1,003 research outputs found
Cosmological tachyon condensation
We consider the prospects for dark matter/energy unification in k-essence
type theories. General mappings are established between the k-essence scalar
field, the hydrodynamic and braneworld descriptions. We develop an extension of
the general relativistic dust model that incorporates the effects of both
pressure and the associated acoustic horizon. Applying this to a tachyon model,
we show that this inhomogeneous "variable Chaplygin gas" does evolve into a
mixed system containing cold dark matter like gravitational condensate in
significant quantities. Our methods can be applied to any dark energy model as
well as to mixtures of dark energy and traditional dark matter.Comment: 22 pages, 3 figures, title changed, typos corrected, accepted in
Phys. Rev.
Chaplygin Gas Cosmology - Unification of Dark Matter and Dark Energy
The models that unify dark matter and dark energy based upon the Chaplygin
gas fail owing to the suppression of structure formation by the adiabatic speed
of sound. Including string theory effects, in particular the Kalb-Ramond field,
we show how nonadiabatic perturbations allow a successful structure formation.Comment: 7 pages, presented by N. B. at IRGAC 2006, Barcelona, 11-15 July
2006, typos corrected, concluding paragraph slightly expanded, final version,
accepted in J. Phys. A, special issu
Spectral signatures of many-body localization with interacting photons
Statistical mechanics is founded on the assumption that a system can reach
thermal equilibrium, regardless of the starting state. Interactions between
particles facilitate thermalization, but, can interacting systems always
equilibrate regardless of parameter values\,? The energy spectrum of a system
can answer this question and reveal the nature of the underlying phases.
However, most experimental techniques only indirectly probe the many-body
energy spectrum. Using a chain of nine superconducting qubits, we implement a
novel technique for directly resolving the energy levels of interacting
photons. We benchmark this method by capturing the intricate energy spectrum
predicted for 2D electrons in a magnetic field, the Hofstadter butterfly. By
increasing disorder, the spatial extent of energy eigenstates at the edge of
the energy band shrink, suggesting the formation of a mobility edge. At strong
disorder, the energy levels cease to repel one another and their statistics
approaches a Poisson distribution - the hallmark of transition from the
thermalized to the many-body localized phase. Our work introduces a new
many-body spectroscopy technique to study quantum phases of matter
On the Nature of the X-ray Emission from the Ultraluminous X-ray Source, M33 X-8: New Constraints from NuSTAR and XMM-Newton
We present nearly simultaneous NuSTAR and XMM-Newton observations of the
nearby (832 kpc) ultraluminous X-ray source (ULX) M33 X-8. M33 X-8 has a 0.3-10
keV luminosity of LX ~ 1.4 x 10^39 erg/s, near the boundary of the
"ultraluminous" classification, making it an important source for understanding
the link between typical Galactic X-ray binaries and ULXs. Past studies have
shown that the 0.3-10 keV spectrum of X-8 can be characterized using an
advection-dominated accretion disk model. We find that when fitting to our
NuSTAR and XMM-Newton observations, an additional high-energy (>10 keV)
Comptonization component is required, which allows us to rule out single
advection-dominated disk and classical sub-Eddington models. With our new
constraints, we analyze XMM-Newton data taken over the last 17 years to show
that small (~30%) variations in the 0.3-10 keV flux of M33 X-8 result in
spectral changes similar to those observed for other ULXs. The two most likely
phenomenological scenarios suggested by the data are degenerate in terms of
constraining the nature of the accreting compact object (i.e., black hole
versus neutron star). We further present a search for pulsations using our
suite of data; however, no clear pulsations are detected. Future observations
designed to observe M33 X-8 at different flux levels across the full 0.3-30 keV
range would significantly improve our constraints on the nature of this
important source.Comment: Accepted for publication in ApJ (15 pages, 4 tables, 6 figures
Three-Dimensional FDTD Simulation of Biomaterial Exposure to Electromagnetic Nanopulses
Ultra-wideband (UWB) electromagnetic pulses of nanosecond duration, or
nanopulses, have been recently approved by the Federal Communications
Commission for a number of various applications. They are also being explored
for applications in biotechnology and medicine. The simulation of the
propagation of a nanopulse through biological matter, previously performed
using a two-dimensional finite difference-time domain method (FDTD), has been
extended here into a full three-dimensional computation. To account for the UWB
frequency range, a geometrical resolution of the exposed sample was ,
and the dielectric properties of biological matter were accurately described in
terms of the Debye model. The results obtained from three-dimensional
computation support the previously obtained results: the electromagnetic field
inside a biological tissue depends on the incident pulse rise time and width,
with increased importance of the rise time as the conductivity increases; no
thermal effects are possible for the low pulse repetition rates, supported by
recent experiments. New results show that the dielectric sample exposed to
nanopulses behaves as a dielectric resonator. For a sample in a cuvette, we
obtained the dominant resonant frequency and the -factor of the resonator.Comment: 15 pages, 8 figure
General-Relativistic Thomas-Fermi model
A system of self-gravitating massive fermions is studied in the framework of
the general-relativistic Thomas-Fermi model. We study the properties of the
free energy functional and its relation to Einstein's field equations. A
self-gravitating fermion gas we then describe by a set of Thomas-Fermi type
self-consistency equations.Comment: 7 pages, LaTex, to appear in Gen. Rel. Gra
Effect of high-pressure hot-water washing treatment on fruit quality, insects, and disease in apples and pears Part III. Use of silicone-based materials and mechanical methods to eliminate surface pests
Surface arthropods on pome fruits can cause export problems and disrupt commercial markets. Eliminating insects and mites on the packing line would be the last opportunity to provide for pest-free produce. In this study, an experimental packing line was used to evaluate techniques using different surfactant baths, pressurized water sprays, and styles of rotating brushes to remove field-collected and laboratory-reared grape mealybug, Pseudococcus maritimus (Ehrhorn) (Homoptera: Pseudococcidae), the diapausing two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) and the woolly apple aphid, Eriosoma lanigerum (Hausman) (Homoptera: Aphididae). The organosilicone Silwet L-77 was no more effective than a silicone-based food grade defoamer in aiding removal. Mechanical methods, such as the style of rotating brushes and pressurized sprays, were significantly effective in removing surface arthropods. No improvement in removal occurred when pressure was increased beyond 420 kPa. These techniques can be easily adapted to commercial facilities and will reduce the incidence of surface arthropods on marketed fresh fruits
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