2,914 research outputs found
Vacuum Polarization of a Massless Scalar Field in the Background of a Global Monopole with Finite Core
In this paper we analyze the vacuum polarization effects of a massless scalar
field in the background of a global monopole considering a inner structure to
it. Specifically we investigate the effect of its structure on the vacuum
expectation value of the square of the field operator, ,
admitting a non-minimal coupling between the field with the geometry: $\xi
{\cal{R}}\hat{\Phi}^2$. Also we calculate the corrections on the vacuum
expectation value of the energy-momentum tensor, , due to
the inner structure of the monopole. In order to develop these analysis, we
calculate the Euclidean Green function associated with the system for points in
the region outside the core. As we shall see, for specific value of the
coupling parameter , the corrections caused by the inner structure of the
monopole can provide relevant contributions on these vacuum polarizations.Comment: Accepted for publication in Classical and Quantum Gravity. Added
references. 22 pages, 1 figur
Neutral Hydrogen 21cm Absorption at Redshift 0.673 towards 1504+377
We detect the 21 cm line of neutral hydrogen in absorption at a redshift of
0.673 towards the 1 Jy radio source 1504+377. The 1504+377 radio source is
located toward the center of what appears to be an inclined disk galaxy at z =
0.674. The 21 cm absorption line shows multiple velocity components over a
velocity range of about 100 km sec, with a total HI column density:
N(HI) = cm. The
velocity-integrated optical depth of this system is the largest yet seen for
redshifted HI 21 cm absorption line systems (Carilli 1995). The 21 cm
absorption line is coincident in redshift with a previously detected broad
molecular absorption line system (Wiklind and Combes 1996). We do not detect HI
21 cm absorption associated with the narrow molecular absorption line system at
z = 0.67150, nor do we detect absorption at these redshifts by the 18 cm lines
of OH, nor by the 2 cm transition of HCO. There is no evidence for a bright
optical AGN in 1504+377, suggesting significant obscuration through the disk --
a hypothesis supported by the strong absorption observed. The 1504+377 system
resembles the ``red quasar'' PKS 1413+135, which has been modeled as a
optically obscured AGN with a very young radio jet in the center of a gas rich
disk galaxy (Perlman et al. 1996). The presence of very bright radio jets at
the centers of these two disk galaxies presents a challenge to unification
schemes for extragalactic radio sources and to models for the formation of
radio loud AGN.Comment: 17 pages, postscrip
Large area pulse ionization chamber for measurement of extremely heavy cosmic rays
Parallel plate ionization chamber for identifying relativistic cosmic ray nucle
Dynamics of CrO3âFe2O3 catalysts during the high-temperature water-gas shift reaction: molecular structures and reactivity
A series of supported CrO3/Fe2O3 catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (Oâ)2Cr6+O2 species and a bulk Fe2O3 phase containing some Cr3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 ,and surface dioxo (Oâ)2Cr6+O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromiumâiron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3â-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe0 and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C16O2/H2 â C18O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromiumâiron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect
The Universe out of an Elementary Particle?
We consider a model of an elementary particle as a 2 + 1 dimensional brane
evolving in a 3 + 1 dimensional space. Introducing gauge fields that live in
the brane as well as normal surface tension can lead to a stable "elementary
particle" configuration. Considering the possibility of non vanishing vacuum
energy inside the bubble leads, when gravitational effects are considered, to
the possibility of a quantum decay of such "elementary particle" into an
infinite universe. Some remarkable features of the quantum mechanics of this
process are discussed, in particular the relation between possible boundary
conditions and the question of instability towards Universe formation is
analyzed
Light-like Signals in General relativity and Cosmology
The modelling of light-like signals in General Relativity taking the form of
impulsive gravitational waves and light-like shells of matter is examined.
Systematic deductions from the Bianchi identities are made. These are based
upon Penrose's hierarchical classification of the geometry induced on the null
hypersurface history of the surface by its imbedding in the space-times to the
future and to the past of it. The signals are not confined to propagate in a
vacuum and thus their interaction with matter (a burst of radiation propagating
through a cosmic fluid, for example) is also studied. Results are accompanied
by illustrative examples using cosmological models, vacuum space-times, the de
sitter univers and Minkowskian space-time.Comment: 21 pages, latex, no figure
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