281 research outputs found
Novel Modifications of Parallel Jacobi Algorithms
We describe two main classes of one-sided trigonometric and hyperbolic
Jacobi-type algorithms for computing eigenvalues and eigenvectors of Hermitian
matrices. These types of algorithms exhibit significant advantages over many
other eigenvalue algorithms. If the matrices permit, both types of algorithms
compute the eigenvalues and eigenvectors with high relative accuracy.
We present novel parallelization techniques for both trigonometric and
hyperbolic classes of algorithms, as well as some new ideas on how pivoting in
each cycle of the algorithm can improve the speed of the parallel one-sided
algorithms. These parallelization approaches are applicable to both
distributed-memory and shared-memory machines.
The numerical testing performed indicates that the hyperbolic algorithms may
be superior to the trigonometric ones, although, in theory, the latter seem
more natural.Comment: Accepted for publication in Numerical Algorithm
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
One-Parameter Squeezed Gaussian States of Time-Dependent Harmonic Oscillator and Selection Rule for Vacuum States
By using the invariant method we find one-parameter squeezed Gaussian states
for both time-independent and time-dependent oscillators. The squeezing
parameter is expressed in terms of energy expectation value for
time-independent case and represents the degree of mixing positive and negative
frequency solutions for time-dependent case. A {\it minimum uncertainty
proposal} is advanced to select uniquely vacuum states at each moment of time.
We show that the Gaussian states with minimum uncertainty coincide with the
true vacuum state for time-independent oscillator and the Bunch-Davies vacuum
for a massive scalar field in a de Sitter spacetime.Comment: 13 Pages, ReVTeX, no figure
Do semiclassical zero temperature black holes exist?
The semiclassical Einstein equations are solved to first order in for the case of a Reissner-Nordstr\"{o}m black hole perturbed by the
vacuum stress-energy of quantized free fields. Massless and massive fields of
spin 0, 1/2, and 1 are considered. We show that in all physically realistic
cases, macroscopic zero temperature black hole solutions do not exist. Any
static zero temperature semiclassical black hole solutions must then be
microscopic and isolated in the space of solutions; they do not join smoothly
onto the classical extreme Reissner-Nordst\"{o}m solution as .Comment: 5 pages, no figures, minor changes and corrections, to appear in
Physical Review Letter
On the Background Field Method Beyond One Loop: A manifestly covariant derivative expansion in super Yang-Mills theories
There are currently many string inspired conjectures about the structure of
the low-energy effective action for super Yang-Mills theories which require
explicit multi-loop calculations. In this paper, we develop a manifestly
covariant derivative expansion of superspace heat kernels and present a scheme
to evaluate multi-loop contributions to the effective action in the framework
of the background field method. The crucial ingredient of the construction is a
detailed analysis of the properties of the parallel displacement propagators
associated with Yang-Mills supermultiples in N-extended superspace.Comment: 32 pages, latex, 7 EPS figures. v2: references, comments added, typos
corrected, incorrect `skeleton' conjecture in sect. 3 replaced by a more
careful treatment. v3: typos corrected, final version published in JHE
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
Widespread platinum anomaly documented at theYounger Dryas onset in North American sedimentary sequences
Previously, a large platinum (Pt) anomaly was reported in the Greenland ice sheet at the Younger Dryas
boundary (YDB) (12,800 Cal B.P.). In order to evaluate its geographic extent, fire-assay and inductively coupled plasma mass spectrometry (FA and ICP-MS) elemental analyses were performed on 11
widely separated archaeological bulk sedimentary sequences. We document discovery of a distinct Pt anomaly spread widely across North America and dating to the Younger Dryas (YD) onset. The apparent synchroneity of this widespread YDB Pt anomaly is consistent with Greenland Ice Sheet Project 2 (GISP2) data that indicated atmospheric input of platinum-rich dust. We expect the Pt anomaly to serve as a widely-distributed time marker horizon (datum) for identification and correlation of the onset of the YD climatic episode at 12,800 Cal B.P. This Pt datum will facilitate the dating and correlating of archaeological, paleontological, and paleoenvironmental data between sequences, especially those with limited age control
Cancer in the offspring of female radiation workers: a record linkage study
This study uses record linkage between the National Registry of Childhood Tumours (NRCT) and the National Registry for Radiation Workers to re-assess our earlier finding that the offspring of women radiation workers exposed to ionising radiation before the child's conception may be at an increased risk of childhood cancer. An additional 16 964 childhood cancer patients taken from the NRCT, together with the same number of matched controls, are included. Pooled analyses, based on the new and original datasets, include 52 612 cases and their matched controls. Relative risks (RRs) for maternal employment as a radiation worker, maternal exposure or not during the relevant pregnancy and pattern of employment relative to conception and diagnosis dates were calculated
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel. In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime, compute the two-point
correlation functions of these perturbations and prove that Minkowski spacetime
is a stable solution of semiclassical gravity. Second, we discuss structure
formation from the stochastic gravity viewpoint. Third, we discuss the
backreaction of Hawking radiation in the gravitational background of a black
hole and describe the metric fluctuations near the event horizon of an
evaporating black holeComment: 100 pages, no figures; an update of the 2003 review in Living Reviews
in Relativity gr-qc/0307032 ; it includes new sections on the Validity of
Semiclassical Gravity, the Stability of Minkowski Spacetime, and the Metric
Fluctuations of an Evaporating Black Hol
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