5,373 research outputs found
Flexible arms provide constant force for pressure switch calibration
In-place calibration of a pressure switch is provided by a system of radially oriented flexing arms which, when rotated at a known velocity, convert the centrifugal force of the arms to a linear force along the shaft. The linear force, when applied to a pressure switch diaphragm, can then be calculated
Thermal/acoustical aircraft insulation material
Attempts made to improve the acoustical properties of low density Fiberfrax foam, an aircraft insulation material, are reported. Characterizations were also made of the physical and thermal properties. Two methods, optimization of fiber blend composition and modification of the foam fabrication process, were examined as possible means of improving foam acoustics. Flame impingement tests were also made; results show performance was satisfactory
Rotating Boson Stars in 5 Dimensions
We study rotating boson stars in five spacetime dimensions. The boson fields
consist of a complex doublet scalar field. Considering boson stars rotating in
two orthogonal planes with both angular momenta of equal magnitude, a special
ansatz for the boson field and the metric allows for solutions with nontrivial
dependence on the radial coordinate only. The charge of the scalar field equals
the sum of the angular momenta. The rotating boson stars are globally regular
and asymptotically flat. For our choice of a sixtic potential the rotating
boson star solutions possess a flat spacetime limit. We study the solutions in
flat and curved spacetime.Comment: 17 pages, 6 figure
Fingerprinting dark energy
Dark energy perturbations are normally either neglected or else included in a
purely numerical way, obscuring their dependence on underlying parameters like
the equation of state or the sound speed. However, while many different
explanations for the dark energy can have the same equation of state, they
usually differ in their perturbations so that these provide a fingerprint for
distinguishing between different models with the same equation of state. In
this paper we derive simple yet accurate approximations that are able to
characterize a specific class of models (encompassing most scalar-field models)
which is often generically called "dark energy". We then use the approximate
solutions to look at the impact of the dark energy perturbations on the dark
matter power spectrum and on the integrated Sachs-Wolfe effect in the cosmic
microwave background radiation.Comment: 11 pages, 5 figures, minor changes to match published versio
CMB power spectra from cosmic strings: predictions for the Planck satellite and beyond
We present a significant improvement over our previous calculations of the
cosmic string contribution to cosmic microwave background (CMB) power spectra,
with particular focus on sub-WMAP angular scales. These smaller scales are
relevant for the now-operational Planck satellite and additional sub-orbital
CMB projects that have even finer resolutions. We employ larger Abelian Higgs
string simulations than before and we additionally model and extrapolate the
statistical measures from our simulations to smaller length scales. We then use
an efficient means of including the extrapolations into our Einstein-Boltzmann
calculations in order to yield accurate results over the multipole range 2 < l
3000 in
the case of the temperature power spectrum, which then allows cautious
extrapolation to even smaller scales. We find that a string contribution to the
temperature power spectrum making up 10% of power at l=10 would be larger than
the Silk-damped primary adiabatic contribution for l > 3500. Astrophysical
contributions such as the Sunyaev-Zeldovich effect also become important at
these scales and will reduce the sensitivity to strings, but these are
potentially distinguishable by their frequency-dependence.Comment: 18 pages, 16 figure
General approach for studying first-order phase transitions at low temperatures
By combining different ideas, a general and efficient protocol to deal with
discontinuous phase transitions at low temperatures is proposed. For small
's, it is possible to derive a generic analytic expression for appropriate
order parameters, whose coefficients are obtained from simple simulations. Once
in such regimes simulations by standard algorithms are not reliable, an
enhanced tempering method, the parallel tempering -- accurate for small and
intermediate system sizes with rather low computational cost -- is used.
Finally, from finite size analysis, one can obtain the thermodynamic limit. The
procedure is illustrated for four distinct models, demonstrating its power,
e.g., to locate coexistence lines and the phases density at the coexistence.Comment: 5 page
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
The Sphaleron Barrier in the Presence of Fermions
We calculate the minimal energy path over the sphaleron barrier in the
pre\-sen\-ce of fermions, assuming that the fermions of a doublet are
degenerate in mass. This allows for spherically symmetric ans\"atze for the
fields, when the mixing angle dependence is neglected. While light fermions
have little influence on the barrier, the presence of heavy fermions ( TeV) strongly deforms the barrier, giving rise to additional sphalerons
for very heavy fermions ( 10 TeV). Heavy fermions form
non-topological solitons in the vacuum sector.Comment: 19 pages, latex, 18 figures in 3 seperate uuencoded postscript files
THU-93/1
Level Crossing Along Sphaleron Barriers
In the electroweak sector of the standard model topologically inequivalent
vacua are separated by finite energy barriers, whose height is given by the
sphale\-ron. For large values of the Higgs mass there exist several sphaleron
solutions and the barriers are no longer symmetric. We construct paths of
classical configurations from one vacuum to a neighbouring one and solve the
fermion equations in the background field configurations along such paths,
choosing the fermions of a doublet degenerate in mass. As in the case of light
Higgs masses we observe the level crossing phenomenon also for large Higgs
masses.Comment: 17 pages, latex, 10 figures in uuencoded postscript files. THU-94/0
Measuring the effective complexity of cosmological models
We introduce a statistical measure of the effective model complexity, called
the Bayesian complexity. We demonstrate that the Bayesian complexity can be
used to assess how many effective parameters a set of data can support and that
it is a useful complement to the model likelihood (the evidence) in model
selection questions. We apply this approach to recent measurements of cosmic
microwave background anisotropies combined with the Hubble Space Telescope
measurement of the Hubble parameter. Using mildly non-informative priors, we
show how the 3-year WMAP data improves on the first-year data by being able to
measure both the spectral index and the reionization epoch at the same time. We
also find that a non-zero curvature is strongly disfavored. We conclude that
although current data could constrain at least seven effective parameters, only
six of them are required in a scheme based on the Lambda-CDM concordance
cosmology.Comment: 9 pages, 4 figures, revised version accepted for publication in PRD,
updated with WMAP3 result
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