48,692 research outputs found
Haloes of k-Essence
We study gravitationally bound static and spherically symmetric
configurations of k-essence fields. In particular, we investigate whether these
configurations can reproduce the properties of dark matter haloes. The classes
of Lagrangians we consider lead to non-isotropic fluids with barotropic and
polytropic equations of state. The latter include microscopic realizations of
the often-considered Chaplygin gases, which we find can cluster into dark
matter halo-like objects with flat rotation curves, while exhibiting a dark
energy-like negative pressure on cosmological scales. We complement our studies
with a series of formal general results about the stability and initial value
formulation of non-canonical scalar field theories, and we also discuss a new
class of de Sitter solutions with spacelike field gradients.Comment: 34pages, single column double spacing, 7 figures, 3 Tables, RevTex4.
Additional references and minor clarifications. To be submitted to JCA
Formulating Weak Lensing from the Boltzmann Equation and Application to Lens-lens Couplings
The Planck mission has conclusively detected lensing of the Cosmic Microwave
Background (CMB) radiation from foreground sources to an overall significance
of greater than . The high precision of this measurement motivates
the development of a more complete formulation of the calculation of this
effect. While most effects on the CMB anisotropies are widely studied through
direct solutions of the Boltzmann equation, the non-linear effect of CMB
lensing is formulated through the solutions of the geodesic equation. In this
paper, we present a new formalism to the calculation of the lensing effect by
\emph{directly solving the Boltzmann equation}, as we did in the calculation of
the CMB anisotropies at recombination. In particular, we developed a
diagrammatic approach to efficiently keep track of all the interaction terms
and calculate all possible non-trivial correlations to arbitrary high orders.
Using this formalism, we explicitly articulate the approximations required to
recover the usual remapping approach used in current studies of the weak
lensing. In addition, we point out additional unexplored corrections that are
manifest in our formalism to which experiments may be sensitive. As an example,
we calculate the correction to the CMB temperature power spectrum for the
\emph{lens-lens} coupling effects which are neglected in standard calculations.
We find that the correction is of the CMB temperature power
spectrum for up to 3000 and thus is comparable to the cosmic variance.Comment: 25 pages, 3 figures, 4 tables, CMB, lensin
Properties of kinematic singularities
The locally rotationally symmetric tilted perfect fluid Bianchi type V
cosmological model provides examples of future geodesically complete spacetimes
that admit a `kinematic singularity' at which the fluid congruence is
inextendible but all frame components of the Weyl and Ricci tensors remain
bounded. We show that for any positive integer n there are examples of Bianchi
type V spacetimes admitting a kinematic singularity such that the covariant
derivatives of the Weyl and Ricci tensors up to the n-th order also stay
bounded. We briefly discuss singularities in classical spacetimes.Comment: 13 pages. Published version. One sentence from version 2 correcte
Quantum random number generation for 1.25 GHz quantum key distribution systems
Security proofs of quantum key distribution (QKD) systems usually assume that
the users have access to source of perfect randomness. State-of-the-art QKD
systems run at frequencies in the GHz range, requiring a sustained GHz rate of
generation and acquisition of quantum random numbers. In this paper we
demonstrate such a high speed random number generator. The entropy source is
based on amplified spontaneous emission from an erbium-doped fibre, which is
directly acquired using a standard small form-factor pluggable (SFP) module.
The module connects to the Field Programmable Gate Array (FPGA) of a QKD
system. A real-time randomness extractor is implemented in the FPGA and
achieves a sustained rate of 1.25 Gbps of provably random bits.Comment: 6 pages, 8 figure
Early and late complications of bariatric operation.
Weight loss surgery is one of the fastest growing segments of the surgical discipline. As with all medical procedures, postoperative complications will occur. Acute care surgeons need to be familiar with the common problems and their management. Although general surgical principles generally apply, diagnoses specific to the various bariatric operations must be considered. There are anatomic considerations which alter management priorities and options for these patients in many instances. These problems present both early or late in the postoperative course. Bariatric operations, in many instances, result in permanent alteration of a patient\u27s anatomy, which can lead to complications at any time during the course of a patient\u27s life. Acute care surgeons diagnosing surgical emergencies in postbariatric operation patients must be familiar with the type of surgery performed, as well as the common postbariatric surgical emergencies. In addition, surgeons must not overlook the common causes of an acute surgical abdomen-acute appendicitis, acute diverticulitis, acute pancreatitis, and gallstone disease-for these are still among the most common etiologies of abdominal pathology in these patients
Time Series Modelling of Tourism Demand from the USA, Japan and Malaysia to Thailand
Even though tourism has been recognized as one of the key sectors for the Thai economy, international tourism demand, or tourist arrivals, to Thailand have recently experienced dramatic fluctuations. The purpose of the paper is to investigate the relationship between the demand for international tourism to Thailand and its major determinants. The paper includes arrivals from the USA, which represents the long haul inbound market, from Japan as the most important medium haul inbound market, and from Malaysia as the most important short haul inbound market. The time series of tourist arrivals and economic determinants from 1971 to 2005 are examined using ARIMA with exogenous variables (ARMAX) models to analyze the relationships between tourist arrivals from these countries to Thailand. The economic determinants and ARMA are used to predict the effects of the economic, financial and political determinants on the numbers of tourists to Thailand.
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