59,264 research outputs found
Proton Decay Constraints on Low Scale AdS/CFT Unification
Dark matter candidates and proton decay in a class of models based on the
AdS/CFT correspondence are discussed. We show that the present bound on the
proton decay lifetime is inconsistent with SUSY, and strongly
constrains non-SUSY, low scale trinification type unification of
orbifolded AdS models.Comment: 10 page
The Physics of 'Now'
The world is four-dimensional according to fundamental physics, governed by
basic laws that operate in a spacetime that has no unique division into space
and time. Yet our subjective experience is divided into present, past, and
future. This paper discusses the origin of this division in terms of simple
models of information gathering and utilizing systems (IGUSes). Past, present,
and future are not properties of four-dimensional spacetime but notions
describing how individual IGUSes process information. Their origin is to be
found in how these IGUSes evolved or were constructed. The past, present, and
future of an IGUS is consistent with the four-dimensional laws of physics and
can be described in four-dimensional terms. The present, for instance, is not a
moment of time in the sense of a spacelike surface in spacetime. Rather there
is a localized notion of present at each point along an IGUS' world line. The
common present of many localized IGUSes is an approximate notion appropriate
when they are sufficiently close to each other and have relative velocities
much less than that of light. But modes of organization that are different from
present, past and future can be imagined that are consistent with the physical
laws. We speculate why the present, past, and future organization might be
favored by evolution and therefore a cognitive universal.Comment: 12 pages, 4 figures, Revtex4, minor correction
The No-Boundary Measure of the Universe
We consider the no-boundary proposal for homogeneous isotropic closed
universes with a cosmological constant and a scalar field with a quadratic
potential. In the semi-classical limit, it predicts classical behavior at late
times if the initial scalar field is more than a certain minimum. If the
classical late time histories are extended back, they may be singular or bounce
at a finite radius. The no-boundary proposal provides a probability measure on
the classical solutions which selects inflationary histories but is heavily
biased towards small amounts of inflation. This would not be compatible with
observations. However we argue that the probability for a homogeneous universe
should be multiplied by exp(3N) where N is the number of e-foldings of slow
roll inflation to obtain the probability for what we observe in our past light
cone. This volume weighting is similar to that in eternal inflation. In a
landscape potential, it would predict that the universe would have a large
amount of inflation and that it would start in an approximately de Sitter state
near a saddle-point of the potential. The universe would then have always been
in the semi-classical regime.Comment: 4 pages, revtex4, minor corrections to accord with published versio
Venus steep-sided domes: Relationships between geological associations and possible petrogenetic models
Venus domes are characterized by steep sides, a circular shape, and a relatively flat summit area. In addition, they are orders of magnitude larger in volume and have a lower height/diameter ratio than terrestrial silicic lava domes. The morphology of the domes is consistent with formation by lava with a high apparent viscosity. Twenty percent of the domes are located in or near tessera (highly deformed highlands), while most other (62 percent) are located in and near coronae (circular deformational features thought to represent local mantle upwelling). These geological associations provide evidence for mechanisms of petrogenesis and several of these models are found to be plausible: remelting of basaltic or evolved crust, differentiation of basaltic melts, and volatile enhancement and eruption of basaltic foams. Hess and Head have shown that the full range of magma compositions existing on the Earth is plausible under various environmental conditions on Venus. Most of the Venera and Vego lander compostional data are consistent with tholeiitic basalt; however, evidence for evolved magmas was provided by Venera 8 data consistent with a quartz monzonite composition. Pieters et al. have examined the color of the Venus surface from Venera lander images and interpret the surface there to be oxidized. Preliminary modeling of dome growth has provided some interpretations of lava rheology. Viscosity values obtained from these models range from 10(exp 14) - 10(exp 17) pa*s, and the yield strength has been calculated to be between 10(exp 4) and 10(exp 6) Pa, consistent with terrestrial silicic rocks. The apparent high viscosity of the dome lavas suggests that the domes have a silicic composition or must augment their viscosity with increased visicularity or crystal content. Sixty-two percent of the Venus domes are associated with coronae, circular features that have been proposed as sites of mantle upwelling, and 20 percent of the domes are located near tessera, relatively high areas of complex deformed terrain. We have investigated several models that are consistent with these geologic associations. The first case involves the differentiation of basalt in a magma reservoir in the crust, perhaps produced by partial melting within a mantle plume. The second case is melting at the base of thickened basaltic crust, and the final case is volatile exsolution and enhancement within a basaltic magma reservoir. The association of domes with tessera might be explained by crustal remelting, while the association with coronae may be consistent with chemical differentiation of a magma reservoir or the exsolution and concentration of volatiles in the reservoir before eruption
Vector Fields in Holographic Cosmology
We extend the holographic formulation of the semiclassical no-boundary wave
function (NBWF) to models with Maxwell vector fields. It is shown that the
familiar saddle points of the NBWF have a representation in which a regular,
Euclidean asymptotic AdS geometry smoothly joins onto a Lorentzian
asymptotically de Sitter universe through a complex transition region. The tree
level probabilities of Lorentzian histories are fully specified by the action
of the AdS region of the saddle points. The scalar and vector matter profiles
in this region are complex from an AdS viewpoint, with universal asymptotic
phases. The dual description of the semiclassical NBWF thus involves complex
deformations of Euclidean CFTs.Comment: 17 pages, 3 fig
C-SIDE: The control-structure interaction demonstration experiment
The Control-Structure Interaction Demonstration Experiment (C-SIDE) is sponsored by the Electro-Optics and Cryogenics Division of Ball Aerospace Systems Group. Our objective is to demonstrate methods of solution to structure control problems utilizing currently available hardware in a system that is an extension of our corporate experience. The larger space structures with which Ball has been associated are the SEASAT radar antenna, Shuttle Imaging Radar (SIR) -A, -B and -C antennas and the Radarsat spacecraft. The motivation for the C-SIDE configuration is to show that integration of active figure control in the radar's system-level design can relieve antenna mechanical design constraints. This presentation is primarily an introduction to the C-SIDE testbed. Its physical and functional layouts, and major components are described. The sensor is of special interest as it enables direct surface figure measurements from a remote location. The Remote Attitude Measurement System (RAMS) makes high-rate, unobtrusive measurements of many locations, several of which may be collocated easily with actuators. The control processor is a 386/25 executing a reduced order model-based algorithm with provision for residual mode filters to compensate for structure interaction. The actuators for the ground demonstration are non-contacting, linear force devices. Results presented illustrate some basic characteristics of control-structure interaction with this hardware. The testbed will be used for evaluation of current technologies and for research in several areas. A brief indication of the evolution of the C-SIDE is given at the conclusion
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