76,368 research outputs found

### Fractional Branes and the Entropy of 4D Black Holes

We reconsider the four dimensional extremal black hole constructed in type
IIB string theory as the bound state of D1-branes, D5-branes, momentum, and
Kaluza-Klein monopoles. Specifically, we examine the case of an arbitrary
number of monopoles. Consequently, the weak coupling calculation of the
microscopic entropy requires a study of the D1-D5 system on an ALE space. We
find that the complete expression for the Bekenstein-Hawking entropy is
obtained by taking into account the massless open strings stretched between the
fractional D-branes which arise in the orbifold limit of the ALE space. The
black hole sector therefore arises as a mixed Higgs-Coulomb branch of an
effective 1+1 dimensional gauge theory.Comment: 12 pages. 1 figure. v2: References adde

### Experimental study of one- and two-component low-turbulence confined coaxial flows

Fluid mechanics experiments to investigate methods for reducing mixing between confined coaxial flows in cylindrical chambers for application to open-cycle gaseous-core nuclear rocket

### Mass and Momentum Turbulent Transport Experiments with Confined Coaxial Jets

Downstream mixing of coaxial jets discharging in an expanded duct was studied to obtain data for the evaluation and improvement of turbulent transport models currently used in a variety of computational procedures throughout the propulsion community for combustor flow modeling. Flow visualization studies showed four major shear regions occurring; a wake region immediately downstream of the inlet jet inlet duct; a shear region further downstream between the inner and annular jets; a recirculation zone; and a reattachment zone. A combination of turbulent momentum transport rate and two velocity component data were obtained from simultaneous measurements with a two color laser velocimeter (LV) system. Axial, radial and azimuthal velocities and turbulent momentum transport rate measurements in the r-z and r-theta planes were used to determine the mean value, second central moment (or rms fluctuation from mean), skewness and kurtosis for each data set probability density function (p.d.f.). A combination of turbulent mass transport rate, concentration and velocity data were obtained system. Velocity and mass transport in all three directions as well as concentration distributions were used to obtain the mean, second central moments, skewness and kurtosis for each p.d.f. These LV/LIF measurements also exposed the existence of a large region of countergradient turbulent axial mass transport in the region where the annular jet fluid was accelerating the inner jet fluid

### Topological gravity localization on a delta-function like Brane

Besides the String Theory context, the quantum General Relativity can be
studied by the use of constrained topological field theories. In the celebrated
Plebanski formalism, the constraints connecting topological field theories and
gravity are imposed in space-times with trivial topology. In the braneworld
context there are two distinct regions of the space-time, namely, the bulk and
the braneworld volume. In this work we show how to construct topological
gravity in a scenario containing one extra dimension and a delta-function like
3-brane which naturally emerges from a spontaneously broken discrete symmetry.
Starting from a D=5 theory we obtain the action for General Relativity in the
Palatini form in the bulk as well as in the braneworld volume. This result is
important for future insights about quantum gravity in brane scenarios.Comment: 4 page

### Semi-automatic crop inventory from sequential ERTS-1 imagery

The detection of a newly introduced crop into the Imperial (California) Valley by sequential ERTS-1 imagery is proving that individual crop types can be identified by remote sensing techniques. Initial results have provided an extremely useful product for water agencies. A system for the identification of field conditions enables the production of a statistical summary within two to three days of receipt of the ERTS-1 imagery. The summary indicates the total acreage of producing crops and irrigated planted crops currently demanding water and further indicates freshly plowed fields that will be demanding water in the near future. Relating the field conditions to the crop calendar of the region by means of computer techniques will provide specific crop identification for the 8000 plus fields

### Optical transitions in highly-charged californium ions with high sensitivity to variation of the fine-structure constant

We study electronic transitions in highly-charged Cf ions that are within the
frequency range of optical lasers and have very high sensitivity to potential
variations in the fine-structure constant, alpha. The transitions are in the
optical despite the large ionisation energies because they lie on the
level-crossing of the 5f and 6p valence orbitals in the thallium isoelectronic
sequence. Cf16+ is a particularly rich ion, having several narrow lines with
properties that minimize certain systematic effects. Cf16+ has very large
nuclear charge and large ionisation energy, resulting in the largest
alpha-sensitivity seen in atomic systems. The lines include positive and
negative shifters

### The Kahler Structure of Supersymmetric Holographic RG Flows

We study the metrics on the families of moduli spaces arising from probing
with a brane the ten and eleven dimensional supergravity solutions
corresponding to renormalisation group flows of supersymmetric large n gauge
theory. In comparing the geometry to the physics of the dual gauge theory, it
is important to identify appropriate coordinates, and starting with the case of
SU(n) gauge theories flowing from N=4 to N=1 via a mass term, we demonstrate
that the metric is Kahler, and solve for the Kahler potential everywhere along
the flow. We show that the asymptotic form of the Kahler potential, and hence
the peculiar conical form of the metric, follows from special properties of the
gauge theory. Furthermore, we find the analogous Kahler structure for the N=4
preserving Coulomb branch flows, and for an N=2 flow. In addition, we establish
similar properties for two eleven dimensional flow geometries recently
presented in the literature, one of which has a deformation of the conifold as
its moduli space. In all of these cases, we notice that the Kahler potential
appears to satisfy a simple universal differential equation. We prove that this
equation arises for all purely Coulomb branch flows dual to both ten and eleven
dimensional geometries, and conjecture that the equation holds much more
generally.Comment: 26 pages. Late

### Giant Leaps and Minimal Branes in Multi-Dimensional Flux Landscapes

There is a standard story about decay in multi-dimensional flux landscapes:
that from any state, the fastest decay is to take a small step, discharging one
flux unit at a time; that fluxes with the same coupling constant are
interchangeable; and that states with N units of a given flux have the same
decay rate as those with -N. We show that this standard story is false. The
fastest decay is a giant leap that discharges many different fluxes in unison;
this decay is mediated by a 'minimal' brane that wraps the internal manifold
and exhibits behavior not visible in the effective theory. We discuss the
implications for the cosmological constant.Comment: Minor updates to agree with published version. 9 pages, 4 figure

### Constraining cosmological ultra-large scale structure using numerical relativity

Cosmic inflation, a period of accelerated expansion in the early universe,
can give rise to large amplitude ultra-large scale inhomogeneities on distance
scales comparable to or larger than the observable universe. The cosmic
microwave background (CMB) anisotropy on the largest angular scales is
sensitive to such inhomogeneities and can be used to constrain the presence of
ultra-large scale structure (ULSS). We numerically evolve nonlinear
inhomogeneities present at the beginning of inflation in full General
Relativity to assess the CMB quadrupole constraint on the amplitude of the
initial fluctuations and the size of the observable universe relative to a
length scale characterizing the ULSS. To obtain a statistically significant
number of simulations, we adopt a toy model in which inhomogeneities are
injected along a preferred direction. We compute the likelihood function for
the CMB quadrupole including both ULSS and the standard quantum fluctuations
produced during inflation. We compute the posterior given the observed CMB
quadrupole, finding that when including gravitational nonlinearities, ULSS
curvature perturbations of order unity are allowed by the data, even on length
scales not too much larger than the size of the observable universe. Our
results illustrate the utility and importance of numerical relativity for
constraining early universe cosmology.Comment: 14 pages, 6 figures v3: Clarifications added regarding the generality
of results - conclusions unchanged, version accepted for publication in PRD,
v2: updated with minor clarifications, submitte

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