18,018 research outputs found
The role of singletons in compactifications
We derive the isometry irrep content of squashed seven-sphere
compactifications of eleven-dimensional supergravity, i.e., the left-squashed
() with and right-squashed () with supersymmetry, in a manner completely independent of the round sphere.
Then we compare this result with the spectrum obtained by Higgsing the round
sphere spectrum. This way we discover features of the spectra which makes it
possible to argue that the only way the round spectrum can be related by a
Higgs mechanism to the one of is if the singletons are included in the
round sphere spectrum. For this to work also in the case it seems that
the gravitino of the spectrum must be replaced by a fermionic singleton
present in the spectrum.Comment: 24 pages including appendix with 12 figure, v2 minor typos correcte
The Low-level Spectrum of the String
We investigate the spectrum of physical states in the string theory, up
to level 2 for a multi-scalar string, and up to level 4 for the two-scalar
string. The (open) string has a photon as its only massless state. By
using screening charges to study the null physical states in the two-scalar
string, we are able to learn about the gauge symmetries of the states in
the multi-scalar string.Comment: 31 pages, Plain Tex, CTP TAMU-70/92, Goteborg ITP 92-43,
Imperial/TP/91-92/22, KCL-TH-92-
Spatially self-similar spherically symmetric perfect-fluid models
Einstein's field equations for spatially self-similar spherically symmetric
perfect-fluid models are investigated. The field equations are rewritten as a
first-order system of autonomous differential equations. Dimensionless
variables are chosen in such a way that the number of equations in the coupled
system is reduced as far as possible and so that the reduced phase space
becomes compact and regular. The system is subsequently analysed qualitatively
with the theory of dynamical systems.Comment: 21 pages, 6 eps-figure
The state space and physical interpretation of self-similar spherically symmetric perfect-fluid models
The purpose of this paper is to further investigate the solution space of
self-similar spherically symmetric perfect-fluid models and gain deeper
understanding of the physical aspects of these solutions. We achieve this by
combining the state space description of the homothetic approach with the use
of the physically interesting quantities arising in the comoving approach. We
focus on three types of models. First, we consider models that are natural
inhomogeneous generalizations of the Friedmann Universe; such models are
asymptotically Friedmann in their past and evolve fluctuations in the energy
density at later times. Second, we consider so-called quasi-static models. This
class includes models that undergo self-similar gravitational collapse and is
important for studying the formation of naked singularities. If naked
singularities do form, they have profound implications for the predictability
of general relativity as a theory. Third, we consider a new class of
asymptotically Minkowski self-similar spacetimes, emphasizing that some of them
are associated with the self-similar solutions associated with the critical
behaviour observed in recent gravitational collapse calculations.Comment: 24 pages, 12 figure
An almost isotropic cosmic microwave temperature does not imply an almost isotropic universe
In this letter we will show that, contrary to what is widely believed, an
almost isotropic cosmic microwave background (CMB) temperature does not imply
that the universe is ``close to a Friedmann-Lemaitre universe''. There are two
important manifestations of anisotropy in the geometry of the universe, (i) the
anisotropy in the overall expansion, and (ii) the intrinsic anisotropy of the
gravitational field, described by the Weyl curvature tensor, although the
former usually receives more attention than the latter in the astrophysical
literature. Here we consider a class of spatially homogeneous models for which
the anisotropy of the CMB temperature is within the current observational
limits but whose Weyl curvature is not negligible, i.e. these models are not
close to isotropy even though the CMB temperature is almost isotropic.Comment: 5 pages (AASTeX, aaspp4.sty), submitted to Astrophysical Journal
Letter
The effects of deformation and pairing correlations on nuclear charge form factor
A set of moderately deformed shell nuclei is employed for testing the
reliability of the nuclear ground state wave functions which are obtained in
the context of a BCS approach and offer a simultaneous consideration of
deformation and pairing correlations effects. In this method, the mean field is
assumed to be an axially symmetric Woods-Saxon potential and the effective
two-body interaction is a monopole pairing force. As quantities of main
interest we have chosen the nuclear form factors, the occupancies of the active
(surface) orbits and the Fermi sea depletion, which provide quite good tests
for microscopic descriptions of nuclei within many body theories. For our
comparisons with results emerging from other similar methods, an axially
deformed harmonic oscillator field is also utilized.Comment: 20 pages, 12 figures, 2 table
Experimental f-value and isotopic structure for the Ni I line blended with [OI] at 6300A
We have measured the oscillator strength of the Ni I line at 6300.34 \AA,
which is known to be blended with the forbidden [O I] 6300 line, used
for determination of the oxygen abundance in cool stars. We give also
wavelengths of the two isotopic line components of Ni and Ni
derived from the asymmetric laboratory line profile. These two line components
of Ni I have to be considered when calculating a line profile of the 6300 \AA\
feature observed in stellar and solar spectra. We also discuss the labelling of
the energy levels involved in the Ni I line, as level mixing makes the
theoretical predictions uncertain.Comment: Accepted for publication in ApJLetter
Localization and clustering in the nuclear Fermi liquid
Using the framework of nuclear energy density functionals we examine the
conditions for single-nucleon localization and formation of cluster structures
in finite nuclei. We propose to characterize localization by the ratio of the
dispersion of single-nucleon wave functions to the average inter-nucleon
distance. This parameter generally increases with mass and describes the
gradual transition from a hybrid phase in light nuclei, characterized by the
spatial localization of individual nucleon states that leads to the formation
of cluster structures, toward the Fermi liquid phase in heavier nuclei. Values
of the localization parameter that correspond to a crystal phase cannot occur
in finite nuclei. Typical length and energy scales in nuclei allow the
formation of liquid drops, clusters, and halo structures.Comment: 6 pages, 3 figure
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