5,438 research outputs found
Competition between Pressure and Gravity Confinement in Lyman-Alpha Forest Observations
A break in the distribution function of Ly clouds (at a typical
redshift of ) has been reported by Petitjean et al. (1993). This feature
is what would be expected from a transition between pressure confinement and
gravity confinement (as predicted in Charlton, Salpeter, and Hogan (1993)). The
column density at which the feature occurs has been used to determine the
external confining pressure, , which could be
due to a hot, intergalactic medium. For models that provide a good fit to the
data, the contribution of the gas in clouds to is small. The specific
shape of the distribution function at the transition (predicted by models to
have a non-monotonic slope) can serve as a diagnostic of the distribution of
dark matter around Ly forest clouds, and the present data already
eliminate certain models.Comment: 10 pages plain TeX, 2 figures available upon request, submitted to
ApJ Letters, PSU-jc-
Strong Gravitational Lensing and Dark Energy Complementarity
In the search for the nature of dark energy most cosmological probes measure
simple functions of the expansion rate. While powerful, these all involve
roughly the same dependence on the dark energy equation of state parameters,
with anticorrelation between its present value w_0 and time variation w_a.
Quantities that have instead positive correlation and so a sensitivity
direction largely orthogonal to, e.g., distance probes offer the hope of
achieving tight constraints through complementarity. Such quantities are found
in strong gravitational lensing observations of image separations and time
delays. While degeneracy between cosmological parameters prevents full
complementarity, strong lensing measurements to 1% accuracy can improve
equation of state characterization by 15-50%. Next generation surveys should
provide data on roughly 10^5 lens systems, though systematic errors will remain
challenging.Comment: 7 pages, 5 figure
Surface-micromachined Ta–Si–N beams for use in micromechanics
Realization and characterization of free-standing surface-microstructures based on Ta-Si-N films are presented. Due to their significant physical and chemical properties, such ternary films are promising candidates for application in microelectromechanical devices
Quantum transport in noncentrosymmetric superconductors and thermodynamics of ferromagnetic superconductors
We consider a general Hamiltonian describing coexistence of itinerant
ferromagnetism, spin-orbit coupling and mixed spin-singlet/triplet
superconducting pairing in the context of mean-field theory. The Hamiltonian is
diagonalized and exact eigenvalues are obtained, thus allowing us to write down
the coupled gap equations for the different order parameters. Our results may
then be applied to any model describing coexistence of any combination of these
three phenomena. As a specific application of our results, we consider
tunneling between a normal metal and a noncentrosymmetric superconductor with
mixed singlet and triplet gaps. The conductance spectrum reveals information
about these gaps in addition to how the influence of spin-orbit coupling is
manifested. We also consider the coexistence of itinerant ferromagnetism and
triplet superconductivity as a model for recently discovered ferromagnetic
superconductors. The coupled gap equations are solved self-consistently, and we
study the conditions necessary to obtain the coexistent regime of
ferromagnetism and superconductivity. Analytical expressions are presented for
the order parameters, and we provide an analysis of the free energy to identify
the preferred system state. Moreover, we make specific predictions concerning
the heat capacity for a ferromagnetic superconductor. In particular, we report
a nonuniversal relative jump in the specific heat, depending on the
magnetization of the system, at the uppermost superconducting phase transition.
[Shortened abstract due to arXiv submission.]Comment: 19 pages, 15 figures (high quality figures available in published
version). Accepted for publication in Phys. Rev.
Testing General Relativity with Current Cosmological Data
Deviations from general relativity, such as could be responsible for the
cosmic acceleration, would influence the growth of large scale structure and
the deflection of light by that structure. We clarify the relations between
several different model independent approaches to deviations from general
relativity appearing in the literature, devising a translation table. We
examine current constraints on such deviations, using weak gravitational
lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background
radiation data of WMAP5, and supernova distance data of Union2. Markov Chain
Monte Carlo likelihood analysis of the parameters over various redshift ranges
yields consistency with general relativity at the 95% confidence level.Comment: 11 pages; 7 figures; typographical errors corrected; this is the
published versio
Exploring the Expanding Universe and Dark Energy using the Statefinder Diagnostic
The coming few years are likely to witness a dramatic increase in high
quality Sn data as current surveys add more high redshift supernovae to their
inventory and as newer and deeper supernova experiments become operational.
Given the current variety in dark energy models and the expected improvement in
observational data, an accurate and versatile diagnostic of dark energy is the
need of the hour. This paper examines the Statefinder diagnostic in the light
of the proposed SNAP satellite which is expected to observe about 2000
supernovae per year. We show that the Statefinder is versatile enough to
differentiate between dark energy models as varied as the cosmological constant
on the one hand, and quintessence, the Chaplygin gas and braneworld models, on
the other. Using SNAP data, the Statefinder can distinguish a cosmological
constant () from quintessence models with and Chaplygin gas
models with at the level if the value of \om is
known exactly. The Statefinder gives reasonable results even when the value of
\om is known to only accuracy. In this case, marginalizing over
\om and assuming a fiducial LCDM model allows us to rule out quintessence
with and the Chaplygin gas with (both at
). These constraints can be made even tighter if we use the
Statefinders in conjunction with the deceleration parameter. The Statefinder is
very sensitive to the total pressure exerted by all forms of matter and
radiation in the universe. It can therefore differentiate between dark energy
models at moderately high redshifts of z \lleq 10.Comment: 21 pages, 17 figures. Minor typos corrected to agree with version
published in MNRAS. Results unchange
The biochemical, physiological, and metabolic evaluation of human subjects in a life support systems evaluator and on a liquid food diet Final report, 12 Jun. 1964 - 23 Feb. 1965
Biochemical, physiological, and metabolic analysis of subjects in life support system on liquid food diets during space environment simulatio
Constraining Parity Violation in Gravity with Measurements of Neutron-Star Moments of Inertia
Neutron stars are sensitive laboratories for testing general relativity,
especially when considering deviations where velocities are relativistic and
gravitational fields are strong. One such deviation is described by dynamical,
Chern-Simons modified gravity, where the Einstein-Hilbert action is modified
through the addition of the gravitational parity-violating Pontryagin density
coupled to a field. This four-dimensional effective theory arises naturally
both in perturbative and non-perturbative string theory, loop quantum gravity,
and generic effective field theory expansions. We calculate here Chern-Simons
modifications to the properties and gravitational fields of slowly spinning
neutron stars. We find that the Chern-Simons correction affects only the
gravitomagnetic sector of the metric to leading order, thus introducing
modifications to the moment of inertia but not to the mass-radius relation. We
show that an observational determination of the moment of inertia to an
accuracy of 10%, as is expected from near-future observations of the double
pulsar, will place a constraint on the Chern-Simons coupling constant of
\xi^{1/4} < 5 km, which is at least three-orders of magnitude stronger than the
previous strongest bound.Comment: 14 pages, 6 figures, replaced with version accepted for publication
in Phys. Rev.
Effect of Void Network on CMB Anisotropy
We study the effect of a void network on the CMB anisotropy in the
Einstein-de Sitter background using Thompson &Vishniac's model. We consider
comprehensively the Sacks-Wolfe effect, the Rees-Sciama effect and the
gravitational lensing effect. Our analysis includes the model of primordial
voids existing at recombination, which is realized in some inflationary models
associated with a first-order phase transition. If there exist primordial voids
whose comoving radius is larger than Mpc at recombination, not
only the Sachs-Wolfe effect but also the Rees-Sciama effect is appreciable even
for multipoles l\lsim1000 of the anisotropy spectrum. The gravitational
lensing effect, on the other hand, slightly smoothes the primary anisotropy;
quantitatively, our results for the void model are similar to the previous
results for a CDM model. All the effects, together, would give some constraints
on the configuration or origin of voids with high-resolution data of the CMB
anisotropy.Comment: 23 pages, latex, 12 eps figures, some calculations and discussions
are added, to appear in ApJ 510 (1999
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