13,147 research outputs found
Growth of covariant perturbations in the contracting phase of a bouncing universe
In this paper we examine the validity of the linear perturbation theory near
a bounce in the covariant analysis. Some linearity parameters are defined to
set up conditions for a linear theory. Linear evolution of density perturbation
and gravitational waves have been computed previously. We have calculated the
vector and scalar induced parts of the shear tensor. For radiationlike and
dustlike single fluid dominated collapsing Friedmann-Lemaitre-Robertson-Walker
background it is shown that the linearity conditions are not satisfied near a
bounce.Comment: 9 pages, final versio
Nonmetallic impurities improve mechanical properties of vapor-deposited tungsten
Mechanical properties of vapor deposited tungsten are improved by selective incorporation of various nonmetallic impurities. Addition of trace quantities of carbon, nitrogen, or oxygen can significantly increase both low and high temperature yield strength without greatly affecting ductile-to-brittle transition temperature
Vapor Deposited Tungsten for Application as a Thermionic Emitter Material
Purity and resistance to grain growth of vapor deposited tungsten tubing for use as thermionic emitte
Effects of additions of nonmetallics on the properties of vapor-deposited tungsten
Nonmetallic additive effects on properties of vapor deposited tungste
Comment on "Does Gluons Carry Half of the Nucleon Momentum?" by X. S. Chen et. al. (PRL103, 062001 (2009))
The authors claim to have found a "proper", "gauge-invariant" definition of a
charged-particle's momentum in gauge theory, which is more "superior" than the
textbook version. I show that their result arises from a misunderstanding of
gauge symmetry by generalizing the Coulomb gauge result indiscriminately and is
not physical
g = 2 as a Gauge Condition
Charged matter spin-1 fields enjoy a nonelectromagnetic gauge symmetry when
interacting with vacuum electromagnetism, provided their gyromagnetic ratio is
2.Comment: 5 pages, REVTeX, submitted to Phys Rev D Brief Report
Asymptotic safety in higher-derivative gravity
We study the non-perturbative renormalization group flow of higher-derivative
gravity employing functional renormalization group techniques. The
non-perturbative contributions to the -functions shift the known
perturbative ultraviolet fixed point into a non-trivial fixed point with three
UV-attractive and one UV-repulsive eigendirections, consistent with the
asymptotic safety conjecture of gravity. The implication of this transition on
the unitarity problem, typically haunting higher-derivative gravity theories,
is discussed.Comment: 8 pages; 1 figure; revised versio
Dark Matter Constraints from the Sagittarius Dwarf and Tail System
2MASS has provided a three-dimensional map of the >360 degree, wrapped tidal
tails of the Sagittarius (Sgr) dwarf spheroidal galaxy, as traced by M giant
stars. With the inclusion of radial velocity data for stars along these tails,
strong constraints exist for dynamical models of the Milky Way-Sgr interaction.
N-body simulations of Sgr disruption with model parameters spanning a range of
initial conditions (e.g., Sgr mass and orbit, Galactic rotation curve, halo
flattening) are used to find parameterizations that match almost every extant
observational constraint of the Sgr system. We discuss the implications of the
Sgr data and models for the orbit, mass and M/L of the Sgr bound core as well
as the strength, flattening, and lumpiness of the Milky Way potential.Comment: 6 pages, 0 figures. Contribution to proceedings of ``IAU Symposium
220: Dark Matter in Galaxies'', eds. S. Ryder, D.J. Pisano, M. Walker, and K.
Freema
Emergent electrodynamics from the Nambu model for spontaneous Lorentz symmetry breaking
After imposing the Gauss law constraint as an initial condition upon the
Hilbert space of the Nambu model, in all its generic realizations, we recover
QED in the corresponding non-linear gauge A_{\mu}A^{\mu}=n^{2}M^{2}. Our result
is non-perturbative in the parameter M for n^{2}\neq 0 and can be extended to
the n^{2}=0 case. This shows that in the Nambu model, spontaneous Lorentz
symmetry breaking dynamically generates gauge invariance, provided the Gauss
law is imposed as an initial condition. In this way electrodynamics is
recovered, with the photon being realized as the Nambu-Goldstone modes of the
spontaneously broken symmetry, which finally turns out to be non-observableComment: 17 page
Non-Gaussian features of primordial magnetic fields in power-law inflation
We show that a conformal-invariance violating coupling of the inflaton to
electromagnetism produces a cross correlation between curvature fluctuations
and a spectrum of primordial magnetic fields. According to this model, in the
case of power-law inflation, a primordial magnetic field is generated with a
nearly flat power spectrum and rms amplitude ranging from nG to pG. We study
the cross correlation, a three-point function of the curvature perturbation and
two powers of the magnetic field, in real and momentum space. The
cross-correlation coefficient, a dimensionless ratio of the three-point
function with the curvature perturbation and magnetic field power spectra, can
be several orders of magnitude larger than expected as based on the amplitude
of scalar metric perturbations from inflation. In momentum space, the
cross-correlation peaks for flattened triangle configurations, and is three
orders of magnitude larger than the squeezed triangle configuration. These
results suggest likely methods for distinguishing the observational signatures
of the model.Comment: 15 pages, 2 figure
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