7,879 research outputs found
A note on the decay of noncommutative solitons
We propose an ansatz for the equations of motion of the noncommutative model
of a tachyonic scalar field interacting with a gauge field, which allows one to
find time-dependent solutions describing decaying solitons. These correspond to
the collapse of lower dimensional branes obtained through tachyon condensation
of unstable brane systems in string theory.Comment: 8 pages, no figures. Extended version, references adde
Local cosmic string in generalised scalar tensor theory
A recent investigation shows that a local gauge string with a
phenomenological energy momentum tensor, as prescribed by Vilenkin, is
inconsistent in Brans-Dicke theory. In this work it has been shown that such a
string is consistent in a more general scalar tensor theory where is
function of the scalar field.A set of solutions of full nonlinear Einstein's
equations for interior region of such a string are presented.Comment: 7 pages, latex format, minor changes according to referee's
suggestions, revised version submitted in Phys.Rev.
A Farey tale for N=4 dyons
We study exponentially suppressed contributions to the degeneracies of
extremal black holes. Within Sen's quantum entropy function framework and
focusing on extremal black holes with an intermediate AdS3 region, we identify
an infinite family of semi-classical AdS2 geometries which can contribute
effects of order exp(S_0/c), where S_0 is the Bekenstein-Hawking-Wald entropy
and c is an integer greater than one. These solutions lift to the extremal
limit of the SL(2,Z) family of BTZ black holes familiar from the "black hole
Farey tail". We test this understanding in N=4 string vacua, where exact dyon
degeneracies are known to be given by Fourier coefficients of Siegel modular
forms. We relate the sum over poles in the Siegel upper half plane to the Farey
tail expansion, and derive a "Farey tale" expansion for the dyon partition
function. Mathematically, this provides a (formal) lift from Hilbert modular
forms to Siegel modular forms with a pole at the diagonal divisor.Comment: 31 page
BPS Spectrum, Indices and Wall Crossing in N=4 Supersymmetric Yang-Mills Theories
BPS states in N=4 supersymmetric SU(N) gauge theories in four dimensions can
be represented as planar string networks with ends lying on D3-branes. We
introduce several protected indices which capture information on the spectrum
and various quantum numbers of these states, give their wall crossing formula
and describe how using the wall crossing formula we can compute all the indices
at all points in the moduli space.Comment: LaTeX file, 33 pages, 15 figure
Interacting viscous ghost tachyon, K-essence and dilaton scalar field models of dark energy
We study the correspondence between the interacting viscous ghost dark energy
model with the tachyon, K-essence and dilaton scalar field models in the
framework of Einstein gravity. We consider a spatially non-flat FRW universe
filled with interacting viscous ghost dark energy and dark matter. We
reconstruct both the dynamics and potential of these scalar field models
according to the evolutionary behavior of the interacting viscous ghost dark
energy model, which can describe the accelerated expansion of the universe. Our
numerical results show that the interaction and viscosity have opposite effects
on the evolutionary properties of the ghost scalar filed models.Comment: 16 pages, 17 figure
Physics and the measurement of continuous variables
Wigner had expressed the opinion that the impossibility of exact measurements
of single operators like position operators rendered the notion of geometrical
points somewhat dubious in physics. Using Sewell's recent resolution of the
measurement problem (collapse of the wave packet) in quantum mechanics and
extending it to the measurement of operators with continuous spectra, we are
able to compare the situation in quantum mechanics with that in quantum
mechanics. Our conclusion is that the notion of a geometrical point is as
meaningful in quantum mechanics as it is in classical mechanics.Comment: 20 page
Two-dimensional superstrings and the supersymmetric matrix model
We present evidence that the supersymmetric matrix model of Marinari and
Parisi represents the world-line theory of N unstable D-particles in type II
superstring theory in two dimensions. This identification suggests that the
matrix model gives a holographic description of superstrings in a
two-dimensional black hole geometry.Comment: 22 pages, 2 figures; v2: corrected eqn 4.6; v3: corrected appendices
and discussion of vacua, added ref
Higher Dimensional Recombination of Intersecting D-branes
We study recombinations of D-brane systems intersecting at more than one
angle using super Yang-Mills theory. We find the condensation of an
off-diagonal tachyon mode relates to the recombination, as was clarified for
branes at one angle in hep-th/0303204. For branes at two angles, after the
tachyon mode between two D2-branes condensed, D2-brane charge is distributed in
the bulk near the intersection point. We also find that, when two intersection
angles are equal, the off-diagonal lowest mode is massless, and a new stable
non-abelian configuration, which is supersymmetric up to a quadratic order in
the fluctuations, is obtained by the deformation by this mode.Comment: 18 pages, 2 figures, JHEP style. v3:references added, minor
corrections, English improve
Towards a developmental state? Provincial economic policy in South Africa
This paper explores the meaning of the developmental state for spatial economic policy in South Africa. Two main questions are addressed: do provincial governments have a role to play in promoting economic prosperity, and to what extent do current provincial policies possess the attributes of a developmental state? These attributes are defined as the ability to plan longer term, to focus key partners on a common agenda, and to mobilise state resources to build productive capabilities. The paper argues that the developmental state must harness the power of government at every level to ensure that each part of the country develops to its potential. However, current provincial capacity is uneven, and weakest where support is needed most. Many provinces seem to have partial strategies and lack the wherewithal for sustained implementation. Coordination across government appears to be poor. The paper concludes by suggesting ways provincial policies could be strengthened
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