure formation is not strongly
modified.Comment: 5 latex pages, 1 postscript figure included, uses revtex.sty in two
column format and epsf.sty, some typos corrected and references added. Links
and further material at http://astro.uchicago.edu/home/web/sigl/r4.htm
Cosmic microwave background and parametric resonance in reheating
The variation of the perturbative 3-curvature parameter, \zeta, is
investigated in the period of reheating after inflation. The two-field model
used has the inflaton, with an extra scalar field coupled to it, and non-linear
effects of both fields are included as well as a slow decay mechanism into the
hydrodynamic fluid of the radiation era. Changes in \zeta occur and persist
into the succeeding cosmic eras to influence the generation of the cosmic
microwave background fluctuations.Comment: 21 pages, 6 figures.Corrects misprinted formula and 2 number
Scalar tilt from broken conformal invariance
Within recently proposed scenario which explains flatness of the spectrum of
scalar cosmological perturbations by a combination of conformal and global
symmetries, we discuss the effect of weak breaking of conformal invariance. We
find that the scalar power spectrum obtains a small tilt which depends on both
the strength of conformal symmetry breaking and the law of evolution of the
scale factor
Parity violating elastic electron scattering and neutron density distributions in the Relativistic Hartree-Bogoliubov model
Parity violating elastic electron scattering on neutron-rich nuclei is
described in the framework of relativistic mean-field theory. Self-consistent
ground state density distributions of Ne, Na, Ni and Sn isotopes are calculated
with the relativistic Hartree- Bogoliubov model, and the resulting neutron
radii are compared with available experimental data. For the elastic scattering
of 850 MeV electrons on these nuclei, the parity-violating asymmetry parameters
are calculated using a relativistic optical model with inclusion of Coulomb
distortion effects. The asymmetry parameters for chains of isotopes are
compared, and their relation to the Fourier transforms of neutron densities is
studied. It is shown that parity-violating asymmetries are sensitive not only
to the formation of the neutron skin, but also to the shell effects of the
neutron density distribution.Comment: RevTeX 17 pages, 18 eps figs, submitted to Phys. Rev.
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