33,501 research outputs found
Laser-based strain measurements for high temperature applications
The Instrumentation and Control Technology Division at NASA Lewis Research Center has developed a high performance optical strain measurement system for high temperature applications using wires and fibers. The system is based on Yamaguchi's two-beam speckle-shift strain measurement technique. The system automatically calculates surface strains at a rate of 5 Hz using a digital signal processor in a high speed micro-computer. The system is fully automated, and can be operated remotely. This report describes the speckle-shift technique and the latest NASA system design. It also shows low temperature strain test results obtained from small diameter tungsten, silicon carbide, and sapphire specimens. These specimens are of interest due to their roles in composite materials research at NASA Lewis
Advanced one-dimensional optical strain measurement system, phase 4
An improved version of the speckle-shift strain measurement system was developed. The system uses a two-dimensional sensor array to maintain speckle correlation in the presence of large off-axis rigid body motions. A digital signal processor (DSP) is used to calculate strains at a rate near the RS-170 camera frame rate. Strain measurements were demonstrated on small diameter wires and fibers used in composite materials research. Accurate values of Young's modulus were measured on tungsten wires, and silicon carbide and sapphire fibers. This optical technique has measured surface strains at specimen temperatures above 750 C and has shown the potential for measurements at much higher temperatures
Primordial black holes in non-Gaussian regimes
Primordial black holes (PBHs) can form in the early Universe from the collapse of rare, large density fluctuations. They have never been observed, but this fact is enough to constrain the amplitude of fluctuations on very small scales which cannot be otherwise probed. Because PBHs form only in very rare large fluctuations, the number of PBHs formed is extremely sensitive to changes in the shape of the tail of the fluctuation distribution - which depends on the amount of non-Gaussianity present. We first study how local non-Gaussianity of arbitrary size up to fifth order affects the abundance and constraints from PBHs, finding that they depend strongly on even small amounts of non-Gaussianity and the upper bound on the allowed amplitude of the power spectrum can vary by several orders of magnitude. The sign of the non-linearity parameters (f_{NL}, g_{NL}, etc) are particularly important. We also study the abundance and constraints from PBHs in the curvaton scenario, in which case the complete non-linear probability distribution is known, and find that truncating to any given order (i.e. to order f_{NL} or g_{NL}, etc) does not give accurate results
Long-short wavelength mode coupling tightens primordial black hole constraints
The effects of non-Gaussianity on the constraints on the primordial curvature perturbation power spectrum from primordial black holes (PBHs) are considered. We extend previous analyses to include the effects of coupling between the modes of the horizon scale at the time the PBH forms and superhorizon modes. We consider terms of up to third order in the Gaussian perturbation. For the weakest constraints on the abundance of PBHs in the early universe (corresponding to a fractional energy density of PBHs of 10−5 at the time of formation), in the case of Gaussian perturbations, constraints on the power spectrum are Pζ<0.05 but can be significantly tighter when even a small amount of non-Gaussianity is considered, to Pζ<0.01, and become approximately Pζ<0.003 in more special cases. Surprisingly, even when there is negative skew (which naively would suggest fewer areas of high density, leading to weaker constraints), we find that the constraints on the power spectrum become tighter than the purely Gaussian case—in strong contrast with previous results. We find that the constraints are highly sensitive to both the non-Gaussianity parameters as well as the amplitude of superhorizon perturbations
Scale-invariant Perturbations from Chaotic Inflation
Vacuum fluctuations in the inflaton field driving chaotic inflation with a
quadratic potential give a red spectrum of primordial density perturbations,
n=0.97. However angular fluctuations in an O(N)-symmetric quadratic potential
have a very nearly scale-invariant spectrum, n=0.9998. We investigate the
possibility that these isocurvature field perturbations could give the dominant
contribution to the primordial density perturbation after inflation.Comment: 10 pages, LaTeX with RevTex, no figure
General formula for the running of local fNL
We compute the scale dependence of fNL for models of multi-field inflation, allowing for an arbitrary field space metric. We show that, in addition to multi-field effects and self interactions, the curved field space metric provides another source of scale dependence, which arises from the field-space Riemann curvature tensor and its derivatives. The scale dependence may be detectable within the near future if the amplitude of fNL is not too far from the current observational bounds
Noisy quantum teleportation: An experimental study on the influence of local environments
We report experimental results on the action of selected local environments
on the fidelity of the quantum teleportation protocol, taking into account
non-ideal, realistic entangled resources. Different working conditions are
theoretically identified, where a noisy protocol can be made almost insensitive
to further addition of noise. We put to test these conditions on a photonic
implementation of the quantum teleportation algorithm, where two polarization
entangled qubits act as the entangled resource and a path qubit on Alice
encodes the state to be teleported. Bob's path qubit is used to implement a
local environment, while the environment on Alice's qubit is simulated as a
weighed average of different pure states. We obtain a good agreement with the
theoretical predictions, we experimentally recreate the conditions to obtain a
noise-induced enhancement of the protocol fidelity, and we identify parameter
regions of increased insensibility to interactions with specific noisy
environments.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
The Standard Model Higgs as the origin of the hot Big Bang
If the Standard Model (SM) Higgs is weakly coupled to the inflationary
sector, the Higgs is expected to be universally in the form of a condensate
towards the end of inflation. The Higgs decays rapidly after inflation - via
non-perturbative effects - into an out-of-equilibrium distribution of SM
species, which thermalize soon afterwards. If the post-inflationary equation of
state of the universe is stiff, , the SM species eventually
dominate the total energy budget. This provides a natural origin for the
relativistic thermal plasma of SM species, required for the onset of the `hot
Big Bang' era. The viability of this scenario requires the inflationary Hubble
scale to be lower than the instability scale for Higgs vacuum decay, the
Higgs not to generate too large curvature perturbations at cosmological scales,
and the SM dominance to occur before Big Bang Nucleosynthesis. We show that
successful reheating into the SM can only be obtained in the presence of a
non-minimal coupling to gravity , with a reheating temperature
of .Comment: 6 pages, 2 figures, minor changes with new figures to match published
version in PL
Review of local non-Gaussianity from multi-field inflation
We review models which generate a large non-Gaussianity of the local form. We
first briefly consider three models which generate the non-Gaussianity either
at or after the end of inflation; the curvaton scenario, modulated (p)reheating
and an inhomogeneous end of inflation. We then focus on ways of generating the
non-Gaussianity during inflation. We derive general conditions which a product
or sum separable potential must satisfy in order to generate a large local
bispectrum during slow-roll inflation. As an application we consider two-field
hybrid inflation. We then derive a formalism not based on slow roll which can
be applied to models in which the slow-roll parameters become large before
inflation ends. An exactly soluble two-field model is given in which this
happens. Finally we also consider further non-Gaussian observables; a scale
dependence of f_NL and the trispectrum.Comment: 22 pages, 2 figures. Invited review for the special issue "Testing
the Gaussianity and Statistical Isotropy of the Universe" for Advances in
Astronomy. v2: Numerous references and comments adde
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