36 research outputs found
Cuscuton: A Causal Field Theory with an Infinite Speed of Sound
We introduce a model of scalar field dark energy, Cuscuton, which can be
realized as the incompressible (or infinite speed of sound) limit of a scalar
field theory with a non-canonical kinetic term (or k-essence). Even though
perturbations of Cuscuton propagate superluminally, we show that they have a
locally degenerate phase space volume (or zero entropy), implying that they
cannot carry any microscopic information, and thus the theory is causal. Even
coupling to ordinary scalar fields cannot lead to superluminal signal
propagation. Furthermore, we show that the family of constant field
hypersurfaces are the family of Constant Mean Curvature (CMC) hypersurfaces,
which are the analogs of soap films (or soap bubbles) in a Euclidian space.
This enables us to find the most general solution in 1+1 dimensions, whose
properties motivate conjectures for global degeneracy of the phase space in
higher dimensions. Finally, we show that the Cuscuton action can model the
continuum limit of the evolution of a field with discrete degrees of freedom
and argue why it is protected against quantum corrections at low energies.
While this paper mainly focuses on interesting features of Cuscuton in a
Minkowski spacetime, a companion paper (astro-ph/0702002) examines cosmology
with Cuscuton dark energy.Comment: 11 pages, 1 figure, added discussion of "coupled cuscuton", matches
the published version in PR
Nozzle Wear and Pressure Rise in Heating Volume of Self-blast Type Ultra-high Pressure Nitrogen Arc
This paper reports on experiments with ultra-high pressure nitrogen arcs in a self-blast type switch design. The effect of filling pressure on nozzle mass loss and pressure-rise in the heating volume were investigated. An arc current peak of 130 A at 190 Hz and a fixed inter-electrode gap of 50 mm were used throughout the experiment. The arc burns inside a polytetrafluoroethylene nozzle with a gas outflow vent in the middle. Nitrogen filling pressure of 1 bar, 20 bar, and 40 bar was tested, which also covers the supercritical region. Moreover, to study the effect of vent size on blow pressure near current zero, three different vent dimensions were investigated. By increasing the filling pressure, the energy deposited in the arc increases as a result of increased arcing voltage. It was observed that the pressure-rise in the heating volume is linked to the filling pressure, while the vent size plays a crucial role in the blow pressure near current zero. The nozzle mass loss per unit energy deposited in the arc is found to be independent of the filling pressure
Experimental investigation of dielectric barrier impact on breakdown voltage enhancement of copper wire-plane electrode systems
Non-pressurized air is extensively used as basic insulation media in medium / high voltage equipments. An inherent property of air-insulated designs is that the systems tend to become physically large. Application of Dielectric barrier can increase the breakdown voltage and therefore decrease the size of the equipments.
In this paper, the impact of dielectric barrier on breakdown voltage enhancement of a copper wire-plane system is investigated. For this purpose, the copper wire is covered with different dielectric materials. Depending on the air gap and dielectric strength of the barrier the breakdown can be initiated in the solid or gas dielectric. Theoretically, free charges are affected by the electric field between the electrodes and accumulated at the dielectric surface, this leads to the reduction of electric field in air gap and enhancement of the ifield in the dielectric layer. Therefore, with appropriate selection of the barrier thickness and material, it is possible to increase the breakdown voltage of the insulation system. The influence of different parameters like inter-electrode spacing, and dielectric material on the break-down voltage is investigated for applied 50 Hz AC and DC voltages. The results indicate that up to 240% increase of the breakdown voltage can be achieved
CMB B-mode polarization from Thomson scattering in the local universe
[Abridged] The polarization of the CMB is widely recognized as a potential
source of information about primordial gravitational waves. The gravitational
wave contribution can be separated from the dominant CMB polarization created
by density perturbations because it generates both E and B polarization modes,
whereas the density perturbations create only E polarization. The limits of our
ability to measure gravitational waves are thus determined by statistical and
systematic errors, foregrounds, and nonlinear evolution effects such as lensing
of the CMB. Usually it is assumed that most foregrounds can be removed because
of their frequency dependence, however Thomson scattering of the CMB quadrupole
by electrons in the Galaxy or nearby structures shares the blackbody frequency
dependence of the CMB. If the optical depth from these nearby electrons is
anisotropic, the polarization generated can include B modes even without tensor
perturbations. We estimate this effect for the Galactic disk and nearby
extragalactic structures, and find that it contributes to the B polarization at
the level of ~(1--2)x10^-4\mu K per logarithmic interval in multipole L for
L<30. This is well below the detectability level even for a future CMB
polarization satellite. Depending on its structure and extent, the Galactic
corona may be a source of B-modes comparable to the residual large-scale
lensing B-mode after the latter has been cleaned using lensing reconstruction
techniques. For an extremely ambitious post-Planck CMB experiment, Thomson
scattering in the Galactic corona is thus a potential contaminant of the
gravitational wave signal; conversely, if the other foregrounds can be cleaned
out, such an experiment might be able to constrain models of the corona.Comment: 10 pages, 4 figures, to be submitted to Phys. Rev.
Cross-Correlation of the Cosmic Microwave Background with the 2MASS Galaxy Survey: Signatures of Dark Energy, Hot Gas, and Point Sources
We cross-correlate the Cosmic Microwave Background (CMB) temperature
anisotropies observed by the Wilkinson Microwave Anisotropy Probe (WMAP) with
the projected distribution of extended sources in the Two Micron All Sky Survey
(2MASS). By modelling the theoretical expectation for this signal, we extract
the signatures of dark energy (Integrated Sachs-Wolfe effect;ISW), hot gas
(thermal Sunyaev-Zeldovich effect;thermal SZ), and microwave point sources in
the cross-correlation. Our strongest signal is the thermal SZ, at the 3.1-3.7
\sigma level, which is consistent with the theoretical prediction based on
observations of X-ray clusters. We also see the ISW signal at the 2.5 \sigma
level, which is consistent with the expected value for the concordance LCDM
cosmology, and is an independent signature of the presence of dark energy in
the universe. Finally, we see the signature of microwave point sources at the
2.7 \sigma level.Comment: 35 pages (preprint format), 8 figures. In addition to minor revisions
based on referee's comments, after correcting for a bug in the code, the SZ
detection is consistent with the X-ray observations. Accepeted for
publication in Physical Review
Waveform Modelling for the Laser Interferometer Space Antenna
LISA, the Laser Interferometer Space Antenna, will usher in a new era in
gravitational-wave astronomy. As the first anticipated space-based
gravitational-wave detector, it will expand our view to the millihertz
gravitational-wave sky, where a spectacular variety of interesting new sources
abound: from millions of ultra-compact binaries in our Galaxy, to mergers of
massive black holes at cosmological distances; from the beginnings of inspirals
that will venture into the ground-based detectors' view to the death spiral of
compact objects into massive black holes, and many sources in between. Central
to realising LISA's discovery potential are waveform models, the theoretical
and phenomenological predictions of the pattern of gravitational waves that
these sources emit. This white paper is presented on behalf of the Waveform
Working Group for the LISA Consortium. It provides a review of the current
state of waveform models for LISA sources, and describes the significant
challenges that must yet be overcome.Comment: 239 pages, 11 figures, white paper from the LISA Consortium Waveform
Working Group, invited for submission to Living Reviews in Relativity,
updated with comments from communit