13,395 research outputs found
From p-branes to Cosmology
We study the relationship between static p-brane solitons and cosmological
solutions of string theory or M-theory. We discuss two different ways in which
extremal p-branes can be generalised to non-extremal ones, and show how wide
classes of recently discussed cosmological models can be mapped into
non-extremal p-brane solutions of one of these two kinds. We also extend
previous discussions of cosmological solutions to include some that make use of
cosmological-type terms in the effective action that can arise from the
generalised dimensional reduction of string theory or M-theory.Comment: Latex, 24 pages, no figur
Null Strings in Schwarzschild Spacetime
The null string equations of motion and constraints in the Schwarzschild
spacetime are given. The solutions are those of the null geodesics of General
Relativity appended by a null string constraint in which the "constants of
motion" depend on the world-sheet spatial coordinate. Because of the extended
nature of a string, the physical interpretation of the solutions is completely
different from the point particle case. In particular, a null string is
generally not propagating in a plane through the origin, although each of its
individual points is. Some special solutions are obtained and their physical
interpretation is given. Especially, the solution for a null string with a
constant radial coordinate moving vertically from the south pole to the
north pole around the photon sphere, is presented. A general discussion of
classical null/tensile strings as compared to massless/massive particles is
given. For instance, tensile circular solutions with a constant radial
coordinate do not exist at all. The results are discussed in relation to
the previous literature on the subject.Comment: 16 pages, REVTEX, no figure
Constraining star cluster disruption mechanisms
Star clusters are found in all sorts of environments and their formation and
evolution is inextricably linked to the star formation process. Their eventual
destruction can result from a number of factors at different times, but the
process can be investigated as a whole through the study of the cluster age
distribution. Observations of populous cluster samples reveal a distribution
following a power law of index approximately -1. In this work we use M33 as a
test case to examine the age distribution of an archetypal cluster population
and show that it is in fact the evolving shape of the mass detection limit that
defines this trend. That is to say, any magnitude-limited sample will appear to
follow a dN/dt=1/t, while cutting the sample according to mass gives rise to a
composite structure, perhaps implying a dependence of the cluster disruption
process on mass. In the context of this framework, we examine different models
of cluster disruption from both theoretical and observational standpoints.Comment: To appear in the proceedings of IAU Symposium 266: "Star Clusters:
Basic Galactic Building Blocks Throughout Time And Space", eds. R. de Grijs
and J. Lepin
Coupled Magnetic Excitations in Single Crystal PrBa2Cu3O6.2
The dispersion of the low-energy magnetic excitations of the Pr sublattice in
PrBa2Cu3O6.2 is determined by inelastic neutron scattering measurements on a
single crystal. The dispersion, which shows the effect of interactions with the
Cu spin-waves, is well described by a model of the coupled Cu-Pr magnetic
system. This enables values for the principal exchange constants to be
determined, which suggest that both Pr-Pr and Cu-Pr interactions are important
in producing the anomalously high ordering temperature of the Pr sublattice.
Measurements of the Cu optic spin wave mode show that the inter-layer Cu-Cu
exchange is significantly lower than in YBa2Cu3O6.2.Comment: To be published Phys. Rev. Let
UHF and VHF radar observations of thunderstorms
A study of thunderstorms was made in the Summer of 1985 with the 430-MHz and 50-MHz radars at the Arecibo Observatory in Puerto Rico. Both radars use the 300-meter dish, which gives a beam width of less than 2 degrees even at these long wavelengths. Though the radars are steerable, only vertical beams were used in this experiment. The height resolution was 300 and 150 meters for the UHF and VHF, respectively. Lightning echoes, as well as returns from precipitation and clear-air turbulence were detected with both wavelengths. Large increases in the returned power were found to be coincident with increasing downward vertical velocities at UHF, whereas at VHF the total power returned was relatively constant during the life of a storm. This was attributed to the fact that the VHF is more sensitive to scattering from the turbulence-induced inhomogeneities in the refractive index and less sensitive to scatter from precipitation particles. On occasion, the shape of the Doppler spectra was observed to change with the occurrence of a lightning discharge in the pulse volume. Though the total power and mean reflectivity weighted Doppler velocity changed little during these events, the power is Doppler frequency bins near that corresponding to the updraft did increase substantially within a fraction of a second after a discharge was detected in the beam. This suggests some interaction between precipitation and lightning
From the WZWN Model to the Liouville Equation: Exact String Dynamics in Conformally Invariant AdS Background
It has been known for some time that the SL(2,R) WZWN model reduces to
Liouville theory. Here we give a direct and physical derivation of this result
based on the classical string equations of motion and the proper string size.
This allows us to extract precisely the physical effects of the metric and
antisymmetric tensor, respectively, on the {\it exact} string dynamics in the
SL(2,R) background. The general solution to the proper string size is also
found. We show that the antisymmetric tensor (corresponding to conformal
invariance) generally gives rise to repulsion, and it precisely cancels the
dominant attractive term arising from the metric.
Both the sinh-Gordon and the cosh-Gordon sectors of the string dynamics in
non-conformally invariant AdS spacetime reduce here to the Liouville equation
(with different signs of the potential), while the original Liouville sector
reduces to the free wave equation. Only the very large classical string size is
affected by the torsion. Medium and small size string behaviours are unchanged.
We also find illustrative classes of string solutions in the SL(2,R)
background: dynamical closed as well as stationary open spiralling strings, for
which the effect of torsion is somewhat like the effect of rotation in the
metric. Similarly, the string solutions in the 2+1 BH-AdS background with
torsion and angular momentum are fully analyzed.Comment: 24 pages including 4 postscript figures. Enlarged version including a
section on string solutions in 2+1 black hole background. To be published in
Phys. Rev. D., December 199
Circular String-Instabilities in Curved Spacetime
We investigate the connection between curved spacetime and the emergence of
string-instabilities, following the approach developed by Loust\'{o} and
S\'{a}nchez for de Sitter and black hole spacetimes. We analyse the linearised
equations determining the comoving physical (transverse) perturbations on
circular strings embedded in Schwarzschild, Reissner-Nordstr\"{o}m and de
Sitter backgrounds. In all 3 cases we find that the "radial" perturbations grow
infinitely for (ring-collapse), while the "angular"
perturbations are bounded in this limit. For we find that
the perturbations in both physical directions (perpendicular to the string
world-sheet in 4 dimensions) blow up in the case of de Sitter space. This
confirms results recently obtained by Loust\'{o} and S\'{a}nchez who considered
perturbations around the string center of mass.Comment: 24 pages Latex + 2 figures (not included). Observatoire de Paris,
Meudon No. 9305
String dynamics in cosmological and black hole backgrounds: The null string expansion
We study the classical dynamics of a bosonic string in the --dimensional
flat Friedmann--Robertson--Walker and Schwarzschild backgrounds. We make a
perturbative development in the string coordinates around a {\it null} string
configuration; the background geometry is taken into account exactly. In the
cosmological case we uncouple and solve the first order fluctuations; the
string time evolution with the conformal gauge world-sheet --coordinate
is given by , where
are given by Eqs.\ (3.15), and is the exponent of the conformal factor
in the Friedmann--Robertson--Walker metric, i.e. . The string
proper size, at first order in the fluctuations, grows like the conformal
factor and the string energy--momentum tensor corresponds to that of
a null fluid. For a string in the black hole background, we study the planar
case, but keep the dimensionality of the spacetime generic. In the null
string expansion, the radial, azimuthal, and time coordinates are
and The first terms of the series represent a
{\it generic} approach to the Schwarzschild singularity at . First and
higher order string perturbations contribute with higher powers of . The
integrated string energy-momentum tensor corresponds to that of a null fluid in
dimensions. As the string approaches the singularity its proper
size grows indefinitely like . We end the paper
giving three particular exact string solutions inside the black hole.Comment: 17 pages, REVTEX, no figure
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