5,444 research outputs found
On the galactic rotation curves problem within an axisymmetric approach
In U. Nucamendi et al. Phys. Rev. D63 (2001) 125016 and K. Lake, Phys. Rev.
Lett. 92 (2004) 051101 it has been shown that galactic potentials can be
kinematically linked to the observed red/blue shifts of the corresponding
galactic rotation curves under a minimal set of assumptions: the emitted
photons come from stable timelike circular geodesic orbits of stars in a static
spherically symmetric gravitational field, and propagate to us along null
geodesics. It is remarkable that this relation can be established without
appealing at all to a concrete theory of gravitational interaction. Here we
generalize this kinematical spherically symmetric approach to the galactic
rotation curves problem to the stationary axisymmetric realm since this is
precisely the symmetry that spiral galaxies possess. Thus, by making use of the
most general stationary axisymmetric metric, we also consider stable circular
orbits of stars that emit signals which travel to a distant observer along null
geodesics and express the galactic red/blue shifts in terms of three arbitrary
metric functions, clarifying the contribution of the rotation as well as the
dragging of the gravitational field. This stationary axisymmetric approach
distinguishes between red and blue shifts emitted by circularly orbiting
receding and approaching stars, respectively, even when they are considered
with respect to the center of a spiral galaxy, indicating the need of precise
measurements in order to confront predictions with observations. We also point
out the difficulties one encounters in the attempt of determining the metric
functions from observations and list some possible strategies to overcome them.Comment: 7 pages in latex (MNRAS format), no figures, discussion and
references adde
Late-time Phase transition and the Galactic halo as a Bose Liquid: (II) the Effect of Visible Matter
In the previous work, we investigated the rotation curves of galaxies
assuming that the dark matter consists of ultra light boson appearing in
late time phase transition' theory. Generalizing this work, we consider the
effect of visible matter and classify the types of rotation curves as we vary
the fraction of the mass and extention of visible matter. We show that visible
matter, in galaxies with flat rotation curves, has mass fraction and it is confined within the distance fraction .Comment: 10 pages, 10 figures included, to appear in Phys. Rev. D50,p365
Audible information design in the New York City subway system: A case study
Presented at 5th International Conference on Auditory Display (ICAD), Glasgow, UK, November 1-4, 1998.Beginning with a detailed presentation of the use of audible signals, in the New York City subway stations and trains, we present an analysis of the information that is communicated by the existing sound design. We show the results of a survey of subway riders regarding their awareness and comprehension of the audible signals in the system. An analysis of the system's sound and information environment is presented, followed by a proposed sonic re-design to better serve both communication and aesthetic needs. We conclude by proposing that the principles of information design, sound design and music must be considered equally with those of acoustics and psychoacoustics when designing audio information and feedback systems
Cascade Birth of Universes in Multidimensional Spaces
The formation mechanism of universes with distinctly different properties is
considered within the framework of pure gravity in a space of D > 4 dimensions.
The emergence of the Planck scale and its relationship to the inflaton mass are
discussed.Comment: 10 p., minor correction
Organics in comet 67P â a first comparative analysis of mass spectra from ROSINAâDFMS, COSAC and Ptolemy
The ESA Rosetta spacecraft followed comet 67P at a close distance for more than 2 yr. In addition, it deployed the lander Philae on to the surface of the comet. The (surface) composition of the comet is of great interest to understand the origin and evolution of comets. By combining measurements made on the comet itself and in the coma, we probe the nature of this surface material and compare it to remote sensing observations. We compare data from the double focusing mass spectrometer (DFMS) of the ROSINA experiment on ESA's Rosetta mission and previously published data from the two mass spectrometers COSAC (COmetary Sampling And Composition) and Ptolemy on the lander. The mass spectra of all three instruments show very similar patterns of mainly CHO-bearing molecules that sublimate at temperatures of 275 K. The DFMS data also show a great variety of CH-, CHN-, CHS-, CHO2- and CHNO-bearing saturated and unsaturated species. Methyl isocyanate, propanal and glycol aldehyde suggested by the earlier analysis of the measured COSAC spectrum could not be confirmed. The presence of polyoxymethylene in the Ptolemy spectrum was found to be unlikely. However, the signature of the aromatic compound toluene was identified in DFMS and Ptolemy data. Comparison with remote sensing instruments confirms the complex nature of the organics on the surface of 67P, which is much more diverse than anticipated
Observation of a New Type of Low Frequency Waves at Comet 67P/Churyumov-Gerasimenko
We report on magnetic field measurements made in the innermost coma of
67P/Churyumov-Gerasimenko in its low activity state. Quasi-coherent,
large-amplitude (), compressional magnetic field
oscillations at 40 mHz dominate the immediate plasma environment of the
nucleus. This differs from previously studied comet-interaction regions where
waves at the cometary ion gyro-frequencies are the main feature. Thus classical
pick-up ion driven instabilities are unable to explain the observations. We
propose a cross-field current instability associated with newborn cometary ion
currents as a possible source mechanism.Comment: 6 pages, 3 Figure
An alternative approach to the galactic dark matter problem
We discuss scenarios in which the galactic dark matter in spiral galaxies is
described by a long range coherent field which settles in a stationary
configuration that might account for the features of the galactic rotation
curves. The simplest possibility is to consider scalar fields, so we discuss in
particular, two mechanisms that would account for the settlement of the scalar
field in a non-trivial configuration in the absence of a direct coupling of the
field with ordinary matter: topological defects, and spontaneous scalarization.Comment: 36 pages, 12 figures, Revtex, a brief discussion added, accepted for
publication in PR
Ground State Energy Fluctuations of a System Coupled to a Bath
It is often argued that a small non-degenerate quantum system coupled to a
bath has a fixed energy in its ground state since a fluctuation in energy would
require an energy supply from the bath. We consider a simple model of a
harmonic oscillator (the system) coupled to a linear string and determine the
mean squared energy fluctuations. We also analyze the two time correlator of
the energy and discuss its behavior for a finite string.Comment: 5 pages, 2 eps figures, minor change
A model for removing the increased recall of recent events from the temporal distribution of autobiographical memory
The reminiscence bump is the tendency to recall relatively many personal events from the period in which the individual was between 10 and 30Â years old. This effect has only been found in autobiographical memory studies that used participants who were older than 40Â years of age. The increased recall of recent events possibly obscures the reminiscence bump in the results of younger participants. In this study, a model was proposed that removes the increase for recent events from the temporal distribution. The model basically estimates a retention function based on the 10 most recent years from the observed distributions and divides the observed distributions by predictions derived from the estimated retention function. The model was examined with three simulated data sets and one experimental data set. The results of the experiment offered two practical examples of how the model could be used to investigate the temporal distribution of autobiographical memories
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