11,139 research outputs found
Fluid injection device for high-pressure systems
Screw activated device, consisting of a compressor, shielded replaceable ampules, a multiple-element rubber gland, and a specially constructed fluid line fitting, injects measured amounts of fluids into a pressurized system. It is sturdy and easily manipulated
Gravitational instantons, extra dimensions and form fields
A broad class of higher dimensional instanton solutions are found for a
theory which contains gravity, a scalar field and antisymmetric tensor fields
of arbitrary rank. The metric used, a warp product of an arbitrary number of
any compact Einstein manifolds, includes many of great interest in particle
physics and cosmology. For example 4D FRW universes with additional dimensions
compactified on a Calabi-Yau three fold, a torus, a compact hyperbolic manifold
or a sphere are all included. It is shown that the solution of this form which
dominates the Hartle Hawking path integral is always a higher dimensional
generalisation of a Hawking Turok instanton when the potential of the scalar
field is such that these instantons can exist. On continuation to Lorentzian
signature such instantons give rise to a spacetime in which all of the spatial
dimensions are of equal size and where the spatial topology is that of a
sphere. The extra dimensions are thus not hidden. In the case where the
potential for the scalar field is generated solely by a dilatonic coupling to
the form fields we find no integrable instantons at all. In particular we find
no integrable solutions of the type under consideration for the supergravity
theories which are the low energy effective field theories of superstrings.Comment: 11 pages, 4 figure
Development of a refrigeration system for lunar surface and spacecraft applications
An evaluation of refrigeration devices suitable for potential lunar surface and spacecraft applications was performed. The following conclusions were reached: (1) the vapor compression system is the best overall refrigeration system for lunar surface and spacecraft applications and the single phase radiator system is generally preferred for earth orbit applications, (2) the vapor compression cycle may have some application for simultaneous heating and cooling, (3) a Stirling cycle refrigerator was selected for the manned cabin of the space shuttle, and (4) significant increases in payload heat rejection can be obtained by a kit vapor compression refrigerator added to the shuttle R-21 loop. The following recommendations were made: (1) a Stirling cycle refrigerator may be used for food freezer and biomedical sample storage, (2) the best system for a food freezer/experiments compartment for an earth orbit space station has not been determined, (3) a deployed radiator system can be designed for large heat loads in earth orbit
How is a Closed String Loop like a Black Hole?
We demonstrate that under plausible assumptions the entropy and temperature
associated with the small oscillations on a circular loop of radius and a
black hole of mass are identical.Comment: 6 pages, uses harvma
Cooling systems for satellite remote sensing instrumentation
The characteristics of a cryogenic cooling system for the Pollution Monitoring Satellite (PMS) are discussed. Studies were conducted to make the following determinations: (1) the characteristics and use of proven and state-of-the-art cryogenic cooling systems for six specified ranges of performance, (2) the system most applicable for each of the six cooling categories, and (3) conceptual designs for candidate system for each of the six representative cooling categories. The six cooling categories of electrical loads are defined. The desired mission life for the cooling system is two years with both continuous and intermittent operating conditions
Cosmological matching conditions
We investigate the evolution of scalar metric perturbations across a sudden
cosmological transition, allowing for an inhomogeneous surface stress at the
transition leading to a discontinuity in the local expansion rate, such as
might be expected in a big crunch/big bang event. We assume that the transition
occurs when some function of local matter variables reaches a critical value,
and that the surface stress is also a function of local matter variables. In
particular we consider the case of a single scalar field and show that a
necessary condition for the surface stress tensor to be perturbed at the
transition is the presence of a non-zero intrinsic entropy perturbation of the
scalar field. We present the matching conditions in terms of gauge-invariant
variables assuming a sudden transition to a fluid-dominated universe with
barotropic equation of state. For adiabatic perturbations the comoving
curvature perturbation is continuous at the transition, while the Newtonian
potential may be discontinuous if there is a discontinuity in the background
Hubble expansion.Comment: 12 pages, no figure
Gravitational instantons and internal dimensions
We Study instanton solutions in general relativity with a scalar field. The
metric ansatz we use is composed of a particular warp product of general
Einstein metrics, such as those found in a number of cosmological settings,
including string cosmology, supergravity compactifications and general Kaluza
Klein reductions. Using the Hartle-Hawking prescription the instantons we
obtain determine whether metrics involving extra compact dimensions of this
type are favoured as initial conditions for the universe. Specifically, we find
that these product metric instantons, viewed as constrained instantons, do have
a local minima in the action. These minima are then compared with the higher
dimensional version of the Hawking-Turok instantons, and we argue that the
latter always have lower action than those associated with these product
metrics.Comment: 10 pages, 5 figure
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