6,610 research outputs found
Galactic Escape Speeds in Mirror and Cold Dark Matter Models
The mirror dark matter (MDM) model of Berezhiani et al. has been shown to
reproduce observed galactic rotational curves for a variety of spiral galaxies,
and has been presented as an alternative to cold dark matter (CDM) models. We
investigate possible additional tests involving the properties of stellar
orbits, which may be used to discriminate between the two models. We
demonstrate that in MDM and CDM models fitted equally well to a galactic
rotational curve, one generally expects predictable differences in escape
speeds from the disc. The recent radial velocity (RAVE) survey of the Milky Way
has pinned down the escape speed from the solar neighbourhood to
km s, placing an additional constraint on dark
matter models. We have constructed an MDM model for the Milky Way based on its
rotational curve, and find an escape speed that is just consistent with the
observed value given the current errors, which lends credence to the viability
of the MDM model. The Gaia-ESO spectroscopic survey is expected to lead to an
even more precise estimate of the escape speed that will further constrain dark
matter models. However, the largest differences in stellar escape speeds
between both models are predicted for dark matter dominated dwarf galaxies such
as DDO 154, and kinematical studies of such galaxies could prove key in
establishing, or abolishing, the validity of the MDM model.Comment: Accepted for publication in the European Physical Journal
Self-similarity in the conformal framework of quiescent cosmology and the Weyl curvature hypothesis
A viable alternative to cosmological inflation is provided by the combined theory of quiescent cosmology and the Weyl curvature hypothesis. We augment the conformal framework of this theory by incorporating the spacetime property of self-similarity. A generalisation of the conformal Killing equation is developed as a definition of asymptotic self-similarity for use in the framework; we derive several propositions and theorems that facilitate the application of this definition, and demonstrate asymptotic self-similarity for FLRW and other models. We also detail the conditions under which self-similarity is preserved by conformal transformations, and investigate its relationship to other symmetry properties in the framework
Geometric creation of quantum vorticity
We consider superfluidity and quantum vorticity in rotating spacetimes. The
system is described by a complex scalar satisfying a nonlinear Klein-Gordon
equation. Rotation terms are identified and found to lead to the transfer of
angular momentum of the spacetime to the scalar field. The scalar field
responds by rotating, physically behaving as a superfluid, through the creation
of quantized vortices. We demonstrate the vortex nucleation through numerical
simulation.Comment: 10 pages, 1 figure, updated to closely resemble published versio
Structural Characterization of Rapid Thermal Oxidized Si\u3csub\u3e1−x−y\u3c/sub\u3eGe\u3csub\u3ex\u3c/sub\u3eC\u3csub\u3ey\u3c/sub\u3e Alloy Films Grown by Rapid Thermal Chemical Vapor Deposition
The structural properties of as-grown and rapid thermal oxidized Si1−x−yGexCy epitaxial layers have been examined using a combination of infrared, x-ray photoelectron, x-ray diffraction, secondary ion mass spectroscopy, and Raman spectroscopy techniques. Carbon incorporation into the Si1−x−yGexCy system can lead to compressive or tensile strain in the film. The structural properties of the oxidized Si1−x−yGexCy film depend on the type of strain (i.e., carbon concentration) of the as-prepared film. For compressive or fully compensated films, the oxidation process drastically reduces the carbon content so that the oxidized films closely resemble to Si1−xGex films. For tensile films, two broad regions, one with carbon content higher and the other lower than that required for full strain compensation, coexist in the oxidized films
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